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          NCSC-TG-021
                                                            Library
          No.  S235,625




          FOREWORD




                    The  National  Computer  Security  Center  is
          issuing   the   Trusted   Database   Management  System
          Interpretation  as  part  of  the  Technical Guidelines
          Program, through which we produce the "Rainbow Series."
          In  the  Rainbow  Series,  we  discuss  in  detail  the
          features  of  the  Trusted  Computer  System Evaluation
          Criteria  (DoD 5200.28-STD)   and provide  guidance for
          meeting   each  requirement.   The   National  Computer
          Security Center, through its Trusted Product Evaluation
          Program, analyzes the security features of commercially
          produced  and  supported  computer  systems.  Together,
          these programs ensure that organizations are capable of
          protecting their  important data with  trusted computer
          systems.

                    The   Trusted   Database   Management  System
          Interpretation  extends the  evaluation classes  of the
          Trusted Computer System  Evaluation Criteria to trusted
          applications   in  general,  and   database  management
          systems in particular.  It serves  as an adjunct to the
          Trusted   Computer   System   Evaluation   Criteria  by
          providing a technical context  for the consideration of
          entire systems  constructed of parts and  by presenting
          database-specific interpretation of topics that require
          direct comment.   Thus, it is relevant  to applications
          which   support  sharing   of  computer   services  and
          resources, and  which enforce access  control policies.
          More  specifically, it  provides insight  into the  the
          design,  implementation, evaluation,  and accreditation
          of database management systems.


                    This document  will be used for  at least one
          year after  the date of signature.   During this period
          the  NCSC  will  gain   experience  using  the  Trusted
          Database  Management Systems Interpretation  in several
          evaluations and continue to  receive comments on issues
          of  technical  accuracy,  clarity  of  exposition,  and
          utility.   After this  trial period,  necessary changes
          will be made and a revised version issued.









          _____________

          PATRICK  R. GALLAGHER,  JR.
	April   1991 
	Director National Computer Security Center



          ACKNOWLEDGMENTS






                    This document represents  the combined effort
          of participants from  the research community, industry,
          and government working over several years.  Three major
          drafts   and   numerous   intermediary   versions  were
          produced,  reviewed, and  commented upon.   To name all
          the contributors would be  impossible.  To single out a
          few  would  be  to  slight  a  host  of others who gave
          unstintingly  of their time  and talent.  To  all those
          who  contributed to  the development  and refinement of
          the   fundamental   concepts,    contributed   to   the
          development  of the  several predecessor  versions, and


          who contributed  valuable personal time  and invaluable
          experience in reviewing and  commenting on the previous
          versions, thanks is extended.




          TABLE OF CONTENTS

          FOREWORD 						i
	ACKNOWLEDGMENTS 					iii
	INTRODUCTION 					1
	          HISTORICAL PERSPECTIVE 			1
                    SCOPE					2
                    PURPOSE 					2
                    STRUCTURE OF THE DOCUMENT 			4

          PART 1 TECHNICAL CONTEXT 				7

                    TC-1 INTRODUCTION 				9
                    TC-2 REFERENCE MONITOR PERSPECTIVE 		10
                    TC-3 NEED FOR EVALUATION BY PARTS 		11
                    TC-4 TCB SUBSETS 				11
                    TC-4.1 INTRODUCTION 			12
                    TC-4.2 TCB SUBSETS CONTEXT 			13
                    TC-4.2.1 DEFINITION OF TCB SUBSETS 		13
                    TC-4.2.2 MOTIVATION 			13
                    TC-4.3 CONDITIONS FOR EVALUATION BY PARTS 	14
                    TC-4.3.1 CANDIDATE TCB SUBSETS 		14
                    TC-4.3.2 POLICY ALLOCATION 			15
                    TC-4.3.3 TRUSTED SUBJECTS INCLUDED 		15
                    TC-4.3.4 TCB SUBSET STRUCTURE 		15
                    TC-4.3.5 SEPARATE SUBSET-DOMAINS 		16
                    TC-4.3.6 SUPPORT FOR RVM ARGUMENTS 		16
                    TC-4.4 EVALUATION ALTERNATIVES 		17
                    TC-5 GENERAL INTERPRETED REQUIREMENTS 		18
                    TC-5.1 OVERVIEW 				18
                    TC-5.2 DETAILED REQUIREMENTS 			18
                    TC-5.2.1 SECURITY POLICY 			18
                    TC-5.2.1.1 Discretionary Access Control 	18
                    TC-5.2.1.2 Object Reuse 			18
                    TC-5.2.1.3 Labels 				19
                    TC-5.2.1.4 Mandatory Access Control 		20
                    TC-5.2.2 ACCOUNTABILITY 			20


                    TC-5.2.2.1 Identification  and Authentication     20
                    TC-5.2.2.2 Audit 				21
                    TC-5.2.3 ASSURANCE 				22
                    TC-5.2.3.1 Operational Assurance 		22
                    TC-5.2.3.2 Life-Cycle Assurance 		23
                    TC-5.2.4 DOCUMENTATION 			24
                    TC-5.2.4.1 Security Features User's Guide 	24
                    TC-5.2.4.2 Trusted Facility Manual 		25
                    TC-5.2.4.3 Test Documentation 		26
                    TC-5.2.4.4 Design Documentation 		26
                    TC-5.3 SUMMARY OF THE REQUIREMENTS 		26
                    TC-5.3.1 LOCAL REQUIREMENTS 			26
                    TC-5.3.2 GLOBAL REQUIREMENTS 			27
                    TC-6 DESIGN CHOICES 			28
                    TC-6.1 OVERVIEW 				28
                    TC-6.2 A SINGLE TCB SUBSET 			28
		TC-6.2.1 ANALYSIS OF THE CONDITIONS 		28
                    TC-6.2.1.1 Condition 1: Candidate TCB Subsets 	28
                    TC-6.2.1.2 Condition 2: Policy Allocation 	29
                    TC-6.2.1.3 Condition 3: Trusted Subjects Included 29
                    TC-6.2.1.4 Condition 4: TCB Subset Structure      29
                    TC-6.2.1.5 Condition 5: Separate Subset-Domains   29
                    TC-6.2.1.6 Condition 6: Support for RVM Arguments 29
                    TC-6.2.2 EVALUATION CONSEQUENCES 		29
                    TC-6.3 TWO TCB SUBSETS,MEETING THE CONDITIONS 	30
                    TC-6.3.1 ANALYSIS OF THE CONDITIONS 		30
                    TC-6.3.1.1 Condition 1: Candidate TCB Subsets 	30
                    TC-6.3.1.2 Condition 2: Policy Allocation 	31
                    TC-6.3.1.3 Condition 3: Trusted Subjects Included 31
                    TC-6.3.1.4 Condition 4: TCB Subset Structure      31
                    TC-6.3.1.5 Condition 5: Separate Subset-Domains 	31
                    TC-6.3.1.6 Condition 6: Support for RVM Arguments 31
                    TC-6.3.2 EVALUATION CONSEQUENCES 		32
                    TC-6.4 TWO TCB SUBSETS, NOT MEETING THE CONDITIONS 33
                    TC-6.4.1 ANALYSIS OF THE CONDITIONS 		34
                    TC-6.4.1.1 Condition 1:  Candidate TCB Subsets 	34
                    TC-6.4.1.2 Condition 2:  Policy Allocation 	34
                    TC-6.4.1.3 Condition 3:  Trusted Subjects Included 34
                    TC-6.4.1.4 Condition 4:  TCB Subset Structure     35
                    TC-6.4.1.5 Condition 5:  Separate Subset-Domains 	35
                    TC-6.4.1.6 Condition 6:  Support for RVM Arguments 35
                    TC-6.4.2 EVALUATION CONSEQUENCES 		35
                    TC-6.5 SUMMARY 				36

          PART 2 INTERPRETED REQUIREMENTS 			37

                    IR-1 OVERVIEW AND CONTEXT 			39
                    IR-2 SUMMARY OF THE INTERPRETATIONS 		39
                    IR-2.1 SECURITY POLICY 			39
                    IR-2.1.1 DISCRETIONARY ACCESS CONTROL 		39
                    IR-2.1.2 OBJECT REUSE 			40
                    IR-2.1.3 LABELS 				40
                    IR-2.1.4 MANDATORY ACCESS CONTROL 		40
                    IR-2.2 ACCOUNTABILITY 			40
                    IR-2.2.1 IDENTIFICATION AND AUTHENTICATION 	40
                    IR-2.2.2 AUDIT 				40
                    IR-2.3 ASSURANCE 				40
                    IR-2.3.1 OPERATIONAL ASSURANCE 		40
                    IR-2.3.1.1 System Architecture 		40
                    IR-2.3.1.2 System Integrity 			40
                    IR-2.3.1.3 Covert Channel Analysis 		41
                    IR-2.3.1.4 Trusted Facility Management 		41
                    IR-2.3.1.5 Trusted Recovery 			41
                    IR-2.3.2 LIFE CYCLE ASSURANCE 		41
                    IR-2.3.2.1 Security Testing 			41
                    IR-2.3.2.2 Design Specification and Verification  41



                    IR-2.3.2.3 Configuration Management 		41
                    IR-2.3.2.4 Trusted Distribution 		41
                    IR-2.4 DOCUMENTATION 			42
                    IR-2.4.1 SECURITY FEATURES USER'S GUIDE 	42
                    IR-2.4.2 TRUSTED FACILITY MANUAL 		42
                    IR-2.4.3 TEST DOCUMENTATION 			42
                    IR-2.4.4 DESIGN DOCUMENTATION 		42
                    IR-3 LABELS 				42
                    IR-3.1 GENERAL DISCUSSION 			42
                    IR-3.2 SPECIFIC INTERPRETATIONS 		43
                    IR-4 AUDIT 				44
                    IR-4.1 GENERAL DISCUSSION 			44
                    IR-4.2 SPECIFIC INTERPRETATIONS 		45
                    IR-5 SYSTEM ARCHITECTURE 			47
                    IR-5.1 GENERAL DISCUSSION 			47
                    IR-5.2 SPECIFIC INTERPRETATIONS 		47
                    IR-6 DESIGN SPECIFICATION AND VERIFICATION 	48
                    IR-6.1 GENERAL DISCUSSION 			48
                    IR-6.2 SPECIFIC INTERPRETATIONS 		52
                    IR-7 DESIGN DOCUMENTATION 			55
                    IR-7.1 GENERAL DISCUSSION 			55
                    IR-7.2 SPECIFIC INTERPRETATIONS 		56

          APPENDIX A 					59

                    CLASS (C1):  				62
                    C1-1 SECURITY POLICY 			62
                    C1-2 ACCOUNTABILITY 			62
                    C1-3 ASSURANCE 				62
                    C1-4 DOCUMENTATION 				63
                    CLASS (C2):  				66
                    C2-1 SECURITY POLICY 			66
                    C2-2 ACCOUNTABILITY 			66
                    C2-3 ASSURANCE 				67
                    C2-4 DOCUMENTATION 				68
                    CLASS (B1):  				70
                    B1-1 SECURITY POLICY 			70
                    B1-2 ACCOUNTABILITY 			71
                    B1-3 ASSURANCE 				73
                    B1-4 DOCUMENTATION 				74
                    CLASS (B2):  				77


                    B2-1 SECURITY POLICY 			77
                    B2-2 ACCOUNTABILITY 			79
                    B2-3 ASSURANCE 				81
                    B2-4 DOCUMENTATION 				85
                    CLASS (B3):  				89
                    B3-1 SECURITY POLICY 			89
                    B3-2 ACCOUNTABILITY 			91
                    B3-3 ASSURANCE 				93
                    B3-4 DOCUMENTATION 				98
                    CLASS (A1):  				102
                    A1-1 SECURITY POLICY 			102
		A1-2 ACCOUNTABILITY 			104
                    A1-3 ASSURANCE 				106
                    A1-4 DOCUMENTATION 				112

          APPENDIX B 					117

                    1.  PERSPECTIVE ON ASSURANCE 			119
                    2.  PROCUREMENT OPTIONS 			120
                    3.  ALTERATION OF PREVIOUSLY EVALUATED TCB        122
                    4.  SATISFYING RVM REQUIREMENTS 		125
                    5.  SUBSET DEPENDENCY 			127
                    6.  TAMPER RESISTANCE ARGUMENTS 		131
                    7.  RATIONALE FOR LOCAL AND GLOBAL REQUIREMENTS 	132
                    8   CONTENT-DEPENDENT AND  CONTEXT-DEPENDENT
     		     ACCESS CONTROL 			136
                    9.  BULK LOADING OF A DATABASE 		137
                    10.  LOCAL ANALYSIS IN SYSTEM ASSESSMENT 	137
                    11.  RATING MORE COMPLEX SYSTEMS 		139

          GLOSSARY 						141


          BIBLIOGRAPHY 					145



          INTRODUCTION



          HISTORICAL PERSPECTIVE

           The  Department  of  Defense  Trusted  Computer System
          Evaluation  Criteria  (TCSEC),  published  in  1983  as
          CSC-STD-001-83, consolidates knowledge about the degree
          of trust one can place  in a computer system to protect
          sensitive information and organizes this knowledge into
          usable criteria  for system evaluation.  The  TCSEC was
          republished  as  a  DoD  standard,  DoD-5200.28-STD, in
          December 1985 to provide a means of evaluating specific
          security features and assurances available in "trusted,
          commercially available automatic data processing system

                    The  TCSEC's  rating  scale  extends  from  a
          minimal to a high level of trust with advanced security
          features   and    sophisticated   assurance   measures.
          Specific criteria determine each rating level and guide
          system builders and evaluators in determining the level
          of trust provided by specific systems.  When the rating
          levels are  combined with environmental  guidelines and
          minimum security protection requirements, accreditation
          decisions for specific installations can be made.

                    The philosophy of  protection embodied in the
          TCSEC  requires  that  the  access  of  subjects (i.e.,
          processes in a domain)  to objects (i.e., containers of
          information) be mediated in accordance with an explicit
          and  well-defined  security   policy.   At  the  higher
          criteria classes,  the "reference monitor  concept" [1]
          is  an essential  part of  the system  and the security
          policy is  modeled.  There are several  security policy
          models  that  represent  the   desired  behavior  of  a
          reference monitor.  The Bell-La  Padula model [4-6] and
          its Multics  interpretation [3] are commonly  used, but
          not mandated.

                    The    computer    security    research   and
          development that  underpin the TCSEC began  in the late
          1960s and concentrated on secure operating systems.  By
          the early 1970s initial  worked examples had provided a
          substantial amount of  information about building trust
          into operating systems.   Research continued throughout


          the   1970s  to    refine  mechanisms,   features,  and
          assurances needed to provide trusted operating systems.

                    Multilevel   database   management   security
          received  far less  research and  development attention
          than did secure operating  systems.  This was primarily
          due  to  the  perception  that  one  could not credibly
          implement  a  multilevel   secure  database  management
          system (DBMS)  on top of an  untrusted operating system
          base.   However,  some  research  in  multilevel secure
          DBMSs  (mostly theoretical)   was conducted  during the
          1970s  [15-16],  and  research  has  continued  to  the
          present  [9-14, 17-19, 22,  25-28].  By the  mid 1980s,
          commercially developed, trusted  operating systems were
          becoming  available that  could provide  the basis  for
          hosting secure  applications such as  multilevel secure
          DBMSs.

                    In June 1986,  the National Computer Security
          Center  (NCSC)  initiated  its  efforts  to address the
          evaluation of trusted  database management systems with
          an   Invitational  Workshop  in   Baltimore,  Maryland.
          Representatives  from  the  research,  database vendor,
          commercial, and  government communities met  to address
          issues of database  management security.  The attendees
          met to discuss aspects of  trust (defined by the TCSEC)
          that  were  sufficiently  well  defined  and capable of
          producing  repeatable and  objective assessments.   The
          NCSC  invited issue  papers and  prepared a  discussion
          agenda.    The  issue   papers  and   the  subcommittee
          summaries  were  published  as  the  Proceedings of the
          National Computer Security Center Invitational Workshop
          on Database Security [20].

                    As  an outgrowth  of this  workshop, the NCSC
          undertook the  task of preparing this  Trusted Database
          Management System Interpretation (TDI)  of the TCSEC to
          focus  on  the  special  problems  posed  by  DBMSs.  A
          working   group    was   assembled   to    draft   this
          Interpretation.   Three  drafts   were  produced,  with
          extensive community review and public discussion.  This
          Interpretation is the result  of the interaction within


          the general  computer security and  database management
          communities.


          SCOPE

                    The  interpretations  in  this  document  are
          intended  to  be  used  in  conjunction  with the TCSEC
          itself;  they  apply  to  application-oriented software
          systems  in general,   and database  management systems
          (DBMSs)  in particular.  Although  the interpretations,
          as noted,  are general enough to apply  to any software
          system  which  supports  sharing  and  needs to enforce
          access  control (e.g., transaction  processing systems,
          electronic   mail   systems),   in   the   interest  of
          simplicity, the  discussion, and thus  the terminology,
          will be directed toward DBMSs.

                    The   interpretations  are  intended   to  be
          applied  primarily  to  commercially  developed trusted
          DBMSs,  but can  also be  applied to  the evaluation of
          existing non-commercial DBMSs  and to the specification
          of security requirements for DBMS acquisitions.


          PURPOSE

                    This  Interpretation of   the TCSEC  has been
          prepared for the following purposes:

                      To provide a  standard to manufacturers for
          security features to build into  their new and  planned commercial
          products in order to provide widely available systems that satisfy
          trust requirements (with  particular emphasis on  preventing the
          disclosure of data) for sensitive applications,



                    To  provide a  metric with  which to evaluate
          the degree of trust that can be placed in computer systems for the
          secure processing of classified and other sensitive information,
          and

                    To provide a  basis for specifying security
          requirements in acquisition specifications.

                    With  respect  to   the  second  purpose  for
          development of the criteria, i.e., providing a security
          evaluation metric,  evaluations can be  delineated into
          two  types:  (1)  evaluations performed  on a  computer
          product   from   a   perspective   that   excludes  the
          application environment; or,  (2) evaluations to assess
          whether appropriate  security measures have  been taken
          to  permit the  system to  be used  operationally in  a
          specific environment.  The former type of evaluation is
          done  by the  National Computer  Security Center (NCSC)
          through the  Trusted Product Evaluation Program  and is
          called "formal product evaluation."

                    The latter  type of evaluation, that  is, one
          done for  the purpose of assessing  a system's security
          attributes  with  respect  to  a  specific  operational
          mission, is  known as a "certification  evaluation."  A
          formal   product   evaluation   does   not   constitute
          certification  or accreditation  for the  system to  be
          used  in  any  specific  application  environment.  The
          system   security   certification    and   the   formal
          approval/accreditation  procedure,  done  in accordance
          with the  applicable policies of the  issuing agencies,
          must still be followed before  a system can be approved
          for  use   in  processing  or  handling   sensitive  or
          classified    information.      Designated    Approving
          Authorities  (DAAs) remain  ultimately responsible  for
          specifying the security of systems they accredit.
                                        3


                    The  TCSEC and   this Interpretation  will be
          used  directly  and  indirectly  in  the  certification
          process.   Along with  applicable policy,  they will be
          used directly  as technical guidance for  evaluation of
          the total system and for specifying system security and
          certification requirements for new acquisitions.  Where
          a  system being  evaluated for  certification employs a
          product that has undergone a formal product evaluation,
          reports from that process will  be used as input to the
          certification   evaluation.   Moreover,   the  National
          Security Agency plans  to publish additional guidelines
          to  assist certifiers  and help  ensure consistency  in
          certifications of systems  processing national security
          informantion.


          STRUCTURE OF THE DOCUMENT

                    The remainder of the  TDI is divided into two
          parts, plus two appendices and a glossary.

                    PART 1, "TECHNICAL CONTEXT," presents general
          information  about the   evaluation of  trusted systems
          that are constructed of several parts.  This discussion
          is  critical  to  trusted   DBMSs  built  upon  trusted
          operating  systems, but is  not limited to  DBMSs only.
          It is included  in the TDI because it  is not currently
          available in  any previously published  document.  This
          section reviews the  central reference monitor concept,
          explains  the  need  to  evaluate  a  system  built  of
          well-defined  parts, and  develops the  concept of  TCB
          subsets.   TCB subsets  provide  a  way of  splitting a
          total TCB  along access control policy  lines such that
          an  evaluation  by  parts  can  be  undertaken.  PART 1
          concludes  with   an  interpretation  of   those  TCSEC
          requirements  which are  relevant to  an evaluation  by
          parts, and  a description of the  process of evaluation
          by parts.

                    PART 2,  "INTERPRETED REQUIREMENTS," provides
          interpretions  of  those  TCSEC  requirements  that are
          either specific to DBMSs  (or applications in general),


          or are  particularly relevant to DBMSs.   The number of
          requirements that are  treated explicitly is relatively
          small, because many of  the TCSEC requirements apply as
          stated    to   database   management    systems.    The
          requirements  treated  explicitly  are  labels,  audit,
          system    architecture,   design    specification   and
          verification, and design documentation.

                    Appendix   A   summarizes   the   interpreted
          requirements  for each TCSEC  class and is  intended to
          provide an easy reference for those requiring a listing
          of  requirements  for  a  specific  class  (e.g.,  B2).
          Appendix  B provides  discussion of  several topics not
          directly  tied to  the requirements  levied on  trusted
          DBMSs by the interpretation of the TCSEC requirements.

                    The  TDI  proper  will  be  supplemented by a
          Companion Document Series (CDS).   The CDS will address
          a wide spectrum of issues  related to trusted DBMSs but
          which are beyond the scope of this document.  Community
          debate about on-going topics  of interest will probably
          result  in gradual  extensions of  what is  known about
          trusted DBMSs.  Thus, volumes in the CDS will be issued
          both regularly (to document  the state of the community
          debate)  and by exception  (to record new  problems and
          new solutions).

          PART 1

          TECHNICAL CONTEXT

                                                  7


          TC-1 INTRODUCTION

                    Modern computing systems are rarely conceived
          and built  by a single organization.   Rather, the rule
          is   that  systems    are  constructed   by  assembling
          parts?hardware,    firmware,   and    software?produced
          independently  by  various  organizations  or  vendors.
          This fact introduces a  fundamental difficulty into the


          task  of evaluating a  "system" for conformance  to the
          trust  requirements  of  the  Trusted  Computer  System
          Evaluation Criteria (TCSEC).  [8] This difficulty stems
          from the  fact that assessment (either  evaluation of a
          product or certification of  a system) entails a global
          perspective of  the entire system  under consideration.
          There are not yet  widely accepted methods of factoring
          the  various aspects  of  a  trust assessment  and then
          reassembling  the results  into a  statement about  the
          whole.


                    These   conflicting  perspectives   of  local
          production   and   global   evaluation   analysis   are
          particularly   evident  in    the  field   of  database
          management, but  they are by  no means limited  to that
          field.   Thus the   publication of  this Interpretation
          requires  consideration  of  how  to  deal with systems
          built in parts in the absence of a general treatment of
          the  topic.  On  the other  hand, any  treatment of the
          issue  in the  context of  trusted database  management
          will have  significant influence in other  fields where
          the  same or  similar problems  arise, just  because of
          community  review and  NCSC publication.   The approach
          taken  in this  document is  to address  the issues  of
          evaluating  systems built  of parts  in a  way that  is
          independent   of   the   field   of   trusted  database
          management.  This  conscious attitude of  generality is
          intended  to  make  clear  the  distinction between the
          larger  system-of-parts issues   and the  more specific
          DBMS issues.

                    PART 1, "TECHNICAL  CONTEXT," is divided into
          six   sections.   Section   TC-2,  "Reference   Monitor
          Perspective,"  summarizes  the  role  of  the reference
          monitor concept in both the  TCSEC and the assessing of
          a system for its  trust characteristics.  Section TC-3,
          "Need for Evaluation by Parts,"  deals with the need to
          extend  the reference  monitor perspective  slightly to
          begin to address the  evaluation of systems constructed
          of separate parts.  Section TC-4, "TCB Subsets," is the
          heart   of   PART   1.    That   section  introduces  a


          conservative  extension  to  the  reference  validation
          mechanism,  TCB subsets.   This extension  provides the
          basis for being able to undertake evaluation of systems
          built  in parts  in a   way that  allows re-use  of the
          results   of   separate   evaluations   (whether  those
          evaluations   were   performed   before   the   current
          evaluation   was   begun   or   whether   the  separate
          evaluations overlap  in time).  Section  TC-5, "General
          Interpreted Requirements," outlines  the application of
          the TCSEC  requirements to individual TCB  subsets when
          an  evaluation by parts  is being done.   Section TC-6,
          "Design  Choices"  describes  the  general  process  of
          applying  TCB subsets  and meeting  the conditions  for
          evaluation by parts.  The  treatment in this section is
          general and not limited  to DBMSs; DBMS-specific issues
          are discussed in the appendices.


          TC-2 REFERENCE MONITOR PERSPECTIVE

                    Building   or   evaluating   a   system   for
          compliance with the requirements  of a particular class
          in the TCSEC is based on the perspective of the Trusted
          Computing Base (TCB).  The notion of the TCB is central
          to the  entire concept of assessing  systems for trust.
          The reference monitor described  in the Anderson report
          [1] is the  basis of the notion of a  TCB, as described
          in the TCSEC:

                    For  convenience,  these  evaluation criteria
          use  the term  Trusted Computing  Base to  refer to the
          reference  validation  mechanism,   be  it  a  security
          kernel,  front-end  security   filter,  or  the  entire
          trusted computer system.  [8, p.  67]

                    Even in those lower  classes (below B2) where
          the reference monitor  concept and reference validation
          mechanisms   are   not    mentioned   explicitly,   the
          perspective   of   the   reference   monitor   and  its
          implementation as  a reference validation  mechanism is
          present.  Specifically, there  are requirements for (1)
          identifying the policy  being enforced, (2) identifying


          subjects and  objects, (3) providing evidence  that the
          operation of the reference validation mechanism matches
          the high-level  description of the user  interface, and
          (4) demonstrating isolation of the TCB.

                    Therefore,  all TCSEC  evaluations share  the
          initial conceptual  steps of identifying  the mediation
          policy, the subjects, and the objects.  Starting from a
          global  system  perspective,  the  initial  step  is to
          identify  the  access  mediation  policy  that  will be
          enforced.  One  must then identify those  active system
          entities that  are candidates for being  the "subjects"
          whose   access  to   objects  the   TCB  will  mediate.
          Similarly,  one must  identify those  passive entities,
          those data repositories, that  are candidates for being
          the "objects," access to which the TCB will mediate.

                    As  usual, the   existence of  an abstraction
          within  a  system  does   not  make  it  necessarily  a
          reference-monitor object, i.e., one  visible at the TCB
          interface.  This  is because the  TCB will make  use of
          system abstractions for both its internal processes and
          its storage requirements.   Those entities, while being
          stored  in system "objects,"  will not be  available to
          untrusted processes (that is,  they are not exported by
          the TCB).   Thus the analytical, iterative  step is the
          separation of candidate subjects and objects into those
          that are mediated by the TCB and those that are not.

                    The    various    trust    classes    include
          requirements,  at varying   levels of  completeness and
          rigor, that the basic  reference monitor perspective of
          mediating access of subjects  to objects be implemented
          in  a  correct,   self-protecting,  and  non-bypassable
          manner.   The core  requirements of  the TCSEC  classes
          focus  on  these  reference  monitor  imperatives.  The
          increasingly  strict requirements  for visibility  into
          the   system  design  and   implementation  (structure,
          documentation, testing, configuration, and distribution
          requirements)  all support  the notion  of checking the
          system's  conformance  to  its  identified  intent with
          regard to the subjects, objects, and policy.


          TC-3 NEED FOR EVALUATION BY PARTS

                    The need  to be able to  evaluate and certify
          systems built  in parts is increasingly  evident.  This
          need is seen in a variety of contexts:

                      The  need to  evaluate and  certify systems
          built  out  of  parts  sold  by  different  vendors,  a
          situation especially prevalent in  the field of trusted
          DBMSs.

                      The  need to   re-assess systems  that have
          undergone either small- or large-scale improvements and
          are  already  evaluated  and  placed  on  the Evaluated
          Products List (EPL).

                      The general problem of "families of
          systems," systems that exist  on a spectrum of hardware
          bases or  that can be customized for  a user's specific
          needs.

                    In all such cases,  two related versions of a
          system are  largely similar.  It should  be possible to
          undertake evaluations and certifications  in such a way
          that the entire assessment does  not have to be re-done
          for every  slight variation that appears.   The current
          state  of technology,   however, places  limitations on
          what  can be  accomplished in  this regard;  it is  not
          currently    possible    to    determine    the   trust
          characteristics  of  a  system   on  the  basis  of  an
          arbitrary collection of subparts.   The overall task of
          trust assessment entails  so many interrelated subtasks
          that the ability to separate and reassemble is not well
          understood.

                    In this circumstance of needing to be able to
          accommodate evaluation  of a system built  in parts and
          the lack of  consensus about how this can be  done in a
          technically sound fashion, a conservative approach must
          be adopted.   The following are required:   (1) a clear
          description  of  what  "parts"  will  be considered for
          separate  evaluation; (2)  a clear  description of  the


          conditions under which such an evaluation by parts will
          be  undertaken;  and  (3)  a  general interpretation of
          TCSEC requirements as they would apply when a system is
          being  evaluated by  parts.  The  "parts" that  will be
          considered  by  separate  evaluation  are  called  "TCB
          subsets," the topic of Section TC-4 below.


          TC-4 TCB SUBSETS


          TC-4.1 INTRODUCTION

                    To  attempt   an  evaluation  of  a   TCB  by
          splitting  it into  parts,  there  must be  available a
          precise  definition of  what parts  are candidates  for
          separate evaluation (that is,  for evaluation by parts)
          as well as any other  conditions that must be satisfied
          before an evaluation by parts will be undertaken.  This
          section   defines  "TCB   subset"  as   a  strict   and
          conservative  extension of  the traditional  concept of
          the reference validation mechanism (RVM) as a method of
          delineating which parts of a  TCB can be candidates for
          separate evaluation.  The definition of TCB subsets, as
          well  as  explanatory  and  motivational  material,  is
          included   below  in   Section  TC-4.2,   "TCB  Subsets
          Context."  Section TC-4.3 addresses the conditions that
          must be  satisfied by a TCB  divided into a set  of TCB
          subsets before evaluation by  parts will be undertaken.
          These  conditions  assure  that  the  structure  of and
          relationships  among  TCB  subsets  will  satisfy TCSEC
          requirements,   especially  those   derived  from   the
          reference monitor concept.


          TC-4.2 TCB SUBSETS CONTEXT


          TC-4.2.1 DEFINITION OF TCB SUBSETS

                     A  TCB  subset  M  is  a  set  of  software,
          firmware, and hardware (where  any of these three could


          be  absent) that  mediates the   access of  a set  S of
          subjects to a set O of objects on the basis of a stated
          access control policy P and satisfies the properties:

                    1) M mediates every access to objects in O by
          subjects in S;
                    2) M is tamper resistant; and
                    3)  M  is  small  enough  to  be  subject  to
          analysis  and tests, the  completeness of which  can be
          assured.

                    M uses resources provided  by an explicit set
          of more primitive TCB subsets  to create the objects of
          O, create  and manage its data  structures, and enforce
          the  policy  P.   If  there  are  no  TCB  subsets more
          primitive than  M, then M uses  only hardware resources
          to  instantiate its objects,  to create and  manage its
          own data structures, and to enforce its policy.

                    The  above  definition  does  not  explicitly
          prohibit an access control policy P that allows trusted
          subjects.  The implications  for the evaluation process
          when  a TCB  subset's policy  allows or  does not allow
          such trusted subjects are substantial and are discussed
          in Section TC-6.4.  As described  in TC-4.3, one of the
          conditions for an evaluation by  parts of a TCB made up
          of TCB subsets is that all the trusted subjects of each
          TCB subset be included in that TCB subset.

          TC-4.2.2 MOTIVATION

                    The definition of "TCB subset" is intended to
          parallel the  definitions of the reference  monitor and
          reference  validation mechanism  found in  the Anderson
          report [1] and included in the TCSEC itself.  "The term
          Trusted  Computing  Base   [refers]  to  the  reference
          validation mechanism, be  it security kernel, front-end
          security  filter,   or  the  entire   trusted  computer
          system."  [8,  p.  67] "TCB subset"  is defined exactly
          like  a reference  validation mechanism,  with only one
          difference, that it does  not necessarily extend to the
          hardware.   Rather, a  TCB subset  uses either hardware


          resources  or  the  resources  provided  by other, more
          primitive  TCB  subsets.   Thus  TCB  subsets  build on
          abstract machines, either physical hardware machines or
          other  TCB  subsets.   Just  like  reference validation
          mechanisms, a TCB subset  must enforce a defined access
          control policy.


          TC-4.3 CONDITIONS FOR EVALUATION BY PARTS

                    Building  or evaluating   a system  using the
          definition of TCB subsets in the section above requires
          meeting  six conditions   in addition  to demonstrating
          that the  candidate TCB subsets satisfy  the properties
          appropriate  to  the   evaluation  target  class.   The
          conditions are as follows:

                              The   candidate  TCB   subsets  are
          identified;
                              The  system policy is  allocated to
          the candidate TCB subsets;
                              Each  candidate   TCB  subset  M[i]
          includes all the trusted subjects   with   respect   to
	its technical policies P[i];
                              The   TCB   subset   structure   or
          architecture is explicitly described;
                              Each  TCB subset  occupies distinct
          subset-domains; and
                              The  more   primitive  TCB  subsets
          provide support for the RVM arguments  for  less  primitve  TCB
          subsets.

          These conditions are described below.

          TC-4.3.1 CANDIDATE TCB SUBSETS

                    The  first  condition  is  that  the relevant
          elements   of  each   candidate  TCB   subset  M[i]  be


          identified.  The hardware, firmware, and software which
          compose the  TCB subset need to  be clearly identified,
          along  with  the  subjects  and  objects.   In terms of
          Section TC-4.2.1, this  condition is the identification
          of  M[i], S[i],  and O[i].    There may  be no  objects
          mediated by  more than one TCB subset.   That is, there
          cannot be an object O that is both in O[i] and O[j].

          TC-4.3.2 POLICY ALLOCATION

                    The next condition  is policy allocation, the
          description  of  the  technical  policy  P[i]  for each
          identified  M[i]  along  with  the  relation  of  these
          policies  to the  system policy  P.  The  policies P[i]
          will  be expressed  in terms  of subjects  in S[i]  and
          objects   in  O[i].    Thus,  to   satisfy  the   TCSEC
          requirement that the (composite) TCB enforce its stated
          policy P, each rule in  P must be traceable through the
          structure  of  the  candidate  TCB  subsets  to the TCB
          subset(s) where that  enforcement occurs.  See Sections
          TC-5.2.1.1 and TC-5.2.1.4.

          TC-4.3.3 TRUSTED SUBJECTS INCLUDED

                    Every  trusted subject  with respect  to P[i]
          must be  part of the  TCB subset M[i].   This condition
          makes possible separate evaluation  of TCB subsets with
          respect to  "local" requirements.  When  this condition
          is not met, evaluation  of candidate TCB subsets cannot
          be  guaranteed on an  evaluation by parts  basis.  This
          situation is treated in Section 6.4.

          TC-4.3.4 TCB SUBSET STRUCTURE

                    The  TCB  subsets  will  exhibit  a structure
          based  on  the  ordering  implied  by  dependency.  TCB
          subset A is  less primitive than TCB subset B  if (a) A
          directly  depends on B  or (b) a  chain of TCB  subsets
          from A to B exists such  that each element of the chain
          directly  depends on  its successor  in the  chain.  In
          this  case we  use the   phrase "TCB  subset B  is more
          primitive than TCB subset A" synonymously.


                    The  sense  of  "directly  depend"  in (a) is
          exactly  the following:  it  is not possible  to verify
          the   implementation   of   A   with   respect  to  its
          specification    without   a   statement    about   the
          specification of B.


                    More precisely, for a candidate TCB subset M,
          let   sM   denote   the   specification   of   M.   The
          specification will include, as a minimum, the statement
          of the technical policy P of M.  Further, let vM denote
          the   (engineering)  demonstrations   of  the   correct
          implementation of M with  respect to its specification.
          A   (candidate)  TCB   subset  A   "depends  (for   its
          correctness)" on  (candidate) TCB subset B  if and only
          if the arguments of vA  assume, wholly or in part, that
          sB  has been  implemented correctly.   (See Appendix B,
          item 5 for additional discussion.)

                    The proposed structure of  TCB subsets has to
          be  identified.  The  order  must  be a  partial order.
          (Without  a  partial  order,  there  could  be circular
          chains  of  dependencies  that  would  inhibit separate
          evaluations of the TCB subsets.)

          TC-4.3.5 SEPARATE SUBSET-DOMAINS

                    The  candidate TCB   subsets must  operate in
          near   isolation  from   each  other,   with  the  only
          interaction between them being that explicitly asserted
          as  part of  the interface.   In the  case of reference
          monitors, many existing  implementations have relied on
          the domain  concept [23] which supports  the assertions
          of  non-bypassability and  self protection.   A natural
          extension  of the domain  concept will be  required for
          isolation  of  TCB  subsets  from  each  other and from
          non-TCB entities.

                    A subset-domain  is a set of  system domains.
          Each  candidate  TCB  subset  must  occupy  a  distinct
          subset-domain such that modify-access to a TCB subset's
          subset-domain is permitted only  to that TCB subset and


          (possibly)  to   more  primitive  TCB   subsets.   This
          requirement    ensures    that    the    structure   of
          subset-domains with respect to access is consonant with
          the dependency relation among TCB subsets.

          TC-4.3.6 SUPPORT FOR RVM ARGUMENTS

                    Candidate TCB subsets  must satisfy the three
          RVM properties  included in the definition  in TC-4.2.1
          in order to complete  evaluation by parts successfully.
          TCB subsets  that build on resources  and functionality
          provided  by more  primitive TCB  subsets must  rely on
          assured and  evaluatable characteristics of  those more
          primitive TCB  subsets.  A convincing argument  must be
          advanced   that  the  features,   characteristics,  and
          assurances provided  by the more primitive  TCB subsets
          are  capable of supporting  RVM arguments for  the less
          primitive TCB subsets.

                    The  first property (mediating  every access)
          requires  that  there  is   no  way  of  bypassing  the
          mediation  provided by  TCB subset  M for  its objects,
          either  directly  or   by  unexpected  side-effects  of
          interactions  with  other  TCB  subsets.   A variety of
          approaches   could  suffice   for  demonstrating   this
          property.

                    The   second  property   (tamper  resistance)
          requires that the policy-critical code and data for the
          less  primitive  TCB  subset   be  protected  from  any
          alteration not specifically allowed  by the TCB subset.
          A variety of approaches could suffice for demonstrating
          this property.

                    The  third property (completeness  of testing
          and    analysis   for    correctness)   requires    the
          (engineering)  demonstrations vM[i] of  the correctness
          of  each less primitive  candidate TCB subset  M[i].  A
          convincing argument must therefore be advanced that the
          specifications sM[k] for all  of the more primitive TCB
          subsets  M[k] on  which M[i]  depends will  suffice for
          establishing vM[i].


          TC-4.4 EVALUATION ALTERNATIVES

                    As  noted  earlier,   the  need  to  evaluate
          systems   whose  elements  are   developed  separately,
          possibly by independent developers, results in the need
          to define  TCB subsets.  That  is not to  say, however,
          that  design  by  TCB  subsetting  and  the  subsequent
          evaluation by parts are the only alternatives.  Rather,
          there are three distinct possibilities.

                    A  system  TCB,  regardless  of  any internal
          structure, may be viewed as a single entity.  In such a
          case,  the  evaluation  may  proceed  essentially as an
          evaluation against the TCSEC.  This case is examined in
          Section TC-6.2.

                    A system TCB may  be presented as a subsetted
          architecture  composed  of  a  number  of candidate TCB
          subsets.  Given that each  of the candidate TCB subsets
          satisfies the  conditions set forth in  Section TC-4.3,
          an  evaluation  by  parts  is  possible.   This case is
          described in Section TC-6.3.

                    It  may be possible  to satisfy only  some of
          the conditions for evaluation by parts.  This situation
          might  arise when  a  previously  evaluated TCB  or TCB
          subset is modified  to accommodate the policy-enforcing
          elements of a new application layer.  A special case of
          this situation is described in Section TC-6.4.  In such
          cases,  depending  upon  the  particulars,  it  may  be
          possible to  realize some of the  savings in evaluation
          effort.  However, no general statements can be made for
          these cases.


          TC-5 GENERAL INTERPRETED REQUIREMENTS


          TC-5.1 OVERVIEW

                    This      section      provides      specific
          interpretations  of those  TCSEC requirements  that are


          particularly  relevant for subsetted  architectures and
          evaluation by parts.  Its  organization is derived from
          the  structure  of  the  TCSEC  requirements.  For each
          relevant TCSEC requirement there is a discussion of how
          that  requirement is  interpreted in  an evaluation  by
          parts.

                    For   conciseness,   only   the  requirements
          headings applicable for A1  systems are included below.
          Thus,  the  interpretation  guidance  below  should  be
          applied  only as demanded  by the requirements  for the
          target  class of the  candidate trusted system.   For a
          system targeted  at an evaluation class  below A1, only
          those  requirements that  pertain to  the target  class
          apply to the TCB subsets making up that system.

                    A    listing   of   the    requirements   and
          interpretations by TCSEC class is presented in Appendix
          A.   The rationale for  the applicability of  the TCSEC
          requirements to TCB  subsets alone or to the  TCB as an
          entirety is described in Appendix B, item 7.


          TC-5.2 DETAILED REQUIREMENTS

          TC-5.2.1 SECURITY POLICY

          TC-5.2.1.1 Discretionary Access Control

                    The discretionary access control requirements
          apply as stated in the  TCSEC to every TCB subset whose
          policy  includes discretionary   access control  of its
          subjects to  its objects.  Any TCB  subset whose policy
          does not  include such discretionary access  control is
          exempt from this requirement.

          TC-5.2.1.2 Object Reuse

                    This  requirement applies   as stated  in the
          TCSEC to every TCB subset in the TCB.

          TC-5.2.1.3 Labels


                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          Label Integrity


                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          Exportation of Labeled Information


                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          Subject Sensitivity Labels


                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

          Device Labels


                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory access control of its subjects to its objects
          and has attached physical  or virtual devices.  Any TCB


          subset  whose policy  does not  include such  mandatory
          access control  or has no attached  physical or virtual
          devices   is  exempt   from  this   requirement.   This
          requirement can be satisifed  by the cooperative action
          of more than one TCB subset.

          TC-5.2.1.4 Mandatory Access Control

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          TC-5.2.2 ACCOUNTABILITY

          TC-5.2.2.1 Identification and Authentication

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    If  the TCB is  composed of TCB  subsets, one
          TCB subset  may either rely  on a mechanism  in another
          TCB subset to provide identification and authentication
          services or provide  the services directly.  Similarly,
          that TCB subset may rely on a mechanism in another more
          primitive TCB subset to  ensure that the security level
          of subjects external to the  TCB that may be created to
          act on  behalf of the individual user  are dominated by
          the clearance and authorization of that user.  Each TCB
          subset   may  maintain   its  own   identification  and
          authentication  data or  one central  repository may be
          maintained.  If each TCB subset  has its own data, then
          the  information  for  each  individual  user  must  be
          consistent among all subsets.

          Trusted Path



                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    When  TCB subsets  are used,  the requirement
          for  trusted  path  at  levels  B2  and  above  remains
          applicable  to the  entire TCB.   The need  for trusted
          path "when positive  TCB-to-user connection is required
          (e.g.,  login,  change  subject  security  level)"  can
          require user interaction with  virtually any TCB subset
          within  the TCB.   The implementation  of trusted  path
          could   be   localized   in   a   single   TCB  subset.
          Alternatively, it could be implemented in more than one
          TCB  subset if   the separate  implementations together
          comply with the system security policy.

          TC-5.2.2.2 Audit

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    A  TCB subset  may maintain  its own security
          audit  log,  distinct  from  that  maintained  by  more
          primitive TCB subsets, or it may use an audit interface
          provided by  a different TCB subset  allowing the audit
          records  it  generates  to  be  processed  by  that TCB
          subset.

                    If  the   TCB  subset  uses   different  user
          identifications than a more primitive TCB subset, there
          shall be  a means to associate  audit records generated
          by different  TCB subsets for the  same individual with
          each other,  either at the  time they are  generated or
          later.

                    Any TCB subset wherein  events may occur that
          require  notification  of  the  security  administrator
          shall  be able  to do  the following:   (1) detect  the
          occurrence of these events,  (2) initiate the recording


          of  the  audit  trail   entry,  and  (3)  initiate  the
          notification   of  the  security   administrator.   The
          ability to  terminate events (2)  and (3) above  may be
          provided  either in  the TCB  subset within  which they
          occur, or in the TCB  subset(s) where actions that lead
          to the event were initiated.

                    The monitoring  and notification requirements
          may require  cooperation between multiple  distinct TCB
          subsets  or  multiple  instantiations  of  the same TCB
          subset.   For example,  to detect  the accumulation  of
          events for a single user it may be necessary to collect
          events from several subjects in distinct processes that
          are surrogates for the  same user.  As another example,
          there may  be a single TCB subset  that collects events
          from a  number of other TCB  subset instantiations and,
          based  on its analysis  of them, notifies  the security
          administrator.

          TC-5.2.3 ASSURANCE

          TC-5.2.3.1 Operational Assurance

                    System Architecture

                    This  requirement applies   as stated  in the
          TCSEC   to  every   TCB  subset,   with  the  following
          additional interpretations.

                    The  TCB must  provide domains  for execution
          that are allocated to and used by TCB subsets according
          to the subset-domain condition for evaluation by parts.
          A  most primitive TCB  subset must provide  domains for
          execution.  A  less primitive TCB subset  must make use
          of domains provided by a  more primitive TCB subset.  A
          less primitive TCB subset may provide further execution
          domains but is not required to do so.

                    Similarly,  the  TCB  must  provide  distinct
          address  spaces   for  untrusted  processes.    A  most
          primitive  TCB  subset  must  provide  distinct address
          spaces for  its subjects.  A less  primitive TCB subset


          must make use of the distinct address space provided by
          a  more primitive  TCB  subset.   A less  primitive TCB
          subset may  provide more fine-grained  distinct address
          spaces, but is not required to do so.

                    In    general,   requirements    specifically
          referring  to hardware  or firmware  apply only  to TCB
          subsets  that   include  hardware  or   firmware.   The
          exception  is   the  requirement  that  the   TCB  make
          effective use of  available hardware.  This requirement
          applies  to  those  TCB   subsets  that  use  resources
          provided  by  more  primitive  TCB  subsets  in lieu of
          hardware.   Those  TCB  subsets  are  required  to make
          effective  use  of   the  protection-relevant  features
          exported to it by the more primitive TCB subsets (e.g.,
          execution domains, support for distinct address spaces)
          to separate those elements that are protection-critical
          from those that are not.

          System Integrity


                    This  requirement applies   as stated  in the
          TCSEC  to every  TCB subset  that includes  hardware or
          firmware.   Any  TCB  subset   that  does  not  include
          hardware or firmware is exempt from this requirement.

          Covert Channel Analysis


                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB.    When the  TCB is  made up
          entirely  of  TCB  subsets  meeting  the conditions for
          evaluation  by parts,  analysis of  the individual  TCB
          subsets satisfies this  requirement.  Otherwise, covert
          channel  analysis of the  entire TCB must  be performed
          (even if the results of  covert channel analysis of the
          individual TCB subsets were available).

          Trusted Facility Management



                    This  requirement applies   as stated  in the
          TCSEC   to   the   entire   TCB.    Any  "operator"  or
          "administrator"  functions intrinsic  to an  individual
          TCB subset must be supported by that TCB subset or by a
          more primitive TCB subset.

          Trusted Recovery


                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB   and to  the individual  TCB
          subsets.  The  cooperative recovery actions of  the TCB
          subsets making up the TCB must provide trusted recovery
          for  the  overall  TCB.   Otherwise,  trusted  recovery
          evaluation  must address  the entire  TCB (even  if the
          individual  TCB  subsets   meet  the  trusted  recovery
          requirements).

          TC-5.2.3.2 Life-Cycle Assurance

          Security Testing


                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB.   If a  TCB consists  of TCB
          subsets meeting the conditions for evaluation by parts,
          the satisfaction of the requirements by each TCB subset
          satisfies   the  requirement    for  the   entire  TCB.
          Otherwise, security  testing of the entire  TCB must be
          performed   (even  if   the  results   of  testing  the
          individual TCB subsets were available).

          Design Specification and Verification


                    This  requirement applies   as stated  in the
          TCSEC to every TCB  subset, with the following specific
          additional interpretations.

                    It  must be   demonstrated that  the security
          policy enforced by the  composite TCB is represented by


          the collection of policy  models for the individual TCB
          subsets.

                    The argument  that the descriptive  top level
          specification (DTLS) and formal top level specification
          (FTLS) are consistent with the TCB interface applies to
          the  entire TCB.   There  is  required an  explicit and
          convincing  description of  how  the  set of  top level
          specifications (one for each  TCB subset) describes the
          TCB  interface  in  terms  of  exceptions,  errors, and
          effects.

          Configuration Management


                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB  subset  in  the  TCB,  with  the
          following additional interpretation.

                    Because subsets  of the TCB may  be developed
          independently, a single configuration management system
          may  not  be  feasible.   However,  the  combination of
          configuration  management systems  used to  support all
          the TCB subsets must  meet all the stated requirements.
          The  information describing the  interrelations between
          separate  TCB  subsets  and  separate  security  policy
          models  falls into  the category  of "all documentation
          and  code associated  with the  current version  of the
          TCB" in the TCSEC requirements.

          Trusted Distribution


                    This  requirement applies   as stated  in the
          TCSEC to the  entire TCB.  It can be  met by satisfying
          the  requirements  for  each  TCB  subset if procedures
          exist for  assuring that all  TCB subsets upon  which a
          particular  TCB  subset  depends  (that  is,  the  more
          primitive TCB subsets) are  exactly the same version as
          specified for the TCB subset in question.

          TC-5.2.4 DOCUMENTATION


          TC-5.2.4.1 Security Features User's Guide

                    This  requirement applies   as stated  in the
          TCSEC to every TCB subset  in the TCB.  This collection
          of guides must include descriptions of every TCB subset
          in  the  TCB  and  explicit  cross-references  to other
          related  user's   guides  to  other  TCB   subsets,  as
          required.  In addition, interactions between mechanisms
          within different TCB subsets must be clearly described.

          TC-5.2.4.2 Trusted Facility Manual

                    This  requirement applies   as stated  in the
          TCSEC to the TCB and to every TCB subset in the TCB.

                    This  requirement can  be met  by providing a
          set of manuals, one  for each distinct (non-replicated)
          TCB  subset.  Each  manual shall  address the functions
          and privileges to be  controlled for the associated TCB
          subset.   Additionally,   it  must  clearly   show  the
          interfaces to, and the interaction with, more primitive
          TCB  subsets.  The  manual  for  each TCB  subset shall
          identify  the  functions  and  privileges  of  the  TCB
          subsets  on which  the associated  TCB subset  depends.
          Also, the TCB subset's  manual shall identify which, if
          any,  configuration options  of the  more primitive TCB
          subsets are to be used for the composite TCB to operate
          securely.

                    Any   pre-defined   roles   supported  (e.g.,
          database  administrator)  by  the  TCB  subset shall be
          addressed.

                    The means for correlating multiple audit logs
          and synonymous  user identifications from  multiple TCB
          subsets (if such exist) shall also be addressed.

                    The  trusted facility  manual shall  describe
          the  composite TCB so  that the interactions  among the
          TCB subsets  can be readily determined.   Other manuals
          may be  referenced in this determination.   The manuals
          that address  the distinct modules  of the TCB  and the


          generation of  the TCB need  to be integrated  with the
          other trusted facility manuals  only to the extent that
          they are affected by the  use and operation (versus the
          development) of the composite TCB.

                    The  TCB modules  that contain  the reference
          validation  mechanism must  be associated  with the TCB
          subset  to  which  they   belong.   The  procedure  for
          generating  a  new  TCB  after  modifying  only one (or
          several)  TCB subsets must  be described.  This  may be
          accommodated   by  independent   regeneration  of   the
          individual TCB  subsets or by regeneration  of only the
          affected TCB subsets.

          TC-5.2.4.3 Test Documentation

                    This  requirement applies   as stated  in the
          TCSEC to the composite TCB.

          TC-5.2.4.4 Design Documentation

                    This  requirement applies   as stated  in the
          TCSEC to the composite TCB, with the following specific
          additional interpretations:

                      Requirements  concerning  models,  FTLS and
          DTLS, apply to individual TCB subsets.

                      The requirement  concerning the description
          of interfaces  between modules of the  TCB includes the
          interfaces between TCB subsets.

                      The documentation of  the implementation of
          a   reference   validation   mechanism   must   include
          justification for meeting the conditions for evaluation
          by parts.

                      The  A1   requirement  to  describe
          clearly  non-FTLS internals of  the TCB applies  to TCB
          subsets.



          TC-5.3 SUMMARY OF THE REQUIREMENTS

                    The  requirements interpretations  in Section
          TC-5.2 above can be grouped into two categories:  those
          that  apply to  individual TCB  subsets and  those that
          apply  totally or  in part   to the  overall TCB.   For
          purposes  of exposition,  the former  category will  be
          termed "local  requirements," that is, those  for which
          separate  analysis   of  the  individual   TCB  subsets
          suffices to determine compliance for the composite TCB.
          The latter  are termed "global requirements,"  that is,
          those which  require analysis of the  entire system and
          for  which  separate  analysis  of  the  individual TCB
          subsets does not suffice.

          TC-5.3.1 LOCAL REQUIREMENTS

                    Discretionary Access Control;
                    Object Reuse;
                    Labels (except Subject Sensitivity Labels);
                    Mandatory Access Control;
                    System   Architecture  (except   domains  for
          execution and distinct address spaces);
                    System Integrity;
                    Configuration Management;
                    Security Features User's Guide;
                    Design Documentation
                    models,
                    DTLSs,
                    FTLSs, and
                    non-FTLS internals.

          TC-5.3.2 GLOBAL REQUIREMENTS

                       Subject     Sensitivity    Labels;    
	             Identification and  Authentication;   
		   Trusted  Path; 
	             Audit; 
		   System Architecture domains for execution, and distinct
	       	   address spaces;
          	   Covert   Channel   Analysis;        
		   Trusted   Facility Management;     
		   Trusted   Recovery  (also  applies  to
		   individual TCB subsets);   
		   Security Testing;   
		   Design Specification and Verification
	                    correspondence  between  system
		          policy and the set of TCB subset models
		
			consistency of TCB interface with the
			set of TCB subset DTLSs, and
			
			consistency ofTCB interface with the 
			set of TCB subset FTLSs; 
		   Trusted Distribution;    
		   Trusted Facility Manual (also applies to individual TCB
		   subsets);
		   Test Documentation; and   
		   Design Documentation (except models, DTLSs, FTLSs, and
non-FTLS internals).


         TC-6 DESIGN CHOICE


          TC-6.1 OVERVIEW

                    This  section  examines  the  several  design
          choices available for constructing systems in parts and
          the consequences of each  when attempting to perform an
          evaluation by parts.  The  first case described is that
          of a  TCB evaluated under the TCSEC  without benefit of
          TCB  subset structuring.   This  case  is of  value for
          several  reasons:  it serves  as a reference  point; it
          can  be considered  the degenerate  case of subsetting;
          and  it  is  always  an  option  to  evaluate  any TCB,
          regardless of  internal structure, as a  monolith.  The
          second  and third  cases are  presented in  terms of  a
          configuration    of    exactly    two    subsets;   the
          generalization  to   more  than  two  TCB   subsets  is
          straightforward.   The   second  case  is  that   of  a
          subsetted  architecture  that   exactly  satisfies  the
          conditions  for evaluation  by parts.   The third  case
          represents a special case of  an altered TCB, one which
          is implemented using trusted subjects.

                    Note that no evaluation using TCB subsets and
          evaluation by  parts results in a  TCB subset receiving
          an evaluation rating.  Rather,  the entire system, with
          its composite TCB, is  evaluated and receives a rating.
          However, evaluation  by parts is intended  to allow the


          results of local analysis  of individual TCB subsets to
          be  distinguishable and  separately referencable.   For
          further discussion of this  topic, see Appendix B, item
          10.


          TC-6.2 A SINGLE TCB SUBSET

                    The  evaluation  of  a  TCB  consisting  of a
          single  TCB subset  is equivalent  to a straightforward
          evaluation  against  the  TCSEC.   The  conditions  for
          evaluation   by  parts   (Section  TC-4.3)   reduce  to
          requirements found in the TCSEC itself.

          TC-6.2.1 ANALYSIS OF THE CONDITIONS

          TC-6.2.1.1    Condition    1:     Candidate    TCB
          Subsets

                     The TCB (hardware,  software, and firmware),
          as distinguished from the rest of the computing system,
          must  be identified.  This  is a basic  requirement for
          TCSEC evaluation.  Similarly,  the subjects and objects
          within the system must  be identified.  The requirement
          that no more than one  TCB subset mediate access to any
          particular object  is satisfied, because there  is only
          one TCB subset.

          TC-6.2.1.2 Condition 2:  Policy Allocation

                    The  policy P  enforced by  the TCB  (subset)
          must  be identified.   The demonstration  that the  TCB
          (subset) enforces that policy  will be a description of
          how  the  TCB  performs  access  mediation  between the
          system's subjects and  objects according a system-level
          description  of limitations   on access  (the technical
          policy  P[i] of  the definition).   The tracing  of the
          policy to the system design and behavior is part of the
          stated TCSEC requirements.

          ?TC-6.2.1.3   Condition    3:    Trusted   Subjects
          Included


                    This  condition  is  satisfied  in  the  same
          manner  as  it  is  in  evaluations  under  the  TCSEC.
          Specifically,  the  TCB  boundary  is  shown  to be the
          interface that is presented to untrusted subjects.

          TC-6.2.1.4 Condition 4:  TCB Subset Structure

                    Satisfaction of this  condition (TC-4.3.4) is
          immediate, because there is only one TCB subset.

          TC-6.2.1.5       Condition      5:        Separate
          Subset-Domains

                    Satisfaction  of  the  separate subset-domain
          condition (TC-4.3.5) is identical  to the C1 through A1
          requirement that "the TCB maintain a domain for its own
          execution that  protects it from  external interference
          or tampering."  [8, p.  13 et al.]

          TC-6.2.1.6   Condition   6:    Support   for   RVM
          Arguments

                    Satisfaction of this  condition (TC-4.3.6) is
          immediate, inasmuch as there  are no less primitive TCB
          subsets  that  must  be  demonstrated  to  satisfy  RVM
          requirements.

          TC-6.2.2 EVALUATION CONSEQUENCES

                    In this case, the  evaluation of the (single)
          TCB  subset proceeds  exactly like  an evaluation under
          the  TCSEC.  Demonstration   that the  candidate system
          meets  all the global  and local requirements  (as they
          apply  to  the  target  evaluation  class) includes the
          consideration  of   each  requirement  as   it  applies
          system's    philosophy     of    protection,    design,
          documentation, and implementation.   The system must be
          shown  to   exhibit  the  properties  of   a  reference
          validation mechanism, appropriate to the target class.


          TC-6.3 TWO TCB SUBSETS, MEETING THE CONDITIONS


                    This case  is of a  TCB that consists  of two
          candidate  TCB subsets, A  and B, where  A is the  most
          primitive TCB subset.  That is, B uses the abstractions
          provided  by  A  (the  objects,  in  particular) as its
          resource  for the  construction and  exportation of its
          own  abstractions.    B  also  uses   the  abstractions
          provided by A for its  metadata (that is, internal data
          structures) that make it  possible for B to instantiate
          its exported abstractions as  well as keep records that
          enable it  to correctly enforce its  stated policy.  In
          terms of the discussion of Section TC-4.3.4, TCB subset
          B directly depends on TCB subset A.  It will be assumed
          that TCB subset A  enforces mandatory and discretionary
          policies on its objects and  that TCB subset B enforces
          a  discretionary  policy  on  the  objects  it exports.
          Additionally, all  trusted subjects of A  are contained
          within A.  Thus, every subject  of A (including all the
          active entities  that make up the logic  of B) operates
          at  a single  sensitivity  level.   It will  further be
          assumed  for  th  is  example  that  the mechanisms for
          domains and thus for  subset-domains are independent of
          the mandatory  and discretionary access  control policy
          enforcement mechanisms.

          TC-6.3.1 ANALYSIS OF THE CONDITIONS

          TC-6.3.1.1    Condition    1:     Candidate    TCB
          Subsets

                    The  TCB (hardware, software,  and firmware),
          as distinguished from the rest of the computing system,
          must  be identified.  This  is a basic  requirement for
          TCSEC evaluation.  Similarly,  the subjects and objects
          within the system must be identified.

                    In this case, all the hardware, software, and
          firmware  that make  up the  TCB must  be identified as
          being part of either TCB subset A or TCB subset B.  The
          subjects  and  objects  mediated  by  A  (call them the
          "A-subjects" and "A-objects"  for this discussion) must
          be identified.  Similarly  the B-subjects and B-objects
          must also be identified.


                    The   additional   requirement   in   Section
          TC-4.3.1 that "there may not be any objects mediated by
          more than  one TCB subset"  means that there  can be no
          B-object that is also an A-object.

          TC-6.3.1.2 Condition 2:  Policy Allocation

                    The policy  P enforced by the  whole TCB must
          be identified.   In addition, the policy  enforced by A
          (call  it  the  A-policy),   stated  in  terms  of  the
          A-subjects  and  the  A-objects,  must  be  identified.
          Similarly,  the  B-policy,  stated   in  terms  of  the
          B-subjects and the B-objects, must be identified.

          TC-6.3.1.3    Condition   3:     Trusted   Subjects
          Included

                    As  was stated  above, TCB  Subset A contains
          all  its own  trusted subjects.   There may  be trusted
          subjects with respect to the  policy of A, but all such
          subjects execute in the subset-domain of A.

          TC-6.3.1.4 Condition 4:  TCB Subset Structure

                    Because B directly uses the A-objects and its
          logic is  embodied in A-subjects, the  structure of the
          TCB  subsets  is  precisely   "TCB  subset  A  is  more
          primitive than TCB subset B."  This is a partial order.

          TC-6.3.1.5       Condition      5:        Separate
          Subset-Domains

                    Satisfaction  of  the  separate subset-domain
          condition  requires that a  set of domains  provided by
          the   system  be   identified  as   being  the  domains
          "occupied"  by  A  and  B.   The  domain,  or  domains,
          occupied  by  A  is  exactly  the  "domain  for its own
          execution" found in the TCSEC requirements.  The domain
          or  domains  occupied  by  TCB  subset  B  must  not be
          modifiable  by any code  or other system  entity except
          possibly by TCB subset A.


          TC-6.3.1.6   Condition   6:    Support   for   RVM
          Arguments

                    Satisfying  the condition  for RVM  arguments
          requires demonstrating  the plausibility of  being able
          to establish the three  requisite properties of an RVM.
          The  first  property  requires  that  no  B-subject  be
          allowed  to  access  B-objects  without  those accesses
          being mediated by TCB  subset B.  The tamper resistance
          property requires that TCB subset  A provide a way that
          TCB subset B can be  designed and implemented such that
          A-subjects  that  are  not  part  of B's implementation
          cannot  tamper with  B's policy-critical  code or data.
          The  third  RVM  property  must  be  satisfied  by  the
          individual TCB  subsets.  The degree to  which each TCB
          subset must satisfy this  property is commensurate with
          the evaluation class of the TCB.

          TC-6.3.2 EVALUATION CONSEQUENCES

                    In this  case, the evaluation of  the two TCB
          subsets  requires  that  each  meet  TCSEC requirements
          applicable to  each TCB subset viewed  individually and
          that the two  TCB subsets combine in a way  to meet all
          the TCSEC requirements stated for the target class.

                    All local requirements are imposed on the two
          TCB subsets, A and B, individually.  If each TCB subset
          can meet  the requirements of the  target class, viewed
          as  if it  were a  separate TCB,  the only  areas where
          additional  evaluation or  accreditation work  might be
          required are those areas where  the sum of the analysis
          of   the  parts   is  not   necessarily  complete   and
          convincing.  Those areas  requiring additional work are
          exactly  the set  of global  requirements described  in
          Section TC-5.3.2.

                    Demonstrating that the candidate system meets
          the  TCSEC requirements  (as they  apply to  the target
          evaluation  class)  requires  that  both  A  and  B  be
          evaluated with respect to the local requirements of the
          target  class and that  the composite TCB  be evaluated


          for global requirements.  In this case, full testing of
          TCB subset  A against all the  requirements (both local
          and  global)  simplifies   the  task  of  demonstrating
          satisfaction of the global requirements, both for B and
          for the entire TCB.

                    Suppose, for  example, that TCB subset  A has
          been subjected  to security testing appropriate  to the
          target  class  and  has  been  shown  to  be adequately
          resistant  to  penetration  attacks.   This  means that
          within  the confidence  level provided  by the  testing
          requirement, no  A-subject can subvert  A's enforcement
          of its policy.  In  this situation, every active entity
          in B is an A-subject  and hence B can neither penetrate
          A nor be  induced to do so by any  B-subject.  Thus, no
          further  testing of  A  will  be required  to determine
          whether the entire TCB is resistant to penetration; any
          additional  penetration  testing   can  be  limited  to
          determining the ability of B to withstand assault.

                    Similarly, if A has  been searched for covert
          channels   (as   required    for   its   target   class
          requirements),  then  no   further  search  for  covert
          channels will be required, either  in the analysis of B
          or  in the  overall  consideration  of the  entire TCB.
          Note  that if B  implements a mandatory  access control
          policy (e.g., integrity), then it would be necessary to
          perform a covert channel analysis  on B, but no further
          covert channel analysis of A would be required.

                    The ability of users to determine the current
          sensitivity  level  of  B-subjects  operating  on their
          behalf  will have  to be  shown by  considering the TCB
          subsets  A   and  B  together.   This   requirement  is
          satisfied   immediately   if    the   argument   relies
          exclusively on A meeting the requirement.



          TC-6.4   TWO   TCB    SUBSETS,   NOT   MEETING   THE
          CONDITIONS


                    This case  also concerns a TCB  that consists
          of two candidate  TCB subsets, C and D.  C  is the most
          primitive TCB subset.  That is, D uses the abstractions
          provided  by  C  (the  objects,  in  particular) as its
          resource  for the  construction and  exportation of its
          own  abstractions.    D  also  uses   the  abstractions
          provided by C for its  metadata (that is, internal data
          structures) that make it  possible for D to instantiate
          its exported abstractions as  well as keep records that
          enable it  to correctly enforce its  stated policy.  In
          terms of the discussion of Section TC-4.3.4, TCB subset
          D directly depends on TCB subset C.  Additionally, D is
          trusted  with  respect  to  C.   That  is,  some of the
          C-subjects  which  make  up  TCB  subset  D  execute as
          trusted processes of C.  Here  also, as in the previous
          example, it is assumed  that C implements mandatory and
          discretionary policies over  its objects.  Further, the
          intent of D is to implement a discretionary policy over
          the  objects it  exports.  However,  because D includes
          subjects  which  are  trusted  relative  to  C's policy
          demonstration  of the  full and  correct enforcement of
          the mandatory policy requires analysis  of both C and D
          and is no longer localized to TCB subset C.  It will be
          assumed  that the mechanisms  for domains and  thus for
          subset-domains  are independent   of the  mandatory and
          discretionary   access   control   policy   enforcement
          mechanisms.

                    This case can be viewed  as a special case of
          a  previously  evaluated  TCB  which  has been altered.
          However,  the  alteration  takes  the  form  of  a less
          primitive  subset  which  is  implemented,  at least in
          part,  with   trusted  subjects  (i.e.,  some   of  the
          C-subjects  are trusted  subjects which  execute in the
          subset-domain  of D).   Although this  case may appear,
          intuitively,  to be  different from  that of  arbitrary
          alteration of  a previously evaluated TCB,  the example
          demonstrates that such an  approach makes it impossible
          to perform an evaluation by parts.

          TC-6.4.1 ANALYSIS OF THE CONDITIONS


          TC-6.4.1.1    Condition    1:     Candidate    TCB
          Subsets

                    The  identification  of  the  TCB  (hardware,
          software, and firmware) as  distinguished from the rest
          of  the computing  system  is  a basic  requirement for
          TCSEC evaluation.   Likewise, the subjects  and objects
          within the system must be identified.

                    In this case, all the hardware, software, and
          firmware  that make  up the  TCB must  be identified as
          being part of either TCB subset C or TCB subset D.  The
          C-subjects  and  C-objects  mediated  by  C  have to be
          identified.   Similarly  the  D-subjects  and D-objects
          must also be identified.

                    The   additional   requirement   in   Section
          TC-4.3.1 that "there may not be any objects mediated by
          more  than  one  TCB  subset"  means  there  can  be no
          D-object that is also a C-object.

          TC-6.4.1.2 Condition 2:  Policy Allocation

                    The policy  P enforced by the  whole TCB must
          be  identified.   In  addition,  the  individual policy
          enforced by C (call it the C-policy) must be identified
          in   terms  of   the  C-subjects   and  the  C-objects.
          Similarly,  the  D-policy,  stated   in  terms  of  the
          D-subjects and the D-objects,  must be stated.  In this
          case, the C-policy will  include the strict enforcement
          of  a  mandatory  access  control  policy  that  allows
          trusted subjects to execute in the subset-domains which
          compose TCB subset D.

          TC-6.4.1.3   Condition    3:    Trusted   Subjects
          Included

                    This  condition is   not satisfied  because D
          includes  at least  one  subject  that is  trusted with
          respect  to C.   Hence a  subject that  is trusted with
          respect to the policy of C is outside C, and evaluation
          by  parts  is  not  an  option.   If  TCB  subset C had


          previously been  evaluated, then this is  an example of
          an  altered TCB,  albeit  a  special case.   The change
          consists  of  the  addition  of  one  or  more  trusted
          C-subjects  in D  whose effect   on the  behavior of  C
          cannot  be predicted  a priori.   An assessment  of the
          impact  of  D  on  the  behavior  of  C  cannot be made
          strictly by an examination  of the trusted subjects and
          the definition  of C's interface.  A  global assessment
          of C and D is required.

          TC-6.4.1.4 Condition 4:  TCB Subset Structure

                    Because D directly uses the C-objects and its
          logic is  embodied in C-subjects, the  structure of the
          TCB  subsets  is  precisely   "TCB  subset  C  is  more
          primitive than TCB subset D."  This is a partial order.

          TC-6.4.1.5       Condition      5:        Separate
          Subset-Domains

                    Satisfying    the   separate    subset-domain
          condition  (TC-4.3.5) requires  identifying the  set of
          system  domains   (likely  administered  by   the  most
          primitive TCB subset C) as  the domains "occupied" by C
          and  D.   The  domain,  or  domains,  occupied  by C is
          exactly the "domain for its own execution" found in the
          TCSEC requirements.  The domain  or domains occupied by
          TCB  subset D  must not  be modifiable  by any  code or
          other system  entity except possibly  by a part  of TCB
          subset C.

          TC-6.4.1.6   Condition   6:    Support   for   RVM
          Arguments

                    Satisfying  the condition  for RVM  arguments
          requires demonstrating  the plausibility of  being able
          to establish the three  requisite properties of an RVM.
          The  first  property  requires  that  no  B-subject  be
          allowed  to  access  B-objects  without  those accesses
          being mediated by TCB  subset B.  The tamper resistance
          property requires that TCB subset  A provide a way that
          TCB subset B can be  designed and implemented such that


          A-subjects  that  are  not  part  of B's implementation
          cannot  tamper with  B's policy-critical  code or data.
          The  third  RVM  property  must  be  satisfied  by  the
          individual TCB  subsets.  The degree to  which each TCB
          subset must satisfy this  property is commensurate with
          the evaluation class of the TCB.

          TC-6.4.2 EVALUATION CONSEQUENCES

                    In   this   example,   the   conditions   for
          evaluation  by parts  are not  satisfied and  thus, the
          full potential for savings in evaluation effort cannot,
          in general, be realized.  A  clear option in such cases
          is to view  the system as a monolithic  TCB and proceed
          accordingly.  However, because  this case represents an
          example of an altered TCB, it admits of a wide spectrum
          of  specific sub-cases.  Thus,  if the analysis  of the
          system  proceeds  in  parallel  to  that  required  for
          evaluation  by parts  it  may  be possible,  in special
          cases, to identify elements of the analysis of the more
          primitive   candidate   TCB   subset   which   can   be
          successfully   argued   to   be   unaffected   by   the
          alterations.     Some    evaluation    effort,    often
          significant,  can be  saved, but  unlike evaluation  by
          parts, how much can  only be estimated by consideration
          of the implementation specifics.   For example, in this
          specific  case,  the  local  analysis  of  TCB subset C
          represents  a reasonable   candidate for  analysis that
          need not be redone.


          TC-6.5 SUMMARY

                    The  three cases  described above  illustrate
          the  effects of  various TCB  subsetting situations  as
          they relate to the evaluation requirements.

                    A  monolithic evaluation proceeds  exactly as
          described in the TCSEC, with requirements being applied
          to the entire TCB.
                    When  all the   conditions for  evaluation by
          parts are satisfied, considerable savings in evaluation


          effort  are realized.  The  evaluation of a  new system
          configuration that includes exactly the same TCB subset
          that was previously evaluated  (such as the TCB subsets
          A and B in the Section  TC-6.3) is limited to (a) local
          analysis of the individual TCB subsets (by reference to
          earlier  analysis,  if  available)  and  (b)  a simpler
          global  analysis, because each  TCB subset is  an exact
          analog of a TCB.
                    When the  conditions for evaluation  by parts
          are  not satisfied, no  general statements can  be made
          about the factorability of  analysis or the reusability
          of  previous analysis.   The extent  to which  previous
          evaluation  evidence and  results remain  valid can  be
          determined only  on a case-by-case  basis. 















          PART 2


          INTERPRETED REQUIREMENTS





          IR-1 OVERVIEW AND CONTEXT

                    Part 2,  "INTERPRETED REQUIREMENTS," provides
          specific  interpretations of  those TCSEC  requirements
          which are  deemed to be either  DBMS-specific (or, more
          generally,   application-specific)    or   particularly
          relevant  to DBMSs.   All  of  the requirements  in the
          TCSEC apply in any case.

                    For   the   topics    included   below,   the
          interpretations  provide  clarification  of  the  TCSEC
          requirements.   As  is  the  case  for  the  TCSEC, the
          interpreted  requirements at   any class  include those
          specified for that class in addition to interpretations
          for  lower classes  that  have  not been  superseded or
          altered.

                    Section IR-2  presents an overall  summary of
          the  TCSEC  requirements,  as  interpreted  in the more
          detailed sections  that follow.  Sections  IR-3 through
          IR-7  address  individual  requirements interpretations
          for   labels,   audit,   system   architecture,  design
          specification    and     verification,    and    design
          documentation, respectively.  The  format is an initial
          discussion  of  the  topic   in  general,  followed  by
          specific requirements and interpretations that apply to
          database   management  systems.    A  listing   of  the
          requirements  and  interpretations  organized  by TCSEC
          class is presented in Appendix A.


          IR-2 SUMMARY OF THE INTERPRETATIONS

                    This      section      provides      specific
          interpretations  of those  TCSEC requirements  that are
          particularly  relevant for subsetted  architectures and
          evaluation by parts.  Its  organization is derived from
          the  structure  of  the  TCSEC  requirements.  For each
          relevant TCSEC requirement there is a discussion of how
          that requirement is interpreted for a DBMS.



          IR-2.1 SECURITY POLICY

          IR-2.1.1 DISCRETIONARY ACCESS CONTROL

                    The  requirement   for  discretionary  access
          control is not changed in  the context of this document
          and therefore applies as stated in the TCSEC.

          IR-2.1.2 OBJECT REUSE


                    The  requirement  for  object  reuse  is  not
          changed in  the context of this  document and therefore
          applies as stated in the TCSEC.

          IR-2.1.3 LABELS

                    The  requirement  for  labels  is  treated in
          Section IR-3 of this document.

          IR-2.1.4 MANDATORY ACCESS CONTROL

                    The requirement for  mandatory access control
          is  not changed  in the  context of  this document  and
          therefore  applies as  stated in  the TCSEC.   However,
          there   are  several   subtle  ramifications   of  this
          requirement of which a developer or evaluator should be
          aware.  A brief discussion can  be found in Appendix B,
          item 8, "Content-Dependent and Context-Dependent Access
          Control."


          IR-2.2 ACCOUNTABILITY

          IR-2.2.1 IDENTIFICATION AND AUTHENTICATION

                    The   requirement   for   identification  and
          authentication  is not changed  in the context  of this
          document and therefore applies as stated in the TCSEC.

          IR-2.2.2 AUDIT


                    The  requirement  for  audit  is  treated  in
          Section IR-4 of this document.


          IR-2.3 ASSURANCE

          IR-2.3.1 OPERATIONAL ASSURANCE

          IR-2.3.1.1 System Architecture

                    The  requirement for  system architecture  is
          treated in Section IR-5 of this document.

          IR-2.3.1.2 System Integrity

                    The requirement  for system integrity  is not
          changed in  the context of this  document and therefore
          applies as stated in the TCSEC.

          IR-2.3.1.3 Covert Channel Analysis

                    The  requirement for covert  channel analysis
          is  not changed  in the  context of  this document  and
          therefore applies as stated in the TCSEC.

          IR-2.3.1.4 Trusted Facility Management

                    The   requirement    for   trusted   facility
          management  is  not  changed  in  the  context  of this
          document and therefore applies as stated in the TCSEC.

          IR-2.3.1.5 Trusted Recovery

                    The requirement  for trusted recovery  is not
          changed in  the context of this  document and therefore
          applies as stated in the TCSEC.

          IR-2.3.2 LIFE CYCLE ASSURANCE

          IR-2.3.2.1 Security Testing


                    The requirement  for security testing  is not
          changed in  the context of this  document and therefore
          applies as stated in the TCSEC.

           IR-2.3.2.2      Design     Specification      and
          Verification

                    The requirement for  design specification and
          verification  is  treated  in   Section  IR-6  of  this
          document.

          IR-2.3.2.3 Configuration Management

                    The requirement  for configuration management
          is  not changed  in the  context of  this document  and
          therefore applies as stated in the TCSEC.

          IR-2.3.2.4 Trusted Distribution

                    The  requirement for trusted  distribution is
          not  changed  in  the  context  of  this  document  and
          therefore applies as stated in the TCSEC.


          IR-2.4 DOCUMENTATION

          IR-2.4.1 SECURITY FEATURES USER'S GUIDE

                    The  requirement  for   a  security  features
          user's  guide is  not changed  in the  context of  this
          document and therefore applies as stated in the TCSEC.

          IR-2.4.2 TRUSTED FACILITY MANUAL

                    The requirement for a trusted facility manual
          is  not changed  in the  context of  this document  and
          therefore applies as stated in the TCSEC.

          IR-2.4.3 TEST DOCUMENTATION


                    The requirement for test documentation is not
          changed in  the context of this  document and therefore
          applies as stated in the TCSEC.

          IR-2.4.4 DESIGN DOCUMENTATION

                    The  requirement for design  documentation is
          treated in Section IR-7 of this document.


          IR-3 LABELS


          IR-3.1 GENERAL DISCUSSION

                    The  labels  requirements  of  the  TCSEC are
          entirely  applicable  to  database  management systems.
          The  basic   difference  between  the   TCSEC  labeling
          requirements  as they  apply to  operating systems  and
          DBMSs involves which storage objects are labeled rather
          than  how   the  labels  are  handled.    This  section
          discusses  special considerations  in implementing  and
          evaluating  labeling mechanisms in  database management
          systems.  Of particular importance  is the selection of
          the storage objects that are to be labeled.

                    Beginning at the B1 evaluation class, trusted
          database management  systems are required  to associate
          labels  with all  storage objects  under their control.
          The  granularity  of  storage  objects  to be protected
          shall be chosen by the DBMS designer.

                    Stored view definitions (that is, named query
          commands) that are visible at  the TCB boundary must be
          stored in labeled objects.  The TCB must mediate access
          both   to  the   view  definition   and  to   the  view
          instantiation  (that  is,  the  set  of labeled objects
          accessed as  the result of executing  the query command
          contained in the view definition).

                    It has been proposed  in several designs that
          views be  used as a mechanism to  implement context- or


          content-dependent     labeling.       The     intuitive
          attractiveness of this approach  is undeniable, but the
          implementation details must be  carefully worked out to
          achieve a sound implementation.   A brief discussion of
          this  topic  can  be  found  in  Appendix  B,  item  8,
          "Content-Dependent    and   Context-Dependent    Access
          Control."

                    TCB   designers  and   evaluators  may   make
          distinctions   between  objects  that   are  explicitly
          labeled or  implicitly labeled.  Such  distinctions are
          meaningful  only within  the confines  of the  TCB; all
          storage objects are explicitly  labeled from a point of
          view outside the TCB.   For example, consider an object
          of one type  (e.g., table or file) within  the TCB that
          "contains" many (reference  monitor) objects of another
          type (e.g.,  tuples and records).  The  file could have
          an explicit  label associated with it, and  some of the
          records  could  have  explicit  labels  associated with
          them.  Those records that  have no explicit label would
          be implicitly labeled by the label of the file.

                    For database management  systems, the objects
          that store the base data  of the database (e.g., files,
          records,  relations,  and  tuples),  as  well  as those
          objects  that store   the metadata  (e.g., directories,
          indices,  schemas,   data  dictionaries,  discretionary
          authorization  tables, recovery  logs, and  transaction
          logs),  must  be  labeled.   Objects  that  need not be
          labeled  include internal  resources that  are not user
          visible  (e.g.,   printer  daemon  scratch   files  and
          resource  allocation  tables).    The  requirement  for
          importing data  (labeled and unlabeled) is  the same as
          in the TCSEC.  For additional information, see Appendix
          B, item 9, "Bulk Loading of a Database."


          IR-3.2 SPECIFIC INTERPRETATIONS

                    CLASS (B1):  LABELED SECURITY PROTECTION

          There are no interpretations for this class.


                    CLASS (B2):  STRUCTURED PROTECTION

                      Statement from TCSEC

                    Sensitivity  labels associated with  each ADP
          system resource .  .  .  that is directly or indirectly
          accessible  by subjects  external to  the TCB  shall be
          maintained by the TCB.

                      Interpretation

                    Internal TCB  variables that are  not visible
          to  untrusted subjects  need not  be labeled,  provided
          that they are not  directly or indirectly accessible by
          subjects external to the TCB.  However, it is important
          to understand that such internal variables can function
          as  covert signaling  channels when  untrusted subjects
          are  able  to  detect  changes  in  these  variables by
          observing system behavior.


                    CLASS (B3):  SECURITY DOMAINS

                    There are no additional requirements.


                    CLASS (A1):  VERIFIED DESIGN

                    There are no additional requirements.


          IR-4 AUDIT


          IR-4.1 GENERAL DISCUSSION

                    The audit requirements of  the TCSEC apply to
          database  management systems.   This section  discusses
          special  considerations  in  designing  and  evaluating
          audit mechanisms in database management systems.


                    The  TCB must  be capable  of maintaining  an
          audit trail  of accesses and attempted  accesses to the
          objects  protected by  the mandatory  and discretionary
          security   policies.   Two    examples  are   given  to
          illustrate auditing techniques for discretionary access
          control decisions.

          Example 1.  Consider a DBMS TCB providing discretionary
          controls on defined views of the database.  Because the
          named object presented at the TCB interface is the view
          definition  (not the  records instantiated  through the
          view), all that needs to be auditable is the use of the
          view by any untrusted subject.

          Example   2.   Consider   a  DBMS   TCB  that  enforces
          discretionary   access  control   on  individual   data
          records.  When a user  enters a query, the construction
          of a  response may involve  a search over  many records
          that are not returned to  the user because they did not
          satisfy the  query.  Records that do  satisfy the query
          but to  which the user  does not have  access should be
          auditable.  Records that do  not satisfy the query need
          not be  auditable.  That is,  in the context  of audit,
          access  permission to  the user  and satisfaction  of a
          query are to be kept separate.

                    There is no need to audit operations that are
          strictly internal to the  TCB.  Separate security audit
          logs may be maintained by  the operating system and the
          database   management   system.    Likewise,   separate
          identifications for the same  user may be maintained by
          the  operating  system   and  the  database  management
          system.  The correlation of  separate audit logs may be
          done either at  the time they are generated  or at some
          later time.

                    The  emphasis of  the audit  criterion is  to
          provide individual accountability for actions by users.
          This  goal is  not the  same as  that for  a backup and
          recovery log.  There is no requirement to integrate the
          audit  log with the  backup and recovery  log, although
          such an integrated log is not prohibited.


                    At the designer's discretion,  there may be a
          selectable  capability to  reduce the  number of  audit
          records generated  in response to queries  that involve
          many access control decisions.


          IR-4.2 SPECIFIC INTERPRETATIONS

          CLASS (C2):  CONTROLLED ACCESS PROTECTION

                      Statement from TCSEC

                    The  TCB shall  be able  to create, maintain,
          and protect  from modification .  .  .   an audit trail
          of accesses to the objects it protects.

                      Interpretation

                    Auditable events,  in the case of  a database
          management   system,  are  the   individual  operations
          initiated   by   untrusted    users   (e.g.,   updates,
          retrievals,  and inserts),  not just  the invocation of
          the database management system.  The auditing mechanism
          shall  have  the  capability  of  auditing all mediated
          accesses  which are  visible to  users.  That  is, each
          discretionary access  control policy decision  shall be
          auditable.  Individual operations  performed by trusted
          software, if totally transparent  to the user, need not
          be auditable.  If the total audit requirement is met by
          the  use  of  more  than  one  audit  log,  a method of
          correlation must be available.

          CLASS (B1):  LABELED SECURITY PROTECTION

                      Statement from TCSEC

                    The  TCB shall  be able  to create, maintain,
          and protect  from modification .  .  .   an audit trail
          of accesses to the objects it protects.

                      Interpretation


                    Auditable events,  in the case of  a database
          management   system,  are  the   individual  operations
          initiated   by   untrusted    users   (e.g.,   updates,
          retrievals,  and inserts),  not just  the invocation of
          the database management system.  The auditing mechanism
          shall  have  the  capability  of  auditing all mediated
          accesses which are visible to  the user.  That is, each
          discretionary access  control policy decision  and each
          mandatory  access  control  policy  decision  shall  be
          auditable.  Individual operations  performed by trusted
          software, if totally transparent  to the user, need not
          be auditable.  If the total audit requirement is met by
          the  use  of  more  than  one  audit  log,  a method of
          correlation must be available.

          CLASS (B2):  STRUCTURED PROTECTION

                    There is no interpretation for the additional
          requirements.

          CLASS (B3):  SECURITY DOMAINS

                    There is no interpretation for the additional
          requirements.

          CLASS (A1):  VERIFIED DESIGN

                    There are no additional requirements.


          IR-5 SYSTEM ARCHITECTURE


          IR-5.1 GENERAL DISCUSSION

                    The  system architecture requirements  of the
          TCSEC apply to database management systems.

                    The    interpretations    provided    are   a
          duplication  of  the  general  interpreted requirements
          that  apply  to  an  evaluation  by  parts.   They  are


          included   because  DBMS   evaluations  often   involve
          multiple TCB subsets.


          IR-5.2 SPECIFIC INTERPRETATIONS

                    CLASS    (C1):      DISCRETIONARY    SECURITY
          PROTECTION

                      Statement from TCSEC

                    The TCB  shall maintain a domain  for its own
          execution that  protects it from  external interference
          or tampering.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets, this requirement applies to each TCB subset.

          CLASS (C2):  CONTROLLED ACCESS PROTECTION

                    There is no interpretation for the additional
          requirements.

          CLASS (B1):  LABELED SECURITY PROTECTION

                    There is no interpretation for the additional
          requirements.

          CLASS (B2):  STRUCTURED PROTECTION

                      Statement from TCSEC

                    The  user  interface  to  the  TCB  shall  be
          completely  defined   and  all  elements  of   the  TCB
          identified.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to the interface  between the


          TCB subsets as well as  the user interface to the whole
          TCB.

                      Statement from TCSEC

                    It  shall  make  effective  use  of available
          hardware   to   separate   those   elements   that  are
          protection-critical from those that are not.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          each TCB subset must make use of facilities provided to
          it by  more primitive TCB subsets, such  as support for
          execution domains  and for distinct address  spaces, to
          achieve the required separation.

                    CLASS (B3):  SECURITY DOMAINS

                    There is no interpretation for the additional
          requirements.

                    CLASS (A1):  VERIFIED DESIGN


                    There are no additional requirements.


          IR-6 DESIGN SPECIFICATION AND VERIFICATION


          IR-6.1 GENERAL DISCUSSION

                    The  design  specification  and  verification
          requirements   of   the   TCSEC,   with   the   related
          documentation    requirements,   apply    to   database
          management systems.

                    The   interpretations   provided   include  a
          duplication  of general  interpreted requirements  that
          apply  to an  evaluation by  parts.  They  are included


          because of  the likelihood that a  DBMS evaluation will
          involve multiple TCB subsets.

                    In   the  database    context,  the   set  of
          candidates  for "subject"  and "object"  can be  larger
          than normally encountered in trusted operating systems.
          Where a database system builds on an underlying trusted
          operating  system, for  example, the  set of  candidate
          subjects  for the  two  TCB  subsets includes  both the
          active  entities created  by the  operating system  and
          those active entities created by the trusted portion of
          the DBMS.  The set of  candidates for objects is large.
          Examples  are  files,  segments,  buffers,  structures,
          pages,   relations,  tables,  tuples,   rows,  columns,
          elements,    entities,    relationships,    procedures,
          metadata,  system  tables,  query  trees,  query plans,
          locking   mechanisms,  rollback   mechanisms,  indices,
          recovery    and   backup    mechanisms,   precalculated
          operations  (such  as  joins),  view  definitions, view
          instantiations, constraints, authorization tables, data
          dictionary tables, and audit logs.

                    The requirements in the TCSEC for showing how
          the  various   representations  of  the   system  being
          evaluated fit together can  be represented as in Figure
          IR-1.  For monolithic TCBs,  the policy must be stated;
          the model  must be developed, maintained,  and shown to
          be sufficient to enforce the policy; and the DTLS (FTLS
          for  A1) must  be constructed  and shown  to correspond
          both to the model and to the TCB implementation.  These
          steps allow  a chain of  reasoning to proceed  from the
          stated  policy  to  the  assertion  that  the system in
          question actually enforces the policy.

                    In  the  case  of  multiple  TCB subsets, the
          intent is the same.  Tracing all policy requirements to
          the actual system implementation  must be possible, and
          vice versa.  The current dilemma is that the theory and
          techniques for subdividing a model into a set of models
          (or a top  level specification into a set  of top level
          specifications)   have   not   yet   been  established.
          Likewise  neither   theory  or  techniques   have  been


          established for composing a set  of models or top level
          specifications  into  a  unified  model  or  top  level
          specification.   The absence  of rigorous  methods does
          not preclude an evaluation using a subsetted TCB.

                    The    process   of    mapping   policy    to
          implementation is  possible for each TCB  subset, using
          the same techniques required for a monolithic TCB.  For
          subsetted   TCBs,   designers   and   evaluators   must
          explicitly   show    how   the   policy,    model   and
          specifications  for  each  TCB  subset  meet  the TCSEC
          requirements.    In   addition,   convincing   informal
          arguments must  be given to show how  the collection of
          TCB subsets  enforces the policy of  the composite TCB.
          Because   more   rigorous   composition   methods   are
          unavailable,   convincing    informal   arguments   are
          appropriate for evaluation of  TCBs up to and including
          Class A1.

                    The TCSEC requirements concerning the mapping
          from  policy to  implementation for  a TCB  composed of
          multiple TCB subsets raise these crucial topics:

                       The  allocation  of  policy  to  the  TCB
          subsets,

                       The relation  of the  models for  the TCB
          subsets to the overall system policy, and

                        The   relation    of   the   top   level
          specification for each TCB subset to the entire system.

                    Allocation of policy to  the TCB subsets is a
          precise division  of the policy for  the entire system,
          as  addressed  in  the  policy  allocation condition of
          Section TC-4.3.

                    The  second topic,  above, requires  that the
          policy for each TCB subset be stated.  Additionally, it
          is  required  that  there  be  an  informal  convincing
          argument that  the collection of models  represents the
          policy enforced by the entire system.


                    The third topic, the way  in which the set of
          top level specifications for the individual TCB subsets
          describes the  composite TCB interface with  respect to
          exceptions, errors and effects, is treated in a similar
          fashion.   The top  level specifications  for each  TCB
          subset  must  meet  the   requirement.   There  is,  in
          addition,  a requirement   for an  informal, convincing
          description of how the  set of top level specifications
          describes the TCB interface with respect to exceptions,
          errors,  and effects.   At the  A1 level,  there is  no
          requirement  for  additional  formal  specification  or
          formal  proofs  beyond  the  specification  and  proofs
          specific to the individual TCB subsets.

                    Rather than formally  composing the policies,
          models,  and  specifications  and  performing  a single
          monolithic evaluation, a series of separate evaluations
          may  be  performed  (one  for  each  TCB  subset).  The
          evaluations are  then tied together by  presentation of
          sufficient  informal  arguments   that  the  individual
          policies collectively represent  the policy enforced by
          the   entire  system,    that  the   individual  models
          collectively  represent the  system's policy,  that the
          individual specifications represent  the TCB interface,
          and  that  the  source  code  of  each  TCB  subset  is
          consistent with its top level specification.

                    Note  that satisfactory  completion of  these
          requirements  is logically equivalent  to demonstrating
          that a "unified" model for the entire TCB is consistent
          with  the  policy  enforced   by  the  system,  that  a
          "unified"  top level  specification corresponds  to the
          model,    and   that     the   "unified"    top   level
          specification(s) corresponds to the source code.  These
          interpreted  requirements,  which   do  not  mandate  a
          "unified"  top  level  specification,  are  technically
          achievable   interpretations   of   the  policy-tracing
          requirements in the case of multiple TCB subsets.


          IR-6.2 SPECIFIC INTERPRETATIONS


          CLASS (B1):  LABELED SECURITY PROTECTION

                      Statement from TCSEC

                    An informal  or formal model of  the security
          policy  supported by the  TCB shall be  maintained over
          the life cycle of the ADP system and demonstrated to be
          consistent with its axioms.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this  requirement  applies  to  the  security
          policy of  each TCB subset.  An  informal argument that
          the  set  of  policy  models  represents  the "security
          policy  supported  by  the  [composite]  TCB"  must  be
          provided.

          CLASS (B2):  STRUCTURED PROTECTION

                      Statement from TCSEC

                    A  formal   model  of  the   security  policy
          supported by the TCB shall  be maintained over the life
          cycle  of  the  ADP   system  and  demonstrated  to  be
          consistent with its axioms.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this  requirement  applies  to  the  security
          policy of  each TCB subset.  An  informal argument that
          the  set  of  policy  models  represents  the "security
          policy  supported  by  the  [composite]  TCB"  must  be
          provided.

                      Statement from TCSEC

                    A descriptive  top-level specification (DTLS)
          of  the TCB  shall  be  maintained that  completely and
          accurately  describes the  TCB in  terms of exceptions,
          error messages, and effects.


                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets, this  requirement applies to the  DTLS of each
          TCB subset.  An informal argument that the set of DTLSs
          accurately describes the TCB must be provided.

                    If there is a multifaceted policy (e.g., both
          mandatory  access  control   and  discretionary  access
          control policies) in a  particular TCB subset, then all
          facets must be  represented in the DTLS and  in the TCB
          subset's model.

                      Statement from TCSEC

                    The   descriptive   top-level   specification
          (DTLS) shall be shown to  be an accurate description of
          the TCB interface.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to the interface  between the
          TCB  subsets as well  as to the  user interface of  the
          composite  TCB.  The   TCB interface  description shall
          include an explanation of how to describe the total TCB
          accurately, in  the context of the  multiple TCB subset
          DTLSs.

          CLASS (B3):  SECURITY DOMAINS

                      Statement from TCSEC

                    A convincing argument shall be given that the
          DTLS is consistent with the model.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement   applies to  individual TCB
          subsets.  Enforcement of all  policies must be shown to
          occur  in  all  situations  (e.g.,  state  transitions)


          required by  the formal security policy  model.  In the
          case  of  a  discretionary  access  control policy, the
          presence of the access  control check at all identified
          state transitions is the total  of what is required for
          demonstrating  consistency  between  the  DTLS  and the
          model.  This may be verified  by inspection of the DTLS
          (that  is, by  visually checking  that exception checks
          required by  the model are  present in the  DTLS).  For
          the mandatory  access control policy, the  DTLS must be
          shown  by a convincing  argument to be  consistent with
          the model.

          CLASS (A1):  VERIFIED DESIGN

                      Statement from TCSEC

                    A  formal top-level  specification (FTLS)  of
          the TCB  shall be maintained that  accurately describes
          the  TCB in  terms of  exceptions, error  messages, and
          effects.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets, this  requirement applies to the  FTLS of each
          TCB subset.  An informal argument that the set of FTLSs
          accurately describes the TCB must be provided.

                    If there is a multifaceted policy (e.g., both
          mandatory  access  control   and  discretionary  access
          control policies) in a  particular TCB subset, then all
          facets must be  represented in the FTLS and  in the TCB
          subset's model.

                      Statement from TCSEC

                    The  FTLS shall  be shown  to be  an accurate
          description of the TCB interface.

                      Interpretation


                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to the interface  between the
          TCB  subsets as well  as to the  user interface of  the
          composite  TCB.  The   TCB interface  description shall
          include an explanation of how to describe the total TCB
          accurately, in  the context of the  multiple TCB subset
          DTLSs.

                      Statement from TCSEC

                    .  .  .  a combination of formal and informal
          techniques  shall be  used to   show that  the FTLS  is
          consistent with the model.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement   applies to  individual TCB
          subsets.  Enforcement of all  policies must be shown to
          occur  in  all  situations  (e.g.,  state  transitions)
          required  by the  formal security  policy model  at the
          required  occasions.  In  the case  of a  discretionary
          access  control  policy,  the  presence  of  the access
          control  check at  all identified  state transitions is
          the  total  of  what   is  required  for  demonstrating
          consistency between  the FTLS and the  model.  This may
          be  verified by  inspection of  the FTLS  (that is,  by
          visually checking that exception checks required by the
          model  are present  in  the  FTLS).  For  the mandatory
          access  control policy,  the FTLS  must be  shown by  a
          combination  of formal  and informal  techniques to  be
          consistent with the model.


          IR-7 DESIGN DOCUMENTATION


          IR-7.1 GENERAL DISCUSSION

                    The  design documentation requirement  of the
          TCSEC applies to database management systems.


                    The    interpretations    provided    are   a
          duplication  of  the  general  interpreted requirements
          that  apply  to  an  evaluation  by  parts.   They  are
          included   because  DBMS   evaluations  often   involve
          multiple TCB subsets.


          IR-7.2 SPECIFIC INTERPRETATIONS

          CLASS (C1):  DISCRETIONARY SECURITY PROTECTION

                      Statement from TCSEC

                    If the  TCB is composed of  distinct modules,
          the   interfaces  between    these  modules   shall  be
          described.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this  requirement applies  to each  TCB subset  and the
          interfaces between TCB subsets.

          CLASS (C2):  CONTROLLED ACCESS PROTECTION

                    There are no additional requirements.

          CLASS (B1):  LABELED SECURITY PROTECTION

                      Statement from TCSEC

                    The specific TCB  protection mechanisms shall
          be  identified and  an explanation  given to  show that
          they satisfy the model.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to each TCB  subset and shall
          include  the protection   mechanisms which  support the
          conditions  for  TCB   subset  structure  and  separate
          subset-domains.


          CLASS (B2):  STRUCTURED PROTECTION

                      Statement from TCSEC

                    The interfaces between  the TCB modules shall
          be described.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this  requirement applies  to each  TCB subset  and the
          interfaces between different TCB subsets.

                      Statement from TCSEC

                    The   descriptive   top-level   specification
          (DTLS) shall be shown to  be an accurate description of
          the TCB interface.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to the interface  between the
          TCB  subsets as well  as to the  user interface of  the
          composite  TCB.  The   TCB interface  description shall
          include an explanation of how to describe the total TCB
          accurately, in  the context of the  multiple TCB subset
          DTLSs.

                      Statement from TCSEC

                    Documentation  shall  describe  how  the  TCB
          implements  the reference  monitor concept  and give an
          explanation  of why it  is tamper resistant,  cannot be
          bypassed, and is correctly implemented.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement  is interpreted  to apply to
          each TCB subset with  respect to its specific technical
          policy.   In  addition,  there  must  be  documented an


          informal  argument that  the cooperative  action of the
          TCB  subsets  makes  the  TCB  itself tamper resistant,
          non-bypassable, and correct.

                      Statement from TCSEC

                    Documentation shall  describe how the  TCB is
          structured to  facilitate testing and to  enforce least
          privilege.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement is interpreted  to apply to individual
          TCB subsets as well as to the overall TCB.

          CLASS (B3):  SECURITY DOMAINS

                      Statement from TCSEC

                    The  TCB implementation  shall be  informally
          shown to be consistent with the DTLS.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement  is interpreted  to apply to
          individual TCB subsets.

          CLASS (A1):  VERIFIED DESIGN

                      Statement from TCSEC

                    The  TCB implementation  shall be  informally
          shown to be consistent with the FTLS.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement  is interpreted  to apply to
          individual       TCB        subsets.       
















          APPENDIX A


          SUMMARY OF THE

          INTERPRETATIONS BY CLASS



	This  section is  a presentation  of all  the
          interpreted requirements organized  by TCSEC class.  It
          includes all the requirements which are either relevant
          to subsetted  architectures or are  DBMS-specific.  Any
          TCSEC  requirements  not  explicitly  identified herein
          apply as stated in the TCSEC.

          CLASS (C1):  DISCRETIONARY SECURITY PROTECTION

          C1-1 SECURITY POLICY


          C1-1.1 DISCRETIONARY ACCESS CONTROL

                    The discretionary access control requirements
          apply as stated in the  TCSEC to every TCB subset whose
          policy  includes discretionary   access control  of its
          subjects to  its objects.  Any TCB  subset whose policy
          does not  include such discretionary access  control is
          exempt from this requirement.

          C1-2 ACCOUNTABILITY

          C1-2.1 IDENTIFICATION AND AUTHENTICATION

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    If  the TCB is  composed of TCB  subsets, one
          TCB subset  may either rely  on a mechanism  in another
          TCB subset to provide identification and authentication
          services  or provide  the services  directly.  Each TCB
          subset   may  maintain   its  own   identification  and
          authentication  data or  one central  repository may be
          maintained.  If each TCB subset  has its own data, then
          the  information  for  each  individual  user  must  be
          consistent among all subsets.

          C1-3 ASSURANCE

          C1-3.1 OPERATIONAL ASSURANCE

          C1-3.1.1 SYSTEM ARCHITECTURE

                    This  requirement applies   as stated  in the
          TCSEC   to  every   TCB  subset,   with  the  following
          additional interpretations.

                    The  TCB must  provide domains  for execution
          that are allocated to and used by TCB subsets according
          to the subset-domain condition for evaluation by parts.
          A  most primitive TCB  subset must provide  domains for


          execution.  A  less primitive TCB subset  must make use
          of domains provided by a  more primitive TCB subset.  A
          less primitive TCB subset may provide further execution
          domains but is not required to do so.

                      Statement from TCSEC

                    The TCB  shall maintain a domain  for its own
          execution that  protects it from  external interference
          or tampering.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets, this requirement applies to each TCB subset.

          C1-3.1.2 SYSTEM INTEGRITY

                    This  requirement applies   as stated  in the
          TCSEC  to every  TCB subset  that includes  hardware or
          firmware.   Any  TCB  subset   that  does  not  include
          hardware or firmware is exempt from this requirement.

          C1-3.2 LIFE CYCLE ASSURANCE

          C1-3.2.1 SECURITY TESTING

                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB.   If a  TCB consists  of TCB
          subsets meeting the conditions for evaluation by parts,
          the satisfaction of the requirements by each TCB subset
          satisfies   the  requirement    for  the   entire  TCB.
          Otherwise, security  testing of the entire  TCB must be
          performed   (even  if   the  results   of  testing  the
          individual TCB subsets were available).

          C1-4 DOCUMENTATION

          C1-4.1 SECURITY FEATURES USER'S GUIDE

                    This  requirement applies   as stated  in the
          TCSEC to every TCB subset  in the TCB.  This collection


          of guides must include descriptions of every TCB subset
          in  the  TCB  and  explicit  cross-references  to other
          related  user's   guides  to  other  TCB   subsets,  as
          required.  In addition, interactions between mechanisms
          within different TCB subsets must be clearly described.

          C1-4.2 TRUSTED FACILITY MANUAL

                    This  requirement applies   as stated  in the
          TCSEC to the TCB and to every TCB subset in the TCB.

                    This  requirement can  be met  by providing a
          set of manuals, one  for each distinct (non-replicated)
          TCB  subset.  Each  manual shall  address the functions
          and privileges to be  controlled for the associated TCB
          subset.   Additionally,   it  must  clearly   show  the
          interfaces to, and the interaction with, more primitive
          TCB  subsets.  The  manual  for  each TCB  subset shall
          identify  the  functions  and  privileges  of  the  TCB
          subsets  on which  the associated  TCB subset  depends.
          Also, the TCB subset's  manual shall identify which, if
          any,  configuration options  of the  more primitive TCB
          subsets are to be used for the composite TCB to operate
          securely.

                    Any   pre-defined   roles   supported  (e.g.,
          database  administrator)  by  the  TCB  subset shall be
          addressed.

                    The means for correlating multiple audit logs
          and synonymous  user identifications from  multiple TCB
          subsets (if such exist) shall also be addressed.

                    The  trusted facility  manual shall  describe
          the  composite TCB so  that the interactions  among the
          TCB subsets  can be readily determined.   Other manuals
          may be  referenced in this determination.   The manuals
          that address  the distinct modules  of the TCB  and the
          generation of  the TCB need  to be integrated  with the
          other trusted facility manuals  only to the extent that
          they are affected by the  use and operation (versus the
          development) of the composite TCB.


          C1-4.3 TEST DOCUMENTATION

          This requirement applies as stated  in the TCSEC to the
          composite TCB.

          C1-4.4 DESIGN DOCUMENTATION

          This requirement applies as stated  in the TCSEC to the
          composite TCB.

                      Statement from TCSEC

                    If the  TCB is composed of  distinct modules,
          the   interfaces  between    these  modules   shall  be
          described.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this  requirement applies  to each  TCB subset  and the
          interfaces between TCB subsets.

          CLASS (C2):  CONTROLLED ACCESS PROTECTION

          C2-1 SECURITY POLICY

          C2-1.1 DISCRETIONARY ACCESS CONTROL

                    The discretionary access control requirements
          apply as stated in the  TCSEC to every TCB subset whose
          policy  includes discretionary   access control  of its
          subjects to  its objects.  Any TCB  subset whose policy
          does not  include such discretionary access  control is
          exempt from this requirement.

          C2-1.2 OBJECT REUSE

          This  requirement applies  as  stated  in the  TCSEC to
          every TCB subset in the TCB.

          C2-2 ACCOUNTABILITY



          C2-2.1 IDENTIFICATION AND AUTHENTICATION

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    If  the TCB is  composed of TCB  subsets, one
          TCB subset  may either rely  on a mechanism  in another
          TCB subset to provide identification and authentication
          services  or provide  the services  directly.  Each TCB
          subset   may  maintain   its  own   identification  and
          authentication  data or  one central  repository may be
          maintained.  If each TCB subset  has its own data, then
          the  information  for  each  individual  user  must  be
          consistent among all subsets.

          C2-2.2 AUDIT

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    A  TCB subset  may maintain  its own security
          audit  log,  distinct  from  that  maintained  by  more
          primitive TCB subsets, or it may use an audit interface
          provided by  a different TCB subset  allowing the audit
          records  it  generates  to  be  processed  by  that TCB
          subset.

                    If  the   TCB  subset  uses   different  user
          identifications than a more primitive TCB subset, there
          shall be  a means to associate  audit records generated
          by different  TCB subsets for the  same individual with
          each other,  either at the  time they are  generated or
          later.

                      Statement from TCSEC


                    The  TCB shall  be able  to create, maintain,
          and protect  from modification .  .  .   an audit trail
          of access to the objects it protects.

                      Interpretation

                    Auditable events,  in the case of  a database
          management   system,  are  the   individual  operations
          initiated   by   untrusted    users   (e.g.,   updates,
          retrievals,  and inserts),  not just  the invocation of
          the database management system.  The auditing mechanism
          shall  have  the  capability  of  auditing all mediated
          accesses  which are  visible to  users.  That  is, each
          discretionary access  control policy decision  shall be
          auditable.  Individual operations  performed by trusted
          software, if totally transparent  to the user, need not
          be auditable.  If the total audit requirement is met by
          the  use  of  more  than  one  audit  log,  a method of
          correlation must be available.

          C2-3 ASSURANCE

          C2-3.1 OPERATIONAL ASSURANCE

          C2-3.1.1 SYSTEM ARCHITECTURE

                    This  requirement applies   as stated  in the
          TCSEC   to  every   TCB  subset,   with  the  following
          additional interpretations.

                    The  TCB must  provide domains  for execution
          that are allocated to and used by TCB subsets according
          to the subset-domain condition for evaluation by parts.
          A  most primitive TCB  subset must provide  domains for
          execution.  A  less primitive TCB subset  must make use
          of domains provided by a  more primitive TCB subset.  A
          less primitive TCB subset may provide further execution
          domains but is not required to do so.

                      Statement from TCSEC


                    The TCB  shall maintain a domain  for its own
          execution that  protects it from  external interference
          or tampering.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets, this requirement applies to each TCB subset.

          C2-3.1.2 SYSTEM INTEGRITY

                    This  requirement applies   as stated  in the
          TCSEC  to every  TCB subset  that includes  hardware or
          firmware.   Any  TCB  subset   that  does  not  include
          hardware or firmware is exempt from this requirement.

          C2-3.2 LIFE CYCLE ASSURANCE

          C2-3.2.1 SECURITY TESTING

                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB.   If a  TCB consists  of TCB
          subsets meeting the conditions for evaluation by parts,
          the satisfaction of the requirements by each TCB subset
          satisfies   the  requirement    for  the   entire  TCB.
          Otherwise, security  testing of the entire  TCB must be
          performed   (even  if   the  results   of  testing  the
          individual TCB subsets were available).

          C2-4 DOCUMENTATION

          C2-4.1 SECURITY FEATURES USER'S GUIDE


                    This  requirement applies   as stated  in the
          TCSEC to every TCB subset  in the TCB.  This collection
          of guides must include descriptions of every TCB subset
          in  the  TCB  and  explicit  cross-references  to other
          related  user's   guides  to  other  TCB   subsets,  as
          required.  In addition, interactions between mechanisms
          within different TCB subsets must be clearly described.


          C2-4.2 TRUSTED FACILITY MANUAL

                    This  requirement applies   as stated  in the
          TCSEC to the TCB and to every TCB subset in the TCB.

                    This  requirement can  be met  by providing a
          set of manuals, one  for each distinct (non-replicated)
          TCB  subset.  Each  manual shall  address the functions
          and privileges to be  controlled for the associated TCB
          subset.   Additionally,   it  must  clearly   show  the
          interfaces to, and the interaction with, more primitive
          TCB  subsets.  The  manual  for  each TCB  subset shall
          identify  the  functions  and  privileges  of  the  TCB
          subsets  on which  the associated  TCB subset  depends.
          Also, the TCB subset's  manual shall identify which, if
          any,  configuration options  of the  more primitive TCB
          subsets are to be used for the composite TCB to operate
          securely.

                    Any   pre-defined   roles   supported  (e.g.,
          database  administrator)  by  the  TCB  subset shall be
          addressed.

                    The means for correlating multiple audit logs
          and synonymous  user identifications from  multiple TCB
          subsets (if such exist) shall also be addressed.

                    The  trusted facility  manual shall  describe
          the  composite TCB so  that the interactions  among the
          TCB subsets  can be readily determined.   Other manuals
          may be  referenced in this determination.   The manuals
          that address  the distinct modules  of the TCB  and the
          generation of  the TCB need  to be integrated  with the
          other trusted facility manuals  only to the extent that
          they are affected by the  use and operation (versus the
          development) of the composite TCB.

          C2-4.3 TEST DOCUMENTATION

                    This  requirement applies   as stated  in the
          TCSEC to the composite TCB.


          C2-4.4 DESIGN DOCUMENTATION

                    This  requirement applies   as stated  in the
          TCSEC to the composite TCB.

                      Statement from TCSEC

                    If the  TCB is composed of  distinct modules,
          the interface between these modules shall be described.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this  requirement applies  to each  TCB subset  and the
          interfaces between TCB subsets.

          CLASS (B1):  LABELED SECURITY PROTECTION

          B1-1 SECURITY POLICY

          B1-1.1 DISCRETIONARY ACCESS CONTROL

                    The discretionary access control requirements
          apply as stated in the  TCSEC to every TCB subset whose
          policy  includes discretionary   access control  of its
          subjects to  its objects.  Any TCB  subset whose policy
          does not  include such discretionary access  control is
          exempt from this requirement.

          B1-1.2 OBJECT REUSE

                    This  requirement applies   as stated  in the
          TCSEC to every TCB subset in the TCB.

          B1-1.3 LABELS

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.


          B1-1.3.1 LABEL INTEGRITY

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          B1-1.3.2 EXPORTATION OF LABELED INFORMATION

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          B1-1.4 MANDATORY ACCESS CONTROL

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          B1-2 ACCOUNTABILITY

          B1-2.1 IDENTIFICATION AND AUTHENTICATION

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    If  the TCB is  composed of TCB  subsets, one
          TCB subset  may either rely  on a mechanism  in another
          TCB subset to provide identification and authentication
          services or provide  the services directly.  Similarly,
          that TCB subset may rely on a mechanism in another more
          primitive TCB subset to  ensure that the security level


          of subjects external to the  TCB that may be created to
          act on  behalf of the individual user  are dominated by
          the clearance and authorization of that user.  Each TCB
          subset   may  maintain   its  own   identification  and
          authentication  data or  one central  repository may be
          maintained.  If each TCB subset  has its own data, then
          the  information  for  each  individual  user  must  be
          consistent among all subsets.

          B1-2.2 AUDIT

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    A  TCB subset  may maintain  its own security
          audit  log,  distinct  from  that  maintained  by  more
          primitive TCB subsets, or it may use an audit interface
          provided by  a different TCB subset  allowing the audit
          records  it  generates  to  be  processed  by  that TCB
          subset.

                    If  the   TCB  subset  uses   different  user
          identifications than a more primitive TCB subset, there
          shall be  a means to associate  audit records generated
          by different  TCB subsets for the  same individual with
          each other,  either at the  time they are  generated or
          later.

                      Statement from TCSEC

                    The  TCB shall  be able  to create, maintain,
          and protect  from modification .  .  .   an audit trail
          of access to the objects it protects.

                      Interpretation

                    Auditable events,  in the case of  a database
          management   system,  are  the   individual  operations
          initiated   by   untrusted    users   (e.g.,   updates,
          retrievals,  and inserts),  not just  the invocation of


          the database management system.  The auditing mechanism
          shall  have  the  capability  of  auditing all mediated
          accesses  which are  visible to  users.  That  is, each
          discretionary access  control policy decision  shall be
          auditable.  Individual operations  performed by trusted
          software, if totally transparent  to the user, need not
          be auditable.  If the total audit requirement is met by
          the  use  of  more  than  one  audit  log,  a method of
          correlation must be available.

                      Statement from TCSEC

                    The  TCB shall  be able  to create, maintain,
          and protect  from modification .  .  .   an audit trail
          of accesses to the objects it protects.

                      Interpretation

                    Auditable events,  in the case of  a database
          management   system,  are  the   individual  operations
          initiated   by   untrusted    users   (e.g.,   updates,
          retrievals,  and inserts),  not just  the invocation of
          the database management system.  The auditing mechanism
          shall  have  the  capability  of  auditing all mediated
          accesses which are visible to  the user.  That is, each
          discretionary access  control policy decision  and each
          mandatory  access  control  policy  decision  shall  be
          auditable.  Individual operations  performed by trusted
          software, if totally transparent  to the user, need not
          be auditable.  If the total audit requirement is met by
          the  use  of  more  than  one  audit  log,  a method of
          correlation must be available.

          B1-3 ASSURANCE 

          B1-3.1 OPERATIONAL ASSURANCE

          B1-3.1.1 SYSTEM ARCHITECTURE

                    This  requirement applies   as stated  in the
          TCSEC   to  every   TCB  subset,   with  the  following
          additional interpretations.


                    The  TCB must  provide domains  for execution
          that are allocated to and used by TCB subsets according
          to the subset-domain condition for evaluation by parts.
          A  most primitive TCB  subset must provide  domains for
          execution.  A  less primitive TCB subset  must make use
          of domains provided by a  more primitive TCB subset.  A
          less primitive TCB subset may provide further execution
          domains but is not required to do so.

                    Similarly,  the  TCB  must  provide  distinct
          address  spaces   for  untrusted  processes.    A  most
          primitive  TCB  subset  must  provide  distinct address
          spaces for  its subjects.  A less  primitive TCB subset
          must make use of the distinct address space provided by
          a  more primitive  TCB  subset.   A less  primitive TCB
          subset may  provide more fine-grained  distinct address
          spaces, but is not required to do so.

                      Statement from TCSEC

                    The TCB  shall maintain a domain  for its own
          execution that  protects it from  external interference
          or tampering.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets, this requirement applies to each TCB subset.

          B1-3.1.2 SYSTEM INTEGRITY

                    This  requirement applies   as stated  in the
          TCSEC  to every  TCB subset  that includes  hardware or
          firmware.   Any  TCB  subset   that  does  not  include
          hardware or firmware is exempt from this requirement.

          B1-3.1 LIFE CYCLE ASSURANCE

          B1-3.2.1 SECURITY TESTING

                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB.   If a  TCB consists  of TCB


          subsets meeting the conditions for evaluation by parts,
          the satisfaction of the requirements by each TCB subset
          satisfies   the  requirement    for  the   entire  TCB.
          Otherwise, security  testing of the entire  TCB must be
          performed   (even  if   the  results   of  testing  the
          individual TCB subsets were available).

          B1-3.2.2 DESIGN SPECIFICATION AND VERIFICATION

                    This  requirement applies   as stated  in the
          TCSEC to every TCB  subset, with the following specific
          additional interpretations.

                    It  must be   demonstrated that  the security
          policy enforced by the  composite TCB is represented by
          the collection of policy  models for the individual TCB
          subsets.

                      Statement from TCSEC

                    An informal  or formal model of  the security
          policy  supported by the  TCB shall be  maintained over
          the life cycle of the ADP system and demonstrated to be
          consistent with its axioms.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this  requirement  applies  to  the  security
          policy of  each TCB subset.  An  informal argument that
          the  set  of  policy  models  represents  the "security
          policy  supported  by  the  [composite]  TCB"  must  be
          provided.

          B1-4 DOCUMENTATION

          B1-4.1 SECURITY FEATURES USER'S GUIDE

                    This  requirement applies   as stated  in the
          TCSEC to every TCB subset  in the TCB.  This collection
          of guides must include descriptions of every TCB subset
          in  the  TCB  and  explicit  cross-references  to other


          related  user's   guides  to  other  TCB   subsets,  as
          required.  In addition, interactions between mechanisms
          within different TCB subsets must be clearly described.

          B1-4.2 TRUSTED FACILITY MANUAL

                    This  requirement applies   as stated  in the
          TCSEC to the TCB and to every TCB subset in the TCB.

                    This  requirement can  be met  by providing a
          set of manuals, one  for each distinct (non-replicated)
          TCB  subset.  Each  manual shall  address the functions
          and privileges to be  controlled for the associated TCB
          subset.   Additionally,   it  must  clearly   show  the
          interfaces to, and the interaction with, more primitive
          TCB  subsets.  The  manual  for  each TCB  subset shall
          identify  the  functions  and  privileges  of  the  TCB
          subsets  on which  the associated  TCB subset  depends.
          Also, the TCB subset's  manual shall identify which, if
          any,  configuration options  of the  more primitive TCB
          subsets are to be used for the composite TCB to operate
          securely.

                    Any   pre-defined   roles   supported  (e.g.,
          database  administrator)  by  the  TCB  subset shall be
          addressed.

                    The means for correlating multiple audit logs
          and synonymous  user identifications from  multiple TCB
          subsets (if such exist) shall also be addressed.

                    The  trusted facility  manual shall  describe
          the  composite TCB so  that the interactions  among the
          TCB subsets  can be readily determined.   Other manuals
          may be  referenced in this determination.   The manuals
          that address  the distinct modules  of the TCB  and the
          generation of  the TCB need  to be integrated  with the
          other trusted facility manuals  only to the extent that
          they are affected by the  use and operation (versus the
          development) of the composite TCB.

          B1-4.3 TEST DOCUMENTATION


          This requirement applies as stated  in the TCSEC to the
          composite TCB.

          B1-4.4 DESIGN DOCUMENTATION

                    This  requirement applies   as stated  in the
          TCSEC to the composite TCB, with the following specific
          additional interpretation:

                    Requirements  concerning   models  and  DTLSs
          apply to individual TCB subsets.

                      Statement from TCSEC

                    If the  TCB is composed of  distinct modules,
          the interface between these modules shall be described.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this  requirement applies  to each  TCB subset  and the
          interfaces between TCB subsets.

                      Statement from TCSEC

                    The specific TCB  protection mechanisms shall
          be  identified and  an explanation  given to  show that
          they satisfy the model.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to each TCB  subset and shall
          include  the protection   mechanisms which  support the
          conditions  for  TCB   subset  structure  and  separate
          subset-domains.

          CLASS (B2):  STRUCTURED PROTECTION

          B2-1 SECURITY POLICY

          B2-1.1 DISCRETIONARY ACCESS CONTROL


                    The discretionary access control requirements
          apply as stated in the  TCSEC to every TCB subset whose
          policy  includes discretionary   access control  of its
          subjects to  its objects.  Any TCB  subset whose policy
          does not  include such discretionary access  control is
          exempt from this requirement.

          B2-1.2 OBJECT REUSE

          This  requirement applies  as  stated  in the  TCSEC to
          every TCB subset in the TCB.

          B2-1.3 LABELS

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

                      Statement from TCSEC

                    Sensitivity  labels associated with  each ADP
          system resource .  .  .  that is directly or indirectly
          accessible  by subjects  external to  the TCB  shall be
          maintained by the TCB

                      Interpretation

                    Internal TCB  variables that are  not visible
          to  untrusted subjects  need not  be labeled,  provided
          that they are not  directly or indirectly accessible by
          subjects external to the TCB.  However, it is important
          to understand that such internal variables can function
          as  covert signaling  channels when  untrusted subjects
          are  able  to  detect  changes  in  these  variables by
          observing system behavior.

          B2-1.3.1 LABEL INTEGRITY


                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          B2-1.3.2 EXPORTATION OF LABELED INFORMATION

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          B2-1.3.3 SUBJECT SENSITIVITY LABELS

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

          B2-1.3.4 DEVICE LABELS

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory access control of its subjects to its objects
          and has attached physical  or virtual devices.  Any TCB
          subset  whose policy  does not  include such  mandatory
          access control  or has no attached  physical or virtual
          devices   is  exempt   from  this   requirement.   This
          requirement can be satisifed  by the cooperative action
          of more than one TCB subset.

          B2-1.4 MANDATORY ACCESS CONTROL

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include


          such  mandatory  access  control  is  exempt  from this
          requirement.

          B2-2 ACCOUNTABILITY

          B2-2.1 IDENTIFICATION AND AUTHENTICATION

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    If  the TCB is  composed of TCB  subsets, one
          TCB subset  may either rely  on a mechanism  in another
          TCB subset to provide identification and authentication
          services or provide  the services directly.  Similarly,
          that TCB subset may rely on a mechanism in another more
          primitive TCB subset to  ensure that the security level
          of subjects external to the  TCB that may be created to
          act on  behalf of the individual user  are dominated by
          the clearance and authorization of that user.  Each TCB
          subset   may  maintain   its  own   identification  and
          authentication  data or  one central  repository may be
          maintained.  If each TCB subset  has its own data, then
          the  information  for  each  individual  user  must  be
          consistent among all subsets.

          B2-2.1.1 TRUSTED PATH

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    When  TCB subsets  are used,  the requirement
          for  trusted  path  at  levels  B2  and  above  remains
          applicable  to the  entire TCB.   The implementation of
          trusted path could be localized in a single TCB subset.
          Alternatively, it could be implemented in more than one
          TCB  subset if   the separate  implementations together
          comply with the system security policy.


          B2-2.2 AUDIT

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    A  TCB subset  may maintain  its own security
          audit  log,  distinct  from  that  maintained  by  more
          primitive TCB subsets, or it may use an audit interface
          provided by  a different TCB subset  allowing the audit
          records  it  generates  to  be  processed  by  that TCB
          subset.

                    If  the   TCB  subset  uses   different  user
          identifications than a more primitive TCB subset, there
          shall be  a means to associate  audit records generated
          by different  TCB subsets for the  same individual with
          each other,  either at the  time they are  generated or
          later.

                    Any TCB subset wherein  events may occur that
          require  notification  of  the  security  administrator
          shall be  able to:  (1) detect the  occurrence of these
          events, (2)  initiate the recording of  the audit trail
          entry,  and  (3)  initiate   the  notification  of  the
          security  administrator.   The   ability  to  terminate
          events (2) and (3) above  may be provided either in the
          TCB  subset within  which they   occur, or  in the  TCB
          subset(s)  where actions  that lead  to the  event were
          initiated.

                    The monitoring  and notification requirements
          may require  cooperation between multiple  distinct TCB
          subsets  or  multiple  instantiations  of  the same TCB
          subset.   For example,  to detect  the accumulation  of
          events for a single user it may be necessary to collect
          events from several subjects in distinct processes that
          are surrogates for the  same user.  As another example,
          there may  be a single TCB subset  that collects events
          from a  number of other TCB  subset instantiations and,


          based  on its analysis  of them, notifies  the security
          administrator.


                      Statement from TCSEC

                    The  TCB shall  be able  to create, maintain,
          and protect  from modification .  .  .   an audit trail
          of accesses to the objects it protects.

                      Interpretation

                    Auditable events,  in the case of  a database
          management   system,  are  the   individual  operations
          initiated   by   untrusted    users   (e.g.,   updates,
          retrievals,  inserts), not  just the  invocation of the
          database  management  system.   The  auditing mechanism
          shall  have  the  capability  of  auditing all mediated
          accesses which are visible to  the user.  That is, each
          discretionary access  control policy decision  and each
          mandatory  access  control  policy  decision  shall  be
          auditable.  Individual operations  performed by trusted
          software, if totally transparent  to the user, need not
          be auditable.  If the total audit requirement is met by
          the  use  of  more  than  one  audit  log,  a method of
          correlation must be available.

                      Statement from TCSEC

                    The  TCB shall  be able  to create, maintain,
          and protect  from modification .  .  .   an audit trail
          of accesses to the objects it protects.

                      Interpretation

                    Auditable events,  in the case of  a database
          management   system,  are  the   individual  operations
          initiated   by   untrusted    users   (e.g.,   updates,
          retrievals,  and inserts),  not just  the invocation of
          the database management system.  The auditing mechanism
          shall  have  the  capability  of  auditing all mediated
          accesses which are visible to  the user.  That is, each


          discretionary access  control policy decision  and each
          mandatory  access  control  policy  decision  shall  be
          auditable.  Individual operations  performed by trusted
          software, if totally transparent  to the user, need not
          be auditable.  If the total audit requirement is met by
          the  use  of  more  than  one  audit  log,  a method of
          correlation must be available.

          B2-3 ASSURANCE 

          B2-3.1 OPERATIONAL ASSURANCE

          B2-3.1.1 SYSTEM ARCHITECTURE

                    This  requirement applies   as stated  in the
          TCSEC   to  every   TCB  subset,   with  the  following
          additional interpretations.

                    The  TCB must  provide domains  for execution
          that are allocated to and used by TCB subsets according
          to the subset-domain condition for evaluation by parts.
          A  most primitive TCB  subset must provide  domains for
          execution.  A  less primitive TCB subset  must make use
          of domains provided by a  more primitive TCB subset.  A
          less primitive TCB subset may provide further execution
          domains but is not required to do so.

                    Similarly,  the  TCB  must  provide  distinct
          address  spaces   for  untrusted  processes.    A  most
          primitive  TCB  subset  must  provide  distinct address
          spaces for  its subjects.  A less  primitive TCB subset
          must make use of the distinct address space provided by
          a  more primitive  TCB  subset.   A less  primitive TCB
          subset may  provide more fine-grained  distinct address
          spaces, but is not required to do so.

                    In    general,   requirements    specifically
          referring  to hardware  or firmware  apply only  to TCB
          subsets  that   include  hardware  or   firmware.   The
          exception  is   the  requirement  that  the   TCB  make
          effective use of  available hardware.  This requirement
          applies  to  those  TCB   subsets  that  use  resources


          provided  by  more  primitive  TCB  subsets  in lieu of
          hardware.   Those  TCB  subsets  are  required  to make
          effective  use  of   the  protection-relevant  features
          exported to it by the more primitive TCB subsets (e.g.,
          execution domains, support for distinct address spaces)
          to separate those elements that are protection-critical
          from those that are not.

                      Statement from TCSEC

                    The TCB  shall maintain a domain  for its own
          execution that  protects it from  external interference
          or tampering.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets, this requirement applies to each TCB subset.

                      Statement from TCSEC

                    The  user  interface  to  the  TCB  shall  be
          completely  defined   and  all  elements  of   the  TCB
          identified.


                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to the interface  between the
          TCB subsets as well as  the user interface to the whole
          TCB.

                      Statement from TCSEC

                    It  shall  make  effective  use  of available
          hardware   to   separate   those   elements   that  are
          protection-critical from those that are not.

                      Interpretation


                    If the  TCB is composed of  multiple subsets,
          each TCB subset must make use of facilities provided to
          it by  more primitive TCB subsets, such  as support for
          execution domains  and for distinct address  spaces, to
          achieve the required separation.

          B2-3.1.2 SYSTEM INTEGRITY

                    This  requirement applies   as stated  in the
          TCSEC  to every  TCB subset  that includes  hardware or
          firmware.   Any  TCB  subset   that  does  not  include
          hardware or firmware is exempt from this requirement.

          B2-3.1.3 COVERT CHANNEL ANALYSIS

                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB.    When the  TCB is  made up
          entirely  of  TCB  subsets  meeting  the conditions for
          evaluation  by parts,  analysis of  the individual  TCB
          subsets satisfies this  requirement.  Otherwise, covert
          channel  analysis of the  entire TCB must  be performed
          (even if the results of  covert channel analysis of the
          individual TCB subsets were available).

          B2-3.1.4 TRUSTED FACILITY MANAGEMENT

                    This  requirement applies   as stated  in the
          TCSEC   to   the   entire   TCB.    Any  "operator"  or
          "administrator"  functions intrinsic  to an  individual
          TCB subset must be supported by that TCB subset or by a
          more primitive TCB subset.

          B2-3.2 LIFE CYCLE ASSURANCE

          B2-3.2.1 SECURITY TESTING

                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB.   If a  TCB consists  of TCB
          subsets meeting the conditions for evaluation by parts,
          the satisfaction of the requirements by each TCB subset
          satisfies   the  requirement    for  the   entire  TCB.
          Otherwise, security  testing of the entire  TCB must be


          performed   (even  if   the  results   of  testing  the
          individual TCB subsets were available).

          B2-3.2.2 DESIGN SPECIFICATION AND VERIFICATION

                    This  requirement applies   as stated  in the
          TCSEC to every TCB  subset, with the following specific
          additional interpretations.

                              It  must be  demonstrated that  the
          security  policy  enforced  by  the  composite  TCB  is
          represented by the collection  of policy models for the
          individual TCB subsets.

                    The argument  that the descriptive  top level
          specification  (DTLS)   is  consistent  with   the  TCB
          interface applies to the entire TCB.  There is required
          an explicit  and convincing description of  how the set
          of top  level specifications (one for  each TCB subset)
          describes  the TCB  interface in  terms of  exceptions,
          errors, and effects.

                      Statement from TCSEC

                    A  formal   model  of  the   security  policy
          supported by the TCB shall  be maintained over the life
          cycle  of  the  ADP   system  and  demonstrated  to  be
          consistent with its axioms.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this  requirement  applies  to  the  security
          policy of  each TCB subset.  An  informal argument that
          the  set  of  policy  models  represents  the "security
          policy  supported  by  the  [composite]  TCB"  must  be
          provided.

                      Statement from TCSEC

                    A descriptive  top-level specification (DTLS)
          of  the TCB  shall  be  maintained that  completely and


          accurately  describes the  TCB in  terms of exceptions,
          error messages, and effects.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets, this  requirement applies to the  DTLS of each
          TCB subset.  An informal argument that the set of DTLSs
          accurately describes the TCB must be provided.

                    If there is a multifaceted policy (e.g., both
          mandatory  access  control   and  discretionary  access
          control policies) in a  particular TCB subset, then all
          facets must be  represented in the DTLS and  in the TCB
          subset's model.

                      Statement from TCSEC

                    The   descriptive   top-level   specification
          (DTLS) shall be shown to  be an accurate description of
          the TCB interface.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to the interface  between the
          TCB  subsets as well  as to the  user interface of  the
          composite  TCB.  The   TCB interface  description shall
          include an explanation of how to describe the total TCB
          accurately, in  the context of the  multiple TCB subset
          DTLSs.

          B2-3.2.3 Configuration Management

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB  subset  in  the  TCB,  with  the
          following additional interpretation.

                    Because subsets  of the TCB may  be developed
          independently, a single configuration management system
          may  not  be  feasible.   However,  the  combination of
          configuration  management systems  used to  support all


          the TCB subsets must  meet all the stated requirements.
          The  information describing the  interrelations between
          separate  TCB  subsets  and  separate  security  policy
          models  falls into  the category  of "all documentation
          and  code associated  with the  current version  of the
          TCB" in the TCSEC requirements.

          B2-4 DOCUMENTATION

          B2-4.1 SECURITY FEATURES USER'S GUIDE

                    This  requirement applies   as stated  in the
          TCSEC to every TCB subset  in the TCB.  This collection
          of guides must include descriptions of every TCB subset
          in  the  TCB  and  explicit  cross-references  to other
          related  user's   guides  to  other  TCB   subsets,  as
          required.  In addition, interactions between mechanisms
          within different TCB subsets must be clearly described.

          B2-4.2 TRUSTED FACILITY MANUAL

                    This  requirement applies   as stated  in the
          TCSEC to the TCB and to every TCB subset in the TCB.

                    This  requirement can  be met  by providing a
          set of manuals, one  for each distinct (non-replicated)
          TCB  subset.  Each  manual shall  address the functions
          and privileges to be  controlled for the associated TCB
          subset.   Additionally,   it  must  clearly   show  the
          interfaces to, and the interaction with, more primitive
          TCB  subsets.  The  manual  for  each TCB  subset shall
          identify  the  functions  and  privileges  of  the  TCB
          subsets  on which  the associated  TCB subset  depends.
          Also, the TCB subset's  manual shall identify which, if
          any,  configuration options  of the  more primitive TCB
          subsets are to be used for the composite TCB to operate
          securely.

                    Any   pre-defined   roles   supported  (e.g.,
          database  administrator)  by  the  TCB  subset shall be
          addressed.


                    The means for correlating multiple audit logs
          and synonymous  user identifications from  multiple TCB
          subsets (if such exist) shall also be addressed.

                    The  trusted facility  manual shall  describe
          the  composite TCB so  that the interactions  among the
          TCB subsets  can be readily determined.   Other manuals
          may be  referenced in this determination.   The manuals
          that address  the distinct modules  of the TCB  and the
          generation of  the TCB need  to be integrated  with the
          other trusted facility manuals  only to the extent that
          they are affected by the  use and operation (versus the
          development) of the composite TCB.

                    The  TCB modules  that contain  the reference
          validation  mechanism must  be associated  with the TCB
          subset  to  which  they   belong.   The  procedure  for
          generating  a  new  TCB  after  modifying  only one (or
          several)  TCB subsets must  be described.  This  may be
          accommodated   by  independent   regeneration  of   the
          individual TCB  subsets or by regeneration  of only the
          affected TCB subsets.

          B2-4.3 TEST DOCUMENTATION

                    This  requirement applies   as stated  in the
          TCSEC to the composite TCB.

          B2-4.4 DESIGN DOCUMENTATION

                    This  requirement applies   as stated  in the
          TCSEC to the composite TCB, with the following specific
          addditional interpretations.

                    Requirements  concerning   models  and  DTLSs
          apply to individual TCB subsets.

                    The requirement concerning the description of
          interfaces  between  modules  of  the  TCB includes the
          interfaces between TCB subsets.


                    The documentation of  the implementation of a
          reference    validation    mechanism    must    include
          justification for meeting the conditions for evaluation
          by parts.

                      Statement from TCSEC

                    The interfaces between  the TCB modules shall
          be described.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this  requirement applies  to each  TCB subset  and the
          interfaces between TCB subsets.

                      Statement from TCSEC

                    The specific TCB  protection mechanisms shall
          be  identified and  an explanation  given to  show that
          they satisfy the model.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to each TCB  subset and shall
          include  the protection   mechanisms which  support the
          conditions  for  TCB   subset  structure  and  separate
          subset-domains.

                      Statement from TCSEC

                    The   descriptive   top-level   specification
          (DTLS) shall be shown to  be an accurate description of
          the TCB interface.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to the interface  between the
          TCB  subsets as well  as to the  user interface of  the
          composite  TCB.  The   TCB interface  description shall


          include an explanation of how to describe the total TCB
          accurately, in  the context of the  multiple TCB subset
          DTLSs.

                      Statement from TCSEC

                    Documentation  shall  describe  how  the  TCB
          implements  the reference  monitor concept  and give an
          explanation  of why it  is tamper resistant,  cannot be
          bypassed, and is correctly implemented.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement  is interpreted  to apply to
          each TCB subset with  respect to its specific technical
          policy.   In  addition,  there  must  be  documented an
          informal  argument that  the cooperative  action of the
          TCB  subsets  makes  the  TCB  itself tamper resistant,
          non-bypassable, and correct.

                      Statement from TCSEC

                    Documentation shall  describe how the  TCB is
          structured to  facilitate testing and to  enforce least
          privilege.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement is interpreted  to apply to individual
          TCB subsets as well as to the overall TCB.

          CLASS (B3):  SECURITY DOMAINS

          B3-1 SECURITY POLICY 

          B3-1.1 DISCRETIONARY ACCESS CONTROL

                    The discretionary access control requirements
          apply as stated in the  TCSEC to every TCB subset whose
          policy  includes discretionary   access control  of its


          subjects to  its objects.  Any TCB  subset whose policy
          does not  include such discretionary access  control is
          exempt from this requirement.

          B3-1.2 OBJECT REUSE

          This  requirement applies  as  stated  in the  TCSEC to
          every TCB subset in the TCB.

          B3-1.3 LABELS


                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

                      Statement from TCSEC

                    Sensitivity  labels associated with  each ADP
          system resource .  .  .  that is directly or indirectly
          accessible  by subjects  external to  the TCB  shall be
          maintained by the TCB

                      Interpretation

                    Internal TCB  variables that are  not visible
          to  untrusted subjects  need not  be labeled,  provided
          that they are not  directly or indirectly accessible by
          subjects external to the TCB.  However, it is important
          to understand that such internal variables can function
          as  covert signaling  channels when  untrusted subjects
          are  able  to  detect  changes  in  these  variables by
          observing system behavior.

          B3-1.3.1 LABEL INTEGRITY

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its


          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          B3-1.3.2 EXPORTATION OF LABELED INFORMATION

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          B3-1.3.3 SUBJECT SENSITIVITY LABELS

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

          B3-1.3.4 DEVICE LABELS

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory access control of its subjects to its objects
          and has attached physical  or virtual devices.  Any TCB
          subset  whose policy  does not  include such  mandatory
          access control  or has no attached  physical or virtual
          devices   is  exempt   from  this   requirement.   This
          requirement can be satisifed  by the cooperative action
          of more than one TCB subset.

          B3-1.4 MANDATORY ACCESS CONTROL

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          B3-2 ACCOUNTABILITY


          B3-2.1 IDENTIFICATION AND AUTHENTICATION

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    If  the TCB is  composed of TCB  subsets, one
          TCB subset  may either rely  on a mechanism  in another
          TCB subset to provide identification and authentication
          services or provide  the services directly.  Similarly,
          that TCB subset may rely on a mechanism in another more
          primitive TCB subset to  ensure that the security level
          of subjects external to the  TCB that may be created to
          act on  behalf of the individual user  are dominated by
          the clearance and authorization of that user.  Each TCB
          subset   may  maintain   its  own   identification  and
          authentication  data or  one central  repository may be
          maintained.  If each TCB subset  has its own data, then
          the  information  for  each  individual  user  must  be
          consistent among all subsets.

          B3-2.1.1 TRUSTED PATH

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    When  TCB subsets  are used,  the requirement
          for  trusted  path  at  levels  B2  and  above  remains
          applicable  to the  entire TCB.   The need  for trusted
          path "when positive  TCB-to-user connection is required
          (e.g.,  login,  change  subject  security  level)"  can
          require user interaction with  virtually any TCB subset
          within  the TCB.   The implementation  of trusted  path
          could   be   localized   in   a   single   TCB  subset.
          Alternatively, it could be implemented in more than one
          TCB  subset if   the separate  implementations together
          comply with the system security policy.

          B3-2.2 AUDIT


                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    A  TCB subset  may maintain  its own security
          audit  log,  distinct  from  that  maintained  by  more
          primitive TCB subsets, or it may use an audit interface
          provided by  a different TCB subset  allowing the audit
          records  it  generates  to  be  processed  by  that TCB
          subset.

                    If  the   TCB  subset  uses   different  user
          identifications than a more primitive TCB subset, there
          shall be  a means to associate  audit records generated
          by different  TCB subsets for the  same individual with
          each other, either at the time they are generated or at
          some later time.

                    Any TCB subset wherein  events may occur that
          require  notification  of  the  security  administrator
          shall be  able to:  (1) detect the  occurrence of these
          events, (2)  initiate the recording of  the audit trail
          entry,  and  (3)  initiate   the  notification  of  the
          security  administrator.   The   ability  to  terminate
          events (2) and (3) above  may be provided either in the
          TCB  subset within  which they   occur, or  in the  TCB
          subset(s)  where actions  that lead  to the  event were
          initiated.

                    The monitoring  and notification requirements
          may require  cooperation between multiple  distinct TCB
          subsets  or  multiple  instantiations  of  the same TCB
          subset.   For example,  to detect  the accumulation  of
          events for a single user it may be necessary to collect
          events from several subjects in distinct processes that
          are surrogates for the  same user.  As another example,
          there may  be a single TCB subset  that collects events
          from a  number of other TCB  subset instantiations and,
          based  on its analysis  of them, notifies  the security
          administrator.


                      Statement from TCSEC

                    The  TCB shall  be able  to create, maintain,
          and protect  from modification .  .  .   an audit trail
          of accesses to the objects it protects.

                      Interpretation

                    Auditable events,  in the case of  a database
          management   system,  are  the   individual  operations
          initiated   by   untrusted    users   (e.g.,   updates,
          retrievals,  inserts), not  just the  invocation of the
          database  management  system.   The  auditing mechanism
          shall  have  the  capability  of  auditing all mediated
          accesses which are visible to  the user.  That is, each
          discretionary access  control policy decision  and each
          mandatory  access  control  policy  decision  shall  be
          auditable.  Individual operations  performed by trusted
          software, if totally transparent  to the user, need not
          be audited.   If the total audit requirement  is met by
          the  use  of  more  than  one  audit  log,  a method of
          correlation must be available.

                      Statement from TCSEC

                    The  TCB shall  be able  to create, maintain,
          and protect  from modification .  .  .   an audit trail
          of accesses to the objects it protects.

                      Interpretation

                    Auditable events,  in the case of  a database
          management   system,  are  the   individual  operations
          initiated   by   untrusted    users   (e.g.,   updates,
          retrievals,  and inserts),  not just  the invocation of
          the database management system.  The auditing mechanism
          shall  have  the  capability  of  auditing all mediated
          accesses which are visible to  the user.  That is, each
          discretionary access  control policy decision  and each
          mandatory  access  control  policy  decision  shall  be
          auditable.  Individual operations  performed by trusted
          software, if totally transparent  to the user, need not


          be auditable.  If the total audit requirement is met by
          the  use  of  more  than  one  audit  log,  a method of
          correlation must be available.

          B3-3 ASSURANCE 

          B3-3.1 OPERATIONAL ASSURANCE

          B3-3.1.1 System Architecture

                    This  requirement applies   as stated  in the
          TCSEC   to  every   TCB  subset,   with  the  following
          additional interpretations.

                    The  TCB must  provide domains  for execution
          that are allocated to and used by TCB subsets according
          to the subset-domain condition for evaluation by parts.
          A  most primitive TCB  subset must provide  domains for
          execution.  A  less primitive TCB subset  must make use
          of domains provided by a  more primitive TCB subset.  A
          less primitive TCB subset may provide further execution
          domains but is not required to do so.

                    Similarly,  the  TCB  must  provide  distinct
          address  spaces   for  untrusted  processes.    A  most
          primitive  TCB  subset  must  provide  distinct address
          spaces for  its subjects.  A less  primitive TCB subset
          must make use of the distinct address space provided by
          a  more primitive  TCB  subset.   A less  primitive TCB
          subset may  provide more fine-grained  distinct address
          spaces, but is not required to do so.

                    In    general,   requirements    specifically
          referring  to hardware  or firmware  apply only  to TCB
          subsets  that include  hardware or  firmware.  However,
          the  requirement that  the  TCB  make effective  use of
          hardware requires that a less primitive TCB subset make
          effective  use  of   the  protection-relevant  features
          exported to it by the more primitive TCB subsets (e.g.,
          execution domains, support for distinct address spaces)
          to separate those elements that are protection-critical
          from those that are not.


                      Statement from TCSEC

                    The TCB  shall maintain a domain  for its own
          execution that  protects it from  external interference
          or tampering.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets, this requirement applies to each TCB subset.

                      Statement from TCSEC

                    The  user  interface  to  the  TCB  shall  be
          completely  defined   and  all  elements  of   the  TCB
          identified.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to the interface  between the
          TCB subsets as well as  the user interface to the whole
          TCB.

                      Statement from TCSEC

                    It  shall  make  effective  use  of available
          hardware   to   separate   those   elements   that  are
          protection-critical from those that are not.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          each TCB subset must make use of facilities provided to
          it by  more primitive TCB subsets, such  as support for
          execution domains  and for distinct address  spaces, to
          achieve the required separation.

          B3-3.1.2 SYSTEM INTEGRITY

                    This  requirement applies   as stated  in the
          TCSEC  to every  TCB subset  that includes  hardware or


          firmware.   Any  TCB  subset   that  does  not  include
          hardware or firmware is exempt from this requirement.

          B3-3.1.3 COVERT CHANNEL ANALYSIS

                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB.    When the  TCB is  made up
          entirely  of  TCB  subsets  meeting  the conditions for
          evaluation  by parts,  analysis of  the individual  TCB
          subsets satisfies this  requirement.  Otherwise, covert
          channel  analysis of the  entire TCB must  be performed
          (even if the results of  covert channel analysis of the
          individual TCB subsets were available).

          B3-3.1.4 TRUSTED FACILITY MANAGEMENT

                    This  requirement applies   as stated  in the
          TCSEC   to   the   entire   TCB.    Any  "operator"  or
          "administrator"  functions intrinsic  to an  individual
          TCB subset must be supported by that TCB subset or by a
          more primitive TCB subset.

          B3-3.1.5 TRUSTED RECOVERY

                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB   and to  the individual  TCB
          subsets.  The  cooperative recovery actions of  the TCB
          subsets making up the TCB must provide trusted recovery
          for  the  overall  TCB.   Otherwise,  trusted  recovery
          evaluation  must address  the entire  TCB (even  if the
          individual  TCB  subsets   meet  the  trusted  recovery
          requirements).

          B3-3.2 LIFE CYCLE ASSURANCE

          B3-3.2.1 SECURITY TESTING

                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB.   If a  TCB consists  of TCB
          subsets meeting the conditions for evaluation by parts,
          the satisfaction of the requirements by each TCB subset
          satisfies   the  requirement    for  the   entire  TCB.


          Otherwise, security  testing of the entire  TCB must be
          performed   (even  if   the  results   of  testing  the
          individual TCB subsets were available).

          B3-3.2.2 DESIGN SPECIFICATION AND VERIFICATION

                    This  requirement applies   as stated  in the
          TCSEC to every TCB  subset, with the following specific
          additional interpretations.

                    It  must be   demonstrated that  the security
          policy enforced by the  composite TCB is represented by
          the collection of policy  models for the individual TCB
          subsets.

                    The argument  that the descriptive  top level
          specification  (DTLS)   is  consistent  with   the  TCB
          interface applies to the entire TCB.  There is required
          an explicit  and convincing description of  how the set
          of top  level specifications (one for  each TCB subset)
          describes  the TCB  interface in  terms of  exceptions,
          errors, and effects.

                      Statement from TCSEC

                    A  formal   model  of  the   security  policy
          supported by the TCB shall  be maintained over the life
          cycle  of  the  ADP   system  and  demonstrated  to  be
          consistent with its axioms.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this  requirement  applies  to  the  security
          policy of  each TCB subset.  An  informal argument that
          the  set  of  policy  models  represents  the "security
          policy  supported  by  the  [composite]  TCB"  must  be
          provided.

                      Statement from TCSEC


                    A descriptive  top-level specification (DTLS)
          of  the TCB  shall  be  maintained that  completely and
          accurately  describes the  TCB in  terms of exceptions,
          error messages, and effects.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets, this  requirement applies to the  DTLS of each
          TCB subset.  An informal argument that the set of DTLSs
          accurately describes the TCB must be provided.

                    If there is a multifaceted policy (e.g., both
          mandatory  access  control   and  discretionary  access
          control policies) in a  particular TCB subset, then all
          facets must be  represented in the DTLS and  in the TCB
          subset's model.

                      Statement from TCSEC

                    The   descriptive   top-level   specification
          (DTLS) shall be shown to  be an accurate description of
          the TCB interface.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to the interface  between the
          TCB  subsets as well  as to the  user interface of  the
          composite  TCB.  The   TCB interface  description shall
          include an explanation of how to describe the total TCB
          accurately, in  the context of the  multiple TCB subset
          DTLSs.

                      Statement from TCSEC

                    A convincing argument shall be given that the
          DTLS is consistent with the model.

                      Interpretation


                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement   applies to  individual TCB
          subsets.  Enforcement of all  policies must be shown to
          occur  in  all  situations  (e.g.,  state  transitions)
          required by  the formal security policy  model.  In the
          case  of  a  discretionary  access  control policy, the
          presence of the access  control check at all identified
          state transitions is the total  of what is required for
          demonstrating  consistency  between  the  DTLS  and the
          model.  This may be verified  by inspection of the DTLS
          (that  is, by  visually checking  that exception checks
          required by  the model are  present in the  DTLS).  For
          the mandatory  access control policy, the  DTLS must be
          shown  by a convincing  argument to be  consistent with
          the model.

          B3-3.2.3 CONFIGURATION MANAGEMENT

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB  subset  in  the  TCB,  with  the
          following additional interpretation.

                    Because subsets  of the TCB may  be developed
          independently, a single configuration management system
          may  not  be  feasible.   However,  the  combination of
          configuration  management systems  used to  support all
          the TCB subsets must  meet all the stated requirements.
          The  information describing the  interrelations between
          separate  TCB  subsets  and  separate  security  policy
          models  falls into  the category  of "all documentation
          and  code associated  with the  current version  of the
          TCB" in the TCSEC requirements.

          B3-4 DOCUMENTATION

          B3-4.1 SECURITY FEATURES USER'S GUIDE

                    This  requirement applies   as stated  in the
          TCSEC to every TCB subset  in the TCB.  This collection
          of guides must include descriptions of every TCB subset
          in  the  TCB  and  explicit  cross-references  to other
          related  user's   guides  to  other  TCB   subsets,  as


          required.  In addition, interactions between mechanisms
          within different TCB subsets must be clearly described.

          B3-4.2 TRUSTED FACILITY MANUAL

                    This  requirement applies   as stated  in the
          TCSEC to the TCB and to every TCB subset in the TCB.

                    This  requirement can  be met  by providing a
          set of manuals, one  for each distinct (non-replicated)
          TCB  subset.  Each  manual shall  address the functions
          and privileges to be  controlled for the associated TCB
          subset.   Additionally,   it  must  clearly   show  the
          interfaces to, and the interaction with, more primitive
          TCB  subsets.  The  manual  for  each TCB  subset shall
          identify  the  functions  and  privileges  of  the  TCB
          subsets  on which  the associated  TCB subset  depends.
          Also, the TCB subset's  manual shall identify which, if
          any,  configuration options  of the  more primitive TCB
          subsets are to be used for the composite TCB to operate
          securely.

                    Any   pre-defined   roles   supported  (e.g.,
          database  administrator)  by  the  TCB  subset shall be
          addressed.

                    The means for correlating multiple audit logs
          and synonymous  user identifications from  multiple TCB
          subsets (if such exist) shall also be addressed.

                    The  trusted facility  manual shall  describe
          the  composite TCB so  that the interactions  among the
          TCB subsets  can be readily determined.   Other manuals
          may be  referenced in this determination.   The manuals
          that address  the distinct modules  of the TCB  and the
          generation of  the TCB need  to be integrated  with the
          other trusted facility manuals  only to the extent that
          they are affected by the  use and operation (versus the
          development) of the composite TCB.

                    The  TCB modules  that contain  the reference
          validation  mechanism must  be associated  with the TCB


          subset  to  which  they   belong.   The  procedure  for
          generating  a  new  TCB  after  modifying  only one (or
          several)  TCB subsets must  be described.  This  may be
          accommodated   by  independent   regeneration  of   the
          individual TCB  subsets or by regeneration  of only the
          affected TCB subsets.

          B3-4.3 TEST DOCUMENTATION

                    This  requirement applies   as stated  in the
          TCSEC to the composite TCB.

          B3-4.4 DESIGN DOCUMENTATION

                    This  requirement applies   as stated  in the
          TCSEC  to   the  composite  TCB,  with   the  following
          addtional specific interpretations.

                    Requirements  concerning   models  and  DTLSs
          apply to individual TCB subsets.

                    The requirement concerning the description of
          interfaces  between  modules  of  the  TCB includes the
          interfaces between TCB subsets.

                    The documentation of  the implementation of a
          reference    validation    mechanism    must    include
          justification for meeting the conditions for evaluation
          by parts.

                      Statement from TCSEC

                    The interfaces between  the TCB modules shall
          be described.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this  requirement applies  to each  TCB subset  and the
          interfaces between TCB subsets.

                      Statement from TCSEC


                    The specific TCB  protection mechanisms shall
          be  identified and  an explanation  given to  show that
          they satisfy the model.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to each TCB  subset and shall
          include  the protection   mechanisms which  support the
          conditions  for  TCB   subset  structure  and  separate
          subset-domains.

                      Statement from TCSEC

                    The   descriptive   top-level   specification
          (DTLS) shall be shown to  be an accurate description of
          the TCB interface.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to the interface  between the
          TCB  subsets as well  as to the  user interface of  the
          composite  TCB.  The   TCB interface  description shall
          include an explanation of how to describe the total TCB
          accurately, in  the context of the  multiple TCB subset
          DTLSs.

                      Statement from TCSEC

                    Documentation  shall  describe  how  the  TCB
          implements  the reference  monitor concept  and give an
          explanation  of why it  is tamper resistant,  cannot be
          bypassed, and is correctly implemented.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement  is interpreted  to apply to
          each TCB subset with  respect to its specific technical
          policy.   In  addition,  there  must  be  documented an
          informal  argument that  the cooperative  action of the


          TCB  subsets  makes  the  TCB  itself tamper resistant,
          non-bypassable, and correct.

                      Statement from TCSEC

                    Documentation shall  describe how the  TCB is
          structured to  facilitate testing and to  enforce least
          privilege.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement is interpreted  to apply to individual
          TCB subsets as well as to the overall TCB.

                      Statement from TCSEC

                    The  TCB implementation  shall be  informally
          shown to be consistent with the DTLS.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement  is interpreted  to apply to
          individual TCB subsets.

          CLASS (A1):  VERIFIED DESIGN

          A1-1 SECURITY POLICY 

          A1-1.1 DISCRETIONARY ACCESS CONTROL

                    The discretionary access control requirements
          apply as stated in the  TCSEC to every TCB subset whose
          policy  includes discretionary   access control  of its
          subjects to  its objects.  Any TCB  subset whose policy
          does not  include such discretionary access  control is
          exempt from this requirement.

          A1-1.2 OBJECT REUSE


          This  requirement applies  as  stated  in the  TCSEC to
          every TCB subset in the TCB.

          A1-1.3 LABELS

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

                      Statement from TCSEC

                    Sensitivity  labels associated with  each ADP
          system resource .  .  .  that is directly or indirectly
          accessible  by subjects  external to  the TCB  shall be
          maintained by the TCB

                      Interpretation

                    Internal TCB  variables that are  not visible
          to  untrusted subjects  need not  be labeled,  provided
          that they are not  directly or indirectly accessible by
          subjects external to the TCB.  However, it is important
          to understand that such internal variables can function
          as  covert signaling  channels when  untrusted subjects
          are  able  to  detect  changes  in  these  variables by
          observing system behavior.

          A1-1.3.1 LABEL INTEGRITY

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          A1-1.3.2 EXPORTATION OF LABELED INFORMATION


                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          A1-1.3.3 SUBJECT SENSITIVITY LABELS

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

          A1-1.3.4 DEVICE LABELS

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory access control of its subjects to its objects
          and has attached physical  or virtual devices.  Any TCB
          subset  whose policy  does not  include such  mandatory
          access control  or has no attached  physical or virtual
          devices   is  exempt   from  this   requirement.   This
          requirement can be satisifed  by the cooperative action
          of more than one TCB subset.

          A1-1.4 MANDATORY ACCESS CONTROL

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB   subset  whose  policy  includes
          mandatory  access  control  of   its  subjects  to  its
          objects.  Any TCB subset  whose policy does not include
          such  mandatory  access  control  is  exempt  from this
          requirement.

          A1-2 ACCOUNTABILITY

          A1-2.1 IDENTIFICATION AND AUTHENTICATION

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the


          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    If  the TCB is  composed of TCB  subsets, one
          TCB subset  may either rely  on a mechanism  in another
          TCB subset to provide identification and authentication
          services or provide  the services directly.  Similarly,
          that TCB subset may rely on a mechanism in another more
          primitive TCB subset to  ensure that the security level
          of subjects external to the  TCB that may be created to
          act on  behalf of the individual user  are dominated by
          the clearance and authorization of that user.  Each TCB
          subset   may  maintain   its  own   identification  and
          authentication  data or  one central  repository may be
          maintained.  If each TCB subset  has its own data, then
          the  information  for  each  individual  user  must  be
          consistent among all subsets.

          A1-2.1.1 TRUSTED PATH

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the
          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    When  TCB subsets  are used,  the requirement
          for  trusted  path  at  levels  B2  and  above  remains
          applicable  to the  entire TCB.   The need  for trusted
          path "when positive  TCB-to-user connection is required
          (e.g.,  login,  change  subject  security  level)"  can
          require user interaction with  virtually any TCB subset
          within  the TCB.   The implementation  of trusted  path
          could   be   localized   in   a   single   TCB  subset.
          Alternatively, it could be implemented in more than one
          TCB  subset if   the separate  implementations together
          comply with the system security policy.

          A1-2.2 AUDIT

                    This  requirement applies   as stated  in the
          TCSEC to the entire TCB.  The cooperative action of the


          TCB  subsets  making  up   the  TCB  must  satisfy  the
          requirement.

                    A  TCB subset  may maintain  its own security
          audit  log,  distinct  from  that  maintained  by  more
          primitive TCB subsets, or it may use an audit interface
          provided by  a different TCB subset  allowing the audit
          records  it  generates  to  be  processed  by  that TCB
          subset.

                    If  the   TCB  subset  uses   different  user
          identifications than a more primitive TCB subset, there
          shall be  a means to associate  audit records generated
          by different  TCB subsets for the  same individual with
          each other, either at the time they are generated or at
          some later time.

                    Any TCB subset wherein  events may occur that
          require  notification  of  the  security  administrator
          shall be  able to:  (1) detect the  occurrence of these
          events, (2)  initiate the recording of  the audit trail
          entry,  and  (3)  initiate   the  notification  of  the
          security  administrator.   The   ability  to  terminate
          events (2) and (3) above  may be provided either in the
          TCB  subset within  which they   occur, or  in the  TCB
          subset(s)  where actions  that lead  to the  event were
          initiated.

                    The monitoring  and notification requirements
          may require  cooperation between multiple  distinct TCB
          subsets  or  multiple  instantiations  of  the same TCB
          subset.   For example,  to detect  the accumulation  of
          events for a single user it may be necessary to collect
          events from several subjects in distinct processes that
          are surrogates for the  same user.  As another example,
          there may  be a single TCB subset  that collects events
          from a  number of other TCB  subset instantiations and,
          based  on its analysis  of them, notifies  the security
          administrator.

                      Statement from TCSEC


                    The  TCB shall  be able  to create, maintain,
          and protect  from modification .  .  .   an audit trail
          of accesses to the objects it protects.

                      Interpretation

                    Auditable events,  in the case of  a database
          management   system,  are  the   individual  operations
          initiated   by   untrusted    users   (e.g.,   updates,
          retrievals,  inserts), not  just the  invocation of the
          database  management  system.   The  auditing mechanism
          shall  have  the  capability  of  auditing all mediated
          accesses which are visible to  the user.  That is, each
          discretionary access  control policy decision  and each
          mandatory  access  control  policy  decision  shall  be
          auditable.  Individual operations  performed by trusted
          software, if totally transparent  to the user, need not
          be audited.   If the total audit requirement  is met by
          the  use  of  more  than  one  audit  log,  a method of
          correlation must be available.

                      Statement from TCSEC

                    The  TCB shall  be able  to create, maintain,
          and protect  from modification .  .  .   an audit trail
          of accesses to the objects it protects.

                      Interpretation

                    Auditable events,  in the case of  a database
          management   system,  are  the   individual  operations
          initiated   by   untrusted    users   (e.g.,   updates,
          retrievals,  and inserts),  not just  the invocation of
          the database management system.  The auditing mechanism
          shall  have  the  capability  of  auditing all mediated
          accesses which are visible to  the user.  That is, each
          discretionary access  control policy decision  and each
          mandatory  access  control  policy  decision  shall  be
          auditable.  Individual operations  performed by trusted
          software, if totally transparent  to the user, need not
          be auditable.  If the total audit requirement is met by


          the  use  of  more  than  one  audit  log,  a method of
          correlation must be available.

          A1-3 ASSURANCE 

          A1-3.1 OPERATIONAL ASSURANCE

          A1-3.1.1 SYSTEM ARCHITECTURE

                    This  requirement applies   as stated  in the
          TCSEC   to  every   TCB  subset,   with  the  following
          additional interpretations.

                    The  TCB must  provide domains  for execution
          that are allocated to and used by TCB subsets according
          to the subset-domain condition for evaluation by parts.
          A  most primitive TCB  subset must provide  domains for
          execution.  A  less primitive TCB subset  must make use
          of domains provided by a  more primitive TCB subset.  A
          less primitive TCB subset may provide further execution
          domains but is not required to do so.

                    Similarly,  the  TCB  must  provide  distinct
          address  spaces   for  untrusted  processes.    A  most
          primitive  TCB  subset  must  provide  distinct address
          spaces for  its subjects.  A less  primitive TCB subset
          must make use of the distinct address space provided by
          a  more primitive  TCB  subset.   A less  primitive TCB
          subset may  provide more fine-grained  distinct address
          spaces, but is not required to do so.

                    In    general,   requirements    specifically
          referring  to hardware  or firmware  apply only  to TCB
          subsets  that include  hardware or  firmware.  However,
          the  requirement that  the  TCB  make effective  use of
          hardware requires that a less primitive TCB subset make
          effective  use  of   the  protection-relevant  features
          exported to it by the more primitive TCB subsets (e.g.,
          execution domains, support for distinct address spaces)
          to separate those elements that are protection-critical
          from those that are not.


                      Statement from TCSEC

                    The TCB  shall maintain a domain  for its own
          execution that  protects it from  external interference
          or tampering.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets, this requirement applies to each TCB subset.

                      Statement from TCSEC

                    The  user  interface  to  the  TCB  shall  be
          completely  defined   and  all  elements  of   the  TCB
          identified.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to the interface  between the
          TCB subsets as well as  the user interface to the whole
          TCB.

                      Statement from TCSEC

                    It  shall  make  effective  use  of available
          hardware   to   separate   those   elements   that  are
          protection-critical from those that are not.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          each TCB subset must make use of facilities provided to
          it by  more primitive TCB subsets, such  as support for
          execution domains  and for distinct address  spaces, to
          achieve the required separation.

          A1-3.1.2 SYSTEM INTEGRITY

                    This  requirement applies   as stated  in the
          TCSEC  to every  TCB subset  that includes  hardware or


          firmware.   Any  TCB  subset   that  does  not  include
          hardware or firmware is exempt from this requirement.

          A1-3.1.3 COVERT CHANNEL ANALYSIS

          This requirement applies as stated  in the TCSEC to the
          entire TCB.   When the TCB  is made up  entirely of TCB
          subsets meeting the conditions for evaluation by parts,
          analysis of  the individual TCB subsets  satisfies this
          requirement.  Otherwise, covert channel analysis of the
          entire TCB  must be performed  (even if the  results of
          covert channel  analysis of the individual  TCB subsets
          were available).

          A1-3.1.4 TRUSTED FACILITY MANAGEMENT

                    This  requirement applies   as stated  in the
          TCSEC   to   the   entire   TCB.    Any  "operator"  or
          "administrator"  functions intrinsic  to an  individual
          TCB subset must be supported by that TCB subset or by a
          more primitive TCB subset.

          A1-3.1.5 TRUSTED RECOVERY

                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB   and to  the individual  TCB
          subsets.  The  cooperative recovery actions of  the TCB
          subsets making up the TCB must provide trusted recovery
          for  the  overall  TCB.   Otherwise,  trusted  recovery
          evaluation  must address  the entire  TCB (even  if the
          individual  TCB  subsets   meet  the  trusted  recovery
          requirements).

          A1-3.2 LIFE CYCLE ASSURANCE

          A1-3.2.1 SECURITY TESTING

                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB.   If a  TCB consists  of TCB
          subsets meeting the conditions for evaluation by parts,
          the satisfaction of the requirements by each TCB subset
          satisfies   the  requirement    for  the   entire  TCB.


          Otherwise, security  testing of the entire  TCB must be
          performed   (even  if   the  results   of  testing  the
          individual TCB subsets were available).

          A1-3.2.2 DESIGN SPECIFICATION AND VERIFICATION

                    This  requirement applies   as stated  in the
          TCSEC to every TCB  subset, with the following specific
          additional interpretations.

                    It  must be   demonstrated that  the security
          policy enforced by the  composite TCB is represented by
          the collection of policy  models for the individual TCB
          subsets.

                    The argument  that the descriptive  top level
          specification (DTLS) and formal top level specification
          (FTLS) are consistent with the TCB interface applies to
          the  entire TCB.   There  is  required an  explicit and
          convincing  description of  how  the  set of  top level
          specifications (one for each  TCB subset) describes the
          TCB  interface  in  terms  of  exceptions,  errors, and
          effects.

                      Statement from TCSEC

                    A  formal   model  of  the   security  policy
          supported by the TCB shall  be maintained over the life
          cycle  of  the  ADP   system  and  demonstrated  to  be
          consistent with its axioms.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this  requirement  applies  to  the  security
          policy of  each TCB subset.  An  informal argument that
          the  set  of  policy  models  represents  the "security
          policy  supported  by  the  [composite]  TCB"  must  be
          provided.

                      Statement from TCSEC


                    A descriptive  top-level specification (DTLS)
          of  the TCB  shall  be  maintained that  completely and
          accurately  describes the  TCB in  terms of exceptions,
          error messages, and effects.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets, this  requirement applies to the  DTLS of each
          TCB subset.  An informal argument that the set of DTLSs
          accurately describes the TCB must be provided.

                    If there is a multifaceted policy (e.g., both
          mandatory  access  control   and  discretionary  access
          control policies) in a  particular TCB subset, then all
          facets must be  represented in the DTLS and  in the TCB
          subset's model.

                      Statement from TCSEC

                    A  formal top-level  specification (FTLS)  of
          the TCB  shall be maintained that  accurately describes
          the  TCB in  terms of  exceptions, error  messages, and
          effects.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets, this  requirement applies to the  FTLS of each
          TCB subset.  An informal argument that the set of FTLSs
          accurately describes the TCB must be provided.

                    If there is a multifaceted policy (e.g., both
          mandatory  access  control   and  discretionary  access
          control policies) in a  particular TCB subset, then all
          facets must be  represented in the FTLS and  in the TCB
          subset's model.

                      Statement from TCSEC

                    The  FTLS shall  be shown  to be  an accurate
          description of the TCB interface.


                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to the interface  between the
          TCB  subsets as well  as to the  user interface of  the
          composite  TCB.  The   TCB interface  description shall
          include an explanation of how to describe the total TCB
          accurately, in  the context of the  multiple TCB subset
          DTLSs.

                      Statement from TCSEC

                    A convincing argument shall be given that the
          DTLS is consistent with the model.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement   applies to  individual TCB
          subsets.  Enforcement of all  policies must be shown to
          occur  in  all  situations  (e.g.,  state  transitions)
          required by  the formal security policy  model.  In the
          case  of  a  discretionary  access  control policy, the
          presence of the access  control check at all identified
          state transitions is the total  of what is required for
          demonstrating  consistency  between  the  DTLS  and the
          model.  This may be verified  by inspection of the DTLS
          (that  is, by  visually checking  that exception checks
          required by  the model are  present in the  DTLS).  For
          the mandatory  access control policy, the  DTLS must be
          shown  by a convincing  argument to be  consistent with
          the model.

                      Statement from TCSEC

                    .  .  .  a combination of formal and informal
          techniques  shall be  used to   show that  the FTLS  is
          consistent with the model.

                      Interpretation


                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement   applies to  individual TCB
          subsets.  Enforcement of all  policies must be shown to
          occur  in  all  situations  (e.g.,  state  transitions)
          required  by the  formal security  policy model  at the
          required  occasions.  In  the case  of a  discretionary
          access  control  policy,  the  presence  of  the access
          control  check at  all identified  state transitions is
          the  total  of  what   is  required  for  demonstrating
          consistency between  the FTLS and the  model.  This may
          be  verified by  inspection of  the FTLS  (that is,  by
          visually checking that exception checks required by the
          model  are present  in  the  FTLS).  For  the mandatory
          access  control policy,  the FTLS  must be  shown by  a
          combination  of formal  and informal  techniques to  be
          consistent with the model.

          A1-3.2.3 CONFIGURATION MANAGEMENT

                    This  requirement applies   as stated  in the
          TCSEC  to  every  TCB  subset  in  the  TCB,  with  the
          following additional interpretation.

                    Because subsets  of the TCB may  be developed
          independently, a single configuration management system
          may  not  be  feasible.   However,  the  combination of
          configuration  management systems  used to  support all
          the TCB subsets must  meet all the stated requirements.
          The  information describing the  interrelations between
          separate  TCB  subsets  and  separate  security  policy
          models  falls into  the category  of "all documentation
          and  code associated  with the  current version  of the
          TCB" in the TCSEC requirements.

          A1-3.2.4 TRUSTED DISTRIBUTION

                     This  requirement applies  as stated  in the
          TCSEC to the  entire TCB.  It can be  met by satisfying
          the  requirements  for  each  TCB  subset if procedures
          exist for  assuring that all  TCB subsets upon  which a
          particular  TCB  subset  depends  (that  is,  the  more


          primitive TCB subsets) are  exactly the same version as
          specified for the TCB subset in question.

          A1-4 DOCUMENTATION

          A1-4.1 SECURITY FEATURES USER'S GUIDE

                    This  requirement applies   as stated  in the
          TCSEC to every TCB subset  in the TCB.  This collection
          of guides must include descriptions of every TCB subset
          in  the  TCB  and  explicit  cross-references  to other
          related  user's   guides  to  other  TCB   subsets,  as
          required.  In addition, interactions between mechanisms
          within different TCB subsets must be clearly described.

          A1-4.2 TRUSTED FACILITY MANUAL

                    This  requirement applies   as stated  in the
          TCSEC to the TCB and to every TCB subset in the TCB.

                    This  requirement can  be met  by providing a
          set of manuals, one  for each distinct (non-replicated)
          TCB  subset.  Each  manual shall  address the functions
          and privileges to be  controlled for the associated TCB
          subset.   Additionally,   it  must  clearly   show  the
          interfaces to, and the interaction with, more primitive
          TCB  subsets.  The  manual  for  each TCB  subset shall
          identify  the  functions  and  privileges  of  the  TCB
          subsets  on which  the associated  TCB subset  depends.
          Also, the TCB subset's  manual shall identify which, if
          any,  configuration options  of the  more primitive TCB
          subsets are to be used for the composite TCB to operate
          securely.

                    Any   pre-defined   roles   supported  (e.g.,
          database  administrator)  by  the  TCB  subset shall be
          addressed.

                    The means for correlating multiple audit logs
          and synonymous  user identifications from  multiple TCB
          subsets (if such exist) shall also be addressed.


                    The  trusted facility  manual shall  describe
          the  composite TCB so  that the interactions  among the
          TCB subsets  can be readily determined.   Other manuals
          may be  referenced in this determination.   The manuals
          that address  the distinct modules  of the TCB  and the
          generation of  the TCB need  to be integrated  with the
          other trusted facility manuals  only to the extent that
          they are affected by the  use and operation (versus the
          development) of the composite TCB.

                    The  TCB modules  that contain  the reference
          validation  mechanism must  be associated  with the TCB
          subset  to  which  they   belong.   The  procedure  for
          generating  a  new  TCB  after  modifying  only one (or
          several)  TCB subsets must  be described.  This  may be
          accommodated   by  independent   regeneration  of   the
          individual TCB  subsets or by regeneration  of only the
          affected TCB subsets.

          A1-4.3 TEST DOCUMENTATION

                    This  requirement applies   as stated  in the
          TCSEC to the composite TCB.

          A1-4.4 DESIGN DOCUMENTATION

                    This  requirement applies   as stated  in the
          TCSEC to the composite TCB, with the following specific
          additional interpretations:

                    Requirements  concerning   models,  FTLS  and
          DTLS, apply to individual TCB subsets.

                    The requirement concerning the description of
          interfaces  between  modules  of  the  TCB includes the
          interfaces between TCB subsets.

                    The documentation of  the implementation of a
          reference    validation    mechanism    must    include
          justification for meeting the conditions for evaluation
          by parts.


                    The   A1  requirement  to   describe  clearly
          non-FTLS internals of the TCB applies to TCB subsets.

                      Statement from TCSEC

                    The interfaces between  the TCB modules shall
          be described.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this  requirement applies  to each  TCB subset  and the
          interfaces between TCB subsets.

                      Statement from TCSEC

                    The specific TCB  protection mechanisms shall
          be  identified and  an explanation  given to  show that
          they satisfy the model.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to each TCB  subset and shall
          include  the protection   mechanisms which  support the
          conditions  for  TCB   subset  structure  and  separate
          subset-domains.

                      Statement from TCSEC

                    The   descriptive   top-level   specification
          (DTLS) shall be shown to  be an accurate description of
          the TCB interface.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement  applies to the interface  between the
          TCB  subsets as well  as to the  user interface of  the
          composite  TCB.  The   TCB interface  description shall
          include an explanation of how to describe the total TCB


          accurately, in  the context of the  multiple TCB subset
          DTLSs.

                      Statement from TCSEC

                    Documentation  shall  describe  how  the  TCB
          implements  the reference  monitor concept  and give an
          explanation  of why it  is tamper resistant,  cannot be
          bypassed, and is correctly implemented.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement  is interpreted  to apply to
          each TCB subset with  respect to its specific technical
          policy.   In  addition,  there  must  be  documented an
          informal  argument that  the cooperative  action of the
          TCB  subsets  makes  the  TCB  itself tamper resistant,
          non-bypassable, and correct.

                      Statement from TCSEC

                    The  TCB implementation  shall be  informally
          shown to be consistent with the DTLS.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement  is interpreted  to apply to
          individual TCB subsets.

                      Statement from TCSEC

                    The  TCB implementation  shall be  informally
          shown to be consistent with the FTLS.

                      Interpretation

                    If  the  TCB  is  composed  of  multiple  TCB
          subsets,  this requirement  is interpreted  to apply to
          individual TCB subsets.


                      Statement from TCSEC

                    Documentation shall  describe how the  TCB is
          structured to  facilitate testing and to  enforce least
          privilege.

                      Interpretation

                    If the  TCB is composed of  multiple subsets,
          this requirement is interpreted  to apply to individual
          TCB subsets as well as to the overall TCB. 

APPENDIX  B

          GENERAL ITEMS


          1.  PERSPECTIVE ON ASSURANCE

                    This   Trusted  Database   Management  System
          Interpretation  (TDI) of   the Trusted  Computer System
          Evaluation Criteria (TCSEC)  derives its perspective on


          assurance directly  from the reference  monitor concept
          as recorded in the Anderson  Report [1] and as embodied
          in  the TCSEC.  In  both the reference  monitor concept
          and in the TCSEC, the  assessment of a system for trust
          characteristics involves a single, global review of the
          system  at issue.   That same  perspective of  an even,
          global   review  of    a  candidate   trusted  database
          management system (DBMS) is present in the TDI, in that
          only   complete   systems   will   be   considered  for
          assessment.   That  is,  neither  software  packages in
          isolation nor systems that satisfy only a subset of the
          TCSEC.  requirements will  be considered for evaluation
          or accreditation.  The  interpretation of requirements,
          both  in  Part  1,  "Technical  Context,"  and  Part 2,
          "Interpreted  Requirements,"  is  consistent  with both
          community experience in designing and assessing trusted
          systems,  and the  precedents of  the reference monitor
          concept and the TCSEC.  The interpretations, therefore,
          provide special guidance for the task of evaluating (or
          accrediting)    candidate     systems    composed    of
          distinguishable  parts.   However,  the interpretations
          neither attenuate nor delete TCSEC requirements.

                    It is  worth noting that the  introduction of
          the TCSEC  with its metric for the  evaluation of whole
          systems had as one goal  the simplification of the task
          of  accrediting   computer  systems  for  use   in  the
          processing  of sensitive  information.  The  evaluation
          process was  not intended to replace  the accreditation
          process but to provide input  to that process.  It must
          be  recognized  that  there  will  be  occasions when a
          fielded  suite  of  computer  systems,  each  evaluated
          against the  TCSEC requirements at a  particular class,
          will not be  able to be assigned a TCSEC  class, nor is
          it  necessary to  be able  to make  such an assignment.
          The accreditation  decision includes the  assessment of
          risk  of   a  particular  system  configuration   in  a
          particular environment;  a decision to connect  a suite
          of TCSEC evaluated systems may  have to be made without
          a  uniformly  applied  TCSEC-like  assessment  over the
          entire system.



          2.  PROCUREMENT OPTIONS

                    The   Trusted  Computing   System  Evaluation
          Criteria  (TCSEC)  and  its  published interpretations,
          including  this  Trusted   Database  Management  System
          Interpretation,   have  as    a  primary   purpose  the
          "provision  [of]   a  basis  for   specifying  security
          requirements  in acquisition  specifications."  [8,  p.
          2] In the area of trusted DBMS and trusted systems that
          include database  management functionality, there  is a
          range of  options open to an  acquisition organization.
          These options need to  be understood by the acquisition
          managers  and their staffs  to allow them  the greatest
          possible    flexibility    in    matching   operational
          requirements with  a combination of  available products
          and  the state  of the  art in  system integration  and
          development.

                    The  fundamental  point  is  the  distinction
          between the  target trust class  (that is, C1,  C2, B1,
          B2, B3, or A1) needed for a particular installation and
          the  degree  of  trusted  DBMS  functionality  that  is
          required.   Succinctly,   a  system  that   requires  a
          particular  level of trust  (B2, for example)  and DBMS
          functionality does not necessarily require an evaluated
          trusted  DBMS at  the level   of B2.   In fact,  if the
          statement of required capability allows it, meeting the
          requirement  without a trusted  DBMS could well  be the
          preferred  risk-reducing approach.   This is  generally
          true  because the  more sophisticated  the trusted DBMS
          requirement,  the more  likely it  is that  the current
          vendor  base will  not be   able to  supply the  needed
          functionality (with  the requisite assurance)  from the
          normal product line.  Further,  even if one can specify
          carefully  just what  additional assured  capability is
          needed  so that  a program-specific  development can be
          undertaken,  the  lack   of  community  experience  and
          consensus   on  advanced    trusted  DBMS   issues  and
          implementations    introduces    the    potential   for
          significant programmatic, schedule, and cost risks.


                    Although  a full  description of  options for
          the  acquisition manager  is beyond  the scope  of this
          document, a  representative description of some  of the
          options  that could  be considered  is included  below.
          The  options  include  (1)  multiple  copies  of a DBMS
          running   at   different   levels,   each   maintaining
          single-level  databases; (2) a  single copy of  a DBMS,
          but   with  each   database  maintained   at  a  single
          sensitivity  level (i.e.,  no sharing  of data  between
          databases); (3)  a single copy supporting  single level
          databases,  but  with  limited  sharing,  perhaps  of a
          "snapshot" nature;  and (4) DBMSs that  allow databases
          that include data of  several sensitivity levels.  This
          option admits  of subcases from the very  simple to the
          very complex.

                    To  illustrate  the   options  listed  above,
          consider a command post  where a commander's staff uses
          a  single computer system.   Included on the  staff are
          logistics,  weather,  and  intelligence  organizations,
          each  dealing with  information of  differing (maximum)
          sensitivity.    If  all  three   organizations  require
          similar  DBMS functions,  there might  be a  variety of
          ways to provide that functionality.

                    (1) If the product DBMS-1 suited the needs of
          each of the organizations and there were no requirement
          to share data between them, then three copies of DBMS-1
          could  be used,  running at,  for example,  TOP SECRET,
          SECRET, and CONFIDENTIAL, respectively, and maintaining
          separate single-level databases.  If the organizational
          missions  do  not   require  multilevel  operations  or
          sharing, this  option could be perfectly  suited to the
          operational need.   In this case, every  copy of DBMS-1
          would be running as an  application outside the TCB and
          would  not have to  be evaluated at  all, neither as  a
          product nor  as a developed system.   The advantages of
          this option are  that commercial, off-the-shelf systems
          can be  used (both the DBMS and  the underlying trusted
          operating system)  and no evaluation risk  is entailed.
          The disadvantages  are the limited flexibility  and the
          hidden fact  that the installation procedures  for many


          DBMSs require  the insertion of code into  the heart of
          the underlying  computer system, insertions  that would
          un   dermine  the   evaluation  rating   and  thus  the
          confidence in the trusted operating system.

                    (2) A cost advantage could be realized in the
          preceding  case if there  were a product,  DBMS-2, such
          that a single copy  could provide DBMS functionality at
          all  three levels.   This capability  requires that the
          base trusted  operating system and the  DBMS itself are
          designed so  that the DBMS  code uses scratch  space to
          allow the  code to be shared without  the potential for
          mixing control or metadata  in workspaces, indices, and
          stacks.  Not  all commercial DBMSs have  this property,
          so this  option will be  less available than  case (1).
          Note that  in this case also,  the databases themselves
          are  single-level and  the workspace  used by  the DBMS
          itself will have to be single-level.

                    (3) If the  operational requirements are that
          the intelligence and  logistics organizations must have
          access  to the weather  data maintained by  the weather
          organization, the simplest  technical solution would be
          to  periodically  provide  a  snapshot  of  the  needed
          weather data  for use by the  other organizations.  The
          database management system DBMS-2 above could provide a
          solution  in this  case if   a portion  of the  weather
          database  could be copied  onto diskette (or  even into
          another   file)   for   the   other   organizations  to
          incorporate  into  their   own  DBMS  operations.   The
          disadvantage of  this approach is that  the information
          will not be as current as that available to the weather
          organization itself.  Frequently, however, that lack of
          currency  may be  a reasonable   price to  pay for  the
          enormous reductions in cost and risk in procurement and
          maintenance.

                    (4) If the  operational requirements will not
          allow   anything  less    than  real-time   sharing  of
          information,  then DBMS-2  will not  be sufficient.  At
          this  point, the  operational requirements  have forced
          the inclusion  of the most sophisticated  solution to a


          trusted   system  with    DBMS  requirements,   a  true
          multilevel DBMS.   In this case, it  remains a valuable
          goal to minimize the  complexity of the needed sharing.
          If  the DBMS  is providing  a functionality  that looks
          like  tables to the  user, then it  is less complex  if
          each table  is at a single  level than if each  row (or
          each column) could be at a different sensitivity level.
          The most  complex situation is  when each entry  in the
          table  could  be  at  a  different  sensitivity  level.
          During  the  procurement  and  development  process, it
          would  be worthwhile  to structure  the procurement  to
          favor solutions  that are as simple as  possible from a
          multilevel trusted DBMS standpoint.


          3.  ALTERATION OF PREVIOUSLY EVALUATED TCB

                    The discussion in Part 1, "Technical Context"
          arose from consideration of  the conditions under which
          it is possible  to add an application layer,  such as a
          DBMS,  to another  TCB in  such a  way as  to allow the
          system rating to be determined by evaluating the system
          elements (i.e.,  the subsets) separately.   The benefit
          to  both  the  applications  vendor  and the evaluators
          derives  from the  fact that  the DBMS  operates as  an
          untrusted subject relative to  the underlying TCB (even
          though the  DBMS enforces its own  policy).  Therefore,
          there  is  no  need  to  re-examine previous evaluation
          evidence;  any   and  all  actions  performed   by  the
          application   layer  are   completely  constrained   in
          accordance  with the  security policy  defined for  the
          underlying TCB.

                    The   savings   in   evaluation   effort   is
          predicated  on the  assumption that  the vendor  of the
          application layer  makes no changes of any  kind to the
          other TCB.  In effect, the  argument made by the vendor
          is as follows:

                    (a) The underlying  TCB enforces policy P[i].
          The validity of the claims about the underlying TCB has
          already been established, and these claims remain valid


          because the underlying TCB has  not been altered in any
          way and  because the DBMS is  completely constrained by
          that  policy  (i.e.,  P[i]  cannot  be  violated by any
          action of the DBMS).

                    (b)  The application  is a  TCB subset  which
          enforces policy P[k].

                    (c)   Thus,  the   policy  enforced   by  the
          composite system (i.e., the policy enforced at the user
          interface of the composite TCB) is merely a combination
          of the policies of the individual TCB subsets.

                    Because   there   is   neither   theory   nor
          experience which  allows one to make  general, a priori
          statements about the effects  of arbitrary changes, any
          alterations to a previously  evaluated product must, in
          general,  be considered  to result  in a  product whose
          security attributes, and thus whose rating, is unknown.
          Thus, if  the previously evaluated product  is altered,
          argument (a)  cannot be made merely  by referencing the
          published   evaluation   report.     It   becomes   the
          responsibility of the DBMS  vendor to state P[i] (i.e.,
          identify  the policy  enforced by  the altered product)
          and  to demonstrate  that the  implementation satisfies
          the  appropriate TCSEC  requirements.  Hence,  at least
          some evaluation evidence focused  on the underlying TCB
          must  be  provided  by  the  vendor  of the application
          layer.   The  amount  of   evidence  required  will  be
          determined by  the type, extent, and  amount of change,
          as well as the characteristics of the original TCB.

                    This  is not  to say,  however, that  changes
          always  invalidate  all  previous  evaluation evidence.
          Rather,  that there  is no  way to  predict what effect
          arbitrary change will have  on that evidence.  Clearly,
          some changes will invalidate a substantial part, if not
          all,  of the  previous evaluation  results, requiring a
          completely  new evaluation.  In  some cases it  will be
          virtually  impossible to  determine the  full effect of
          even  relatively  simple   changes,  whereas  in  other
          instances it  may be possible to  determine the effects


          of  at  least  some  changes  quite  precisely.   In  a
          well-modularized system, changes to  the internals of a
          module might be shown to have no functional or security
          effect  outside of  the  module.   Even changes  to the
          module that alter its interface  might be shown to have
          effects  which do  not propagate  beyond those  modules
          which have a direct interface to the altered module.

                    In  either   case  however,  at   least  some
          evidence must be produced about the underlying, altered
          TCB.  Thus, the vendor who  alters the product which is
          hosting his application becomes responsible for any and
          all evidence required to  substantiate the claims being
          made for both the application and the underlying TCB.

                    In fact, it is always  the case that the DBMS
          vendor is responsible for  all the evidence required to
          demonstrate   that  the   system  (i.e.,   the  trusted
          components of the application  plus the underlying TCB)
          has the level of trust claimed  for it.  In the case of
          strict subsetting for evaluation by parts, in which the
          application    is    layered    onto    an   unaltered,
          previously-evaluated  TCB,  part  of  the  evidence  is
          satisfied   by  referencing  the   previous  evaluation
          results  and the  commercially-available specifications
          for  that  portion  of  the  system.   However,  if the
          previously   evaluated  TCB   has  been   altered,  the
          applications    vendor   is    now   responsible    for
          demonstrating that the underlying  TCB has the level of
          trust being claimed  for it.  How much, if  any, of the
          previous evaluation evidence is still valid is a matter
          to be resolved.

                    The development of  the subsetting notion has
          been motivated by the need for evaluating systems whose
          elements may have been  developed by different vendors.
          Consequently,  the discussion  of TCB  subsets has been
          largely  focused on the  topic of extending  the policy
          enforcement  attributes of  previously evaluated  TCBs.
          However,  the  notion  of  TCB  subsetting  provides  a
          perfectly  general   design  method.   A  TCB   can  be
          structured and policy enforcement allocated to simplify


          both analysis and evaluation.   To the extent that each
          TCB  subset   in  a  subsetted  system   satisfies  the
          conditions  of TC-4.3,  the evaluation  can be factored
          along  policy lines,  and  a  rating for  the composite
          system determined.


          4.  SATISFYING RVM REQUIREMENTS

                    The evaluation of a  system whose TCB is made
          up of a set of TCB  subsets follows a logical flow that
          makes  it  an  orderly  process.   The  initial step is
          satisfying  the  conditions  for  evaluation  by parts.
          Those conditions are as follows:

                        Identification  of  the   candidate  TCB
          subsets;

                       Allocation  of   the  policy  (the  clear
          statement  of the  technical policies  enforced by  the
          individual TCB subsets, stated in terms of the subjects
          and objects for that TCB subset);

                       Demonstration  that  each  candidate  TCB
          subset contains its own trusted subjects;

                      Specification of the  structure of the TCB
          subsets (as a collection of abstract machines);

                      Identification of  sets of domains (called
          "subset-domains")  assigned for   the execution  of the
          individual TCB subsets; and

                       Identification of what  externally stated
          properties  of  TCB  subsets  will  be  used  to  argue
          convincingly  and independently  for the  RVM nature of
          each of the constituent TCB subsets.

                    The  successful  completion   of  this  step,
          especially the "support for  RVM arguments" will result
          in a conditional approval of two items:  a set of goals
          in the evaluation of the more primitive TCB subsets and


          the  feasibility  of  being   able  to  argue  the  RVM
          properties of less primitive  TCB subsets using no more
          information about  the more primitive TCB  subsets than
          the identified goals.  The goals for the more primitive
          TCB   subsets   will   be    a   set   of   mechanisms,
          characteristics,  or properties  whose proper,  assured
          functioning will have to be demonstrated.  Examples are
          domain    mechanisms,   mandatory    integrity   policy
          enforcement  mechanisms,  and  a  special  category  of
          object with associated content-preservation guarantees.
          These mechanisms  or characteristics or  properties are
          strictly  a function of  the more primitive  TCB subset
          and  will  have  to  be  evaluated  and  approved  in a
          (possibly) later  part of the evaluation  process.  The
          conditional approval of the feasibility of constructing
          an  independent  RVM  argument  for  less primitive TCB
          subsets  relies on  an interplay  between the  proposed
          mechanisms  of the  more primitive  TCB subset  and the
          anticipated  needs of  the  RVM  argument for  the less
          primitive TCB subset.

                    The  next steps   of the  evaluation process,
          with  regards  to  the  reference  validation mechanism
          requirements, involve the independent evaluation of the
          TCB subsets.  TCB subsets  that have been identified as
          providing  features or  mechanisms on  which other  TCB
          subsets  will rely  for RVM  arguments will  have to be
          demonstrated to provide the  stated mechanisms with the
          same level of assurance  as the target evaluation class
          of the entire system.  If all the identified mechanisms
          can  be  validated,  the  conditional  approval  of the
          "support   for   RVM   arguments"   condition   remains
          unchanged.   If  some   mechanism  cannot  be  properly
          established  from either  a functional  or an assurance
          perspective,  then  the  conditional  approval  of that
          portion of the "support" condition is withdrawn and the
          evaluation effort must regroup.

                    TCB subsets that were projected to be able to
          complete RVM arguments successfully  using no more than
          the identified  "support" mechanisms and  features will
          have to  have full RVM arguments  advanced and accepted


          by  the  evaluation  team.   There  are  three possible
          outcomes:  (1) it could be shown that the TCB subset in
          question  does not  meet the  RVM requirements;  (2) it
          could  be shown,  using the  external descriptions  and
          assurances of the mechanisms  of the more primitive TCB
          subsets, that  the less primitive TCB  subset does meet
          the RVM requirements; or (3)  it might be impossible to
          determine  whether  or  not  the  TCB  subset meets the
          requirements.   In case (1),  the TCB subset  failed to
          meet  its reference  validation mechanism  requirements
          and the design team must regroup.  In case (2), the TCB
          subset  satisfies  its  reference  validation mechanism
          requirements.  In case (3), the conditional approval of
          the  "support  for  RVM  arguments"  condition  will be
          withdrawn and the design and evaluation teams will have
          to agree on an alternate course of action.

                    In the case that  an attempt to establish RVM
          properties for a less primitive TCB subset could not be
          completed  (case  (3)  above),  it  might  well be that
          additional mechanisms or features of the more primitive
          TCB  subset  would  allow   the  RVM  arguments  to  be
          completed successfully.   In that case,  the evaluation
          team and the design team would have to establish a new,
          augmented   set  of    mechanisms  for   the  "support"
          condition.  The evaluation could then continue with the
          new  mechanism requiring  validation by  the evaluation
          team  and the  argument for  the RVM  properties of the
          less primitive TCB subset having to be re-attempted.

                    It is  important to note that  the dependency
          of  the  less  primitive  TCB  subsets  on  the assured
          policies  and   RVM  supporting  mechanisms   makes  it
          imperative that the evaluation of the whole TCB be done
          by   a  single   evaluation  team   through  the  final
          determination  that the  system complies  with the full
          set  of requirements  for the  target class.   Thus, in
          particular, an evaluation team addressing an evaluation
          by parts (including  the case of a TCB  subset that has
          been previously  evaluated and placed on  the EPL) must
          be  kept  together  for  the  entire evaluation.  Local
          evaluation   of  one   TCB  subset   does  not  justify


          dissolving  the  responsible  subteam.   Later results,
          global or local to another  TCB subset, could require a
          full  evaluation team  current  on  all aspects  of the
          evaluated  configuration.   This   does  not  mean,  of
          course,  that  the  original   evaluation  team  for  a
          previously evaluated EPL product has to be reassembled.
          A  new  team,  part  of  which  may  be delegated prime
          responsibility for  that TCB subset, suffices,  as long
          as  the  full  team  is  kept  together  for  the whole
          evaluation.


          5.  SUBSET DEPENDENCY

                    For  candidate TCB  subset M,  sM denotes the
          specification  of  M,  including   as  a  minimum,  the
          statement of the technical policy  P of M.  The term vM
          denotes   the  (engineering)   demonstrations  of   the
          correctness of the implementation  of M with respect to
          its specification.  A (candidate) TCB subset A "depends
          (for its  correctness)" on (candidate) TCB  subset B if
          and  only  if  the   arguments  within  vA  assume  the
          correctness of the implementation  of B with respect to
          sB.

                    In less  precise terms, the  specification sM
          defines what M is supposed  to do.  M does whatever its
          implementation  allows, features  and all.   How well M
          does  compared  to  what  sM   says  it  should  do  is
          encompassed  in the  engineering arguments  vM.  If, in
          the argument vA, one has to  assume that all or part of
          sB accurately  describes what B does, A  "depends" on B
          for its correctness.

          Example 1:  Use of Provided Objects

                    Suppose  TCB subset  B provides  "file" as  a
          mediated  object under  its technical  policy P[B]  and
          that candidate TCB subset A  uses B-files to store data
          and  executables.  If vA  uses the fact  that different
          B-files are distinct and  access to them is constrained
          by  the  technical  policy  P[B]  (assumptions that are


          nearly  certain to  apply), then  vA is  relying on the
          fact that sB and B match and in this case, A depends on
          B.

          Example 2:  Mutually Suspicious Systems

                    Suppose  A  and  B  are  mutually  suspicious
          airline    reservation    systems    hosted    in   two
          interconnected   systems  belonging  to   two  distinct
          organizations.   It  is  assumed  that  sA  and sB both
          provide  for a  capability to  accept reservations over
          the  network from  "foreign" systems  using a  mutually
          defined  protocol.   The   protocol  is  carefully  and
          completely specified  (within both sA and  sB) and both
          vA  and  vB  demonstrate,   to  the  desired  level  of
          satisfaction, that A and B  are correct with respect to
          sA  and sB,  respectively.  A  does not  depend for its
          correctness on B because  sA is complete:  for whatever
          sequence  of  bits  it  receives  from  B, sA specifies
          exactly   what  behavior   A  must   exhibit,  and   vA
          demonstrates  that  it   does  exhibit  that  behavior.
          Similarly,   B  does   not  depend   upon  A   for  its
          correctness.  Neither A nor B depends on the other.

                    There may well exist a "system specification"
          that  specifies  the  desired  behavior  of  the entire
          system, but that is irrelevant  to the arguments that A
          and  B are individually  correct with respect  to their
          own specifications.  It may even  be the case that both
          A  and B are  individually correct, while  the combined
          system  is  incorrect  with   respect  to  the  "system
          specification."  That is, two correct subsystems can be
          combined improperly with respect to the desired "system
          specification."

          Example 3:  Use of Remotely Provided Functionality

                    Suppose A  is a mail  server and B  a generic
          SQL DBMS.  The specification  sA (as might be expected)
          makes  no mention  of a  DBMS; it  simply specifies the
          interface behavior  (to its human clients)  of the mail
          system.  In vA, however, we  find that the software for


          A uses tables provided by B to store messages.  A and B
          are on  separate, interconnected machines.   Neither sB
          nor vB make  mention of the mail system at  all.  As in
          the  preceding  example,  sB  completely  specifies the
          behavior  of  B  for  all  received  bit  patterns  and
          sequences.   Here, A  depends upon  B, but  B does  not
          depend on A.  Note that data flow in both directions is
          completely  legitimate and  does not  compromise in any
          way the "integrity" (correctness)  of B.  Dependency is
          distinct from "data flow."

                    This   example  shows   that  a   superficial
          examination of the "architecture" of a domain structure
          is  insufficient  to   determine  which  candidate  TCB
          subsets depend upon  other TCB subsets.  Superficially,
          this  architecture  is  the  same  as  the  example  of
          mutually suspicious  systems above, but here  A depends
          on  B.   It  also  shows  that  an  examination  of the
          interface specifications is  insufficient.  Finally, it
          shows that dependency is not  the same as the notion of
          "privilege" (as  normally understood in the  context of
          operating  systems  to  mean  loosened  restrictions on
          allowed  functions and  accesses), because  there is in
          this example no  sense in which B has access  to all of
          A's internal structures.  It only has access to some of
          them.

          Example 4:  Use of Locally Provided Functionality

                    Suppose A  and B are the mail  server and SQL
          DBMS  of  the  preceding  example,  except  that  A  is
          implemented in a less privileged ring than B.  That is,
          the interconnect is replaced by a ring-crossing service
          call.  Obviously, A  still depends on B and  B does not
          depend on  A.  The change  is that now  B has potential
          access to any of A's  structures.  The general rule for
          the use of domain protection mechanisms (such as rings)
          is that  if B is privileged  with respect to A,  then A
          necessarily  depends  on  B  (simply  by  virtue of B's
          privilege  to access any  of A's structures).   Thus, a
          detailed  examination of  sA and  vA is  unnecessary to
          establish dependency.


          Cautionary Example

                    Suppose   that   A   and   B   are  "mutually
          dependent"; that is, A depends on B and B depends on A.
          This  means  that  vA  assumes  that  B  is implemented
          correctly with respect to sB,  and vB assumes that A is
          implemented  correctly  with  respect  to  sA.  Further
          suppose that  both vA and vB are  valid (reasonable and
          compelling).  One would hope  that it follows from this
          that both A and B  are correct.  Unfortunately, this is
          not always the case.

                    If A  and B are both correct  with respect to
          sA  and sB,  then vA  is a  valid argument  with a true
          premise and is therefore true.   The same is true for B
          and vB.

                    Suppose,  however,  that   A  is  implemented
          incorrectly  with respect  to sA  and B  is implemented
          incorrectly with respect to sB.  vA is a valid argument
          with a  false premise; it is thus  valid but (possibly)
          untrue.  Similarly, vB is  valid but (possibly) untrue.
          This case shows that it is quite possible for vA and vB
          to both be valid while  A and B are both (individually)
          incorrectly implemented.

                    What has  been exposed here is  a hidden case
          of circular reasoning:  the  argument that A is correct
          is based on  the assumption that B is  correct, and the
          argument that  B is correct is based  on the assumption
          that  A is correct.   Thus, vA depends  (circularly) on
          the assumption that A is correct, and vA reduces to the
          following tautology:

          if vA is correct with respect  to sA then vA is correct
          with respect to sA.

          It is thus possible in principle for mutually dependent
          subsystems A  and B to have  vA and vB to  be logically
          valid while either A or  B, or both, are incorrect with
          respect to their specifications (sA or sB).


                    To  make this  result concrete,  consider two
          airline  reservation systems,  A  and  B, based  on the
          mutually  suspicious   systems  of  example   2  above.
          Suppose that A maintains  information about all flights
          originating or terminating in the United States while B
          maintains  information  about  flights  originating  or
          terminating in Europe.  Assume  sA includes a statement
          that A will generate  flight itineraries from an origin
          to  a  destination,   with  appropriate  provision  for
          connections.   "Appropriate provision  for connections"
          means  allowing enough  time to  change planes  without
          requiring  passengers to  wait an  excessive period  of
          time.   Further  assume  that  sB  includes  a  similar
          statement.  From  the assumption that A meets  sA and B
          meets  sB, one  can construct  a valid  argument that A
          meets its  specification sA for  itineraries orginating
          or  terminating  in  either  the  U.S.   or  Europe.  A
          similarly  valid argument  can  be  made about  B.  If,
          however,  A stores  flight segment  times in  the local
          time  of the airport  and B stores  them i n  Greenwich
          Mean Time, an itinerary generated by either A or B that
          relies on information from  the other will be incorrect
          because  of the  time  differences.   Thus, A  will not
          generate  accurate,  timely  flight  itineraries,  even
          though   a  valid   argument  that   it  does   can  be
          constructed.    This   difficulty   arises   from   the
          presumption that A and B are mutually dependent.


          6.  TAMPER RESISTANCE ARGUMENTS

                    The    requirement   to    demonstrate   that
          individual TCB subsets exhibit the reference validation
          mechanism  tamper resistance property  deserves special
          attention.  In  a subsetted architecture there  are two
          (related)  aspects to  this requirement.   The first is
          the ability of a less  primitive TCB subset to maintain
          control over  access to the objects  that implement its
          logic and data structures.   The second is establishing
          the      assurance     that      policy-critical     or
          correctness-critical  data  used  by  a  TCB  subset is
          persistent (that is, that it  does not change or become


          contaminated with  other data between the  execution of
          instructions).

                    A  less primitive  TCB subset  will be  using
          objects and  subjects provided by a  more primitive TCB
          subset.  Thus,  policy-critical data will be  stored in
          objects  that are  provided by  the more  primitive TCB
          subset  rather than in  some system entity  created and
          maintained  by the  less primitive  TCB subset  itself.
          One part  of the tamper resistance  argument focuses on
          being able to demonstrate  that no alteration of either
          the policy-critical data or of the TCB subset's code is
          possible.  That is, no system  entity external to a TCB
          subset (with  the possible exception of  more primitive
          TCB subsets upon which it depends) should be capable of
          causing arbitrary changes to  that TCB subset's code or
          data  structures.  If a  third, not more  primitive TCB
          subset (or a subject totally outside the TCB) were able
          to gain access to  an object where policy-critical data
          was stored, the tamper resistance property could not be
          established  for the  TCB  subset  in question.   For a
          most-primitive TCB subset, a wide variety of techniques
          could  be used to  limit access to  an object in  which
          such policy-critical data  is stored (e.g., prohibition
          on the sharing of  objects, strict ownership control of
          the ability to extend  access permission, and mandatory
          access   controls).   Similarly,  the   conditions  for
          evaluation  by parts  require that  the system designer
          identify  a   set  of  mechanisms  and   their  assured
          properties  as  the   basis  for  demonstrating  tamper
          resistance for each TCB subset.

                    The  second topic  is the  assurance that the
          contents  of  the  objects  that  store  a TCB subset's
          policy-critical  data  not  change  except  through the
          execution  of  that  TCB  subset's  logic.   If  a data
          structure that  identified an exported object  (such as
          tables or  tuples or entities) were  to have extraneous
          data  inserted  by  a  more  primitive  TCB subset (for
          example,  as  a  result  of  garbage  collection or the
          random  action  of  memory  management),  then no basis
          would  exist  for   arguments  concerning  the  correct


          implementation of the less primitive TCB subset.  For a
          most  primitive TCB  subset (which  includes supporting
          hardware), the  argument that the  policy-critical data
          is kept current and correct can be made strictly on the
          basis of  that TCB subset.  However, when  a TCB subset
          obtains resources from a more primitive TCB subset, the
          argument cannot  be made strictly  on the basis  of the
          design  of the TCB  subset.  Rather, the  argument must
          proceed  from  assured   mechanisms  provided  by  more
          primitive TCB  subsets.  This is exactl  y analogous to
          the case of a  reference validation mechanism for which
          one relies  on mechanisms not strictly  included in the
          design  of the  policy-enforcing elements  to establish
          the  requisite  properties   of  non-bypassability  and
          tamper  resistance.   A  variety  of  mechanisms  might
          provide   a   basis    for   demonstrating   that   the
          implementation of a TCB  subset is correct, even though
          resources are obtained from a more primitive TCB subset
          (e.g., type-enforcement).


          7.  RATIONALE FOR LOCAL AND GLOBAL REQUIREMENTS

                    Section     TC-5,    "General     Interpreted
          Requirements,"  lists  the  requirements  of  the TCSEC
          according to  how the requirements apply to  a TCB made
          up of more than one  TCB subset.  The general rationale
          for distinguishing which  requirements can be satisfied
          by  independent analysis  and consideration  of the TCB
          subsets was to consider the  requirements one at a time
          to determine whether satisfaction of the requirement by
          the individual TCB subsets  would necessarily mean that
          the  entire system   satisfies the  stated requirement.
          For  some  requirements,  such  as  those  for  certain
          documentation,  it  is  clear  that  the requirement is
          factorable; that is, it  is satisfied for the composite
          TCB  if it  is satisfied  for each  of the  TCB subsets
          individually.    For  other   requirements,  individual
          system  characteristics could  make factorability  of a
          requirement patently obvious, but not all systems would
          enjoy such  simple factorability.  An example  would be
          trusted path requirements  for security-related events:


          if  all  security-related  ev  ents  occur  in the most
          primitive TCB  subset, satisfaction of  the requirement
          by  that TCB  subset suffices  to demonstrate  that the
          system meets the requirement, but for systems that have
          security-relevant events in less primitive TCB subsets,
          some explanation  of the cooperative action  of the TCB
          subsets   would   be   required.    For   still   other
          requirements, one  can expect that the  satisfaction of
          the  requirement  for  the  entire  system  will always
          require some  explanation of the cooperative  action of
          the constituent  TCB subsets.  Provision of  a coherent
          audit record across events in several TCB subsets is in
          this category.

                    In  the paragraphs  below, a  brief rationale
          for  identifying requirements   as wholly  or partially
          global  is provided.   Those requirements  that are not
          listed are considered to be completely local.

          Subject Sensitivity Labels

                    At level B2 and above, the TCSEC requires the
          following:

                    The TCB  shall immediately notify  a terminal
          user  of each change  in the security  level associated
          with  that  user  during  an  interactive  session.   A
          terminal user shall be able to query the TCB as desired
          for  a display  of the  subject's complete  sensitivity
          level.

                    For   subsetted   architectures,   the   user
          interface  could  be  to  a  TCB  subset  that does not
          support  a mandatory  access control  policy.  Thus,  a
          change noted  by a more primitive TCB  subset that does
          support such a  policy would have to be  relayed to the
          user, possibly  through cooperative action of  the full
          set of TCB subsets.  Similarly, a request by a terminal
          user  for  the  complete  sensitivity  level  could  be
          initially  received  by  a  TCB  subset  that  does not
          support  a  mandatory  access  control  policy and will
          require  cooperation between  TCB subsets  to determine


          the complete  subject sensitivity level and  to provide
          that information to the requesting user.

          Identification and Authentication

                    The    identification   and    authentication
          requirements in the TCSEC address the need to correctly
          associate authorizations with subjects.   In a TCB made
          of several TCB subsets, it is possible that only one of
          the  TCB   subsets  will  provide   identification  and
          authentication,  which will  be  used  by all  the less
          primitive  TCB subsets.   Alternatively, identification
          and  authentication may  be provided  directly in  more
          than one  TCB subset.  In either case,  the TCB subsets
          have  to work  cooperatively to  use identification and
          authentication data for  uniquely identifying users and
          for associating users with auditable actions.

          Trusted Path

                    At B2, the only required uses of trusted path
          are  login  and  authentication.    At  B3  and  above,
          occasions  "when a  positive TCB-to-user  connection is
          required (e.g., login,  change subject security level)"
          are  included.   In  both  cases,  a  system vendor may
          choose  to use  trusted path  for situations  where the
          security-relevant event could  be recognized or handled
          in more  than one TCB subset.  On  those occasions, the
          careful coordination of all the involved TCB subsets in
          the correct handling of trusted path situations must be
          shown.  If a single  TCB subset implements trusted path
          and all the invocations of  trusted path are limited to
          that  TCB  subset  (that  is,  the  flow  of control in
          responding  to a  trusted path  initiation never leaves
          the TCB  subset until the response  is completed), then
          nothing further would be  required.  The description of
          the limitation  of trusted path to a  single TCB subset
          will  suffice for the  global part of  the requirement,
          leaving only the demonstration of local satisfaction of
          the requireme nt by the identified TCB subset.

          Audit


                    If  each  of  several  TCB  subsets meets the
          audit requirements locally, then  there is the issue of
          whether the set of audit records meets the requirements
          of  being  able  to  note  and  record  individual user
          actions, and  at B3 and  above, to be  able to initiate
          required action.   If not all the TCB  subsets meet the
          audit requirements locally,  then the requirements must
          be satisfied  by the cooperative  action of the  set of
          TCB subsets.  In both cases, consideration of the audit
          characteristics of  all the TCB subsets has  to be part
          of  determining that  the  entire  TCB meets  the audit
          requirements.

          System Architecture

                    For   many   of   the   system   architecture
          requirements,  demonstrating  that   a  requirement  is
          satisfied  by all  of the  consitituent TCB  subsets is
          sufficient to demonstrate that  it is satisfied for the
          composite  TCB.   The  requirements  for  the "TCB [to]
          maintain a domain for its execution" and for the TCB to
          "maintain  process isolation  through the  provision of
          distinct  address  spaces"  could  be  satisfied by the
          entire  TCB without  each constituent  TCB meeting  the
          requirement.

                    The  requirement for  the TCB  to maintain  a
          domain for its execution implies  the need for each TCB
          subset to have a domain for its own execution, isolated
          from  both untrusted  portions of  the system  and from
          less primitive  TCB subsets.  The exact  wording of the
          TCSEC requirement  could be read as  disallowing a less
          primitive TCB  subset from occupying a  domain provided
          by  a more  primitive TCB  subset and  to allocate  its
          subjects to domains that do  not have access to its own
          domain:   the verb   "maintain" could  be (erroneously)
          read  as  requiring  each  TCB  subset  to  create  and
          maintain  its  own  domain  for  execution.  The proper
          interpretation is that the TCB  as a whole must provide
          and  maintain such  domains for  execution, rather than
          each individual TCB subset.


                    Similarly,  the exact   wording of  the TCSEC
          requirement on the  "maintain[ing] of process isolation
          through the provision of distinct address spaces" could
          be  read  as  requiring  each  TCB  subset  to  provide
          distinct address spaces.   The proper interpretation is
          that  the TCB  as a   whole must  provide and  maintain
          process   isolation,  either   through  provision   and
          subsequent use  of distinct address spaces,  or through
          provision  of  distinct  address  spaces  by  every TCB
          subset.

          Covert Channel Analysis

                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB.    When the  TCB is  made up
          entirely  of  TCB  subsets  meeting  the conditions for
          evaluation  by parts,  analysis of  the individual  TCB
          subsets suffices to  satisfy this analysis requirement.
          Otherwise,  covert channel   analysis must  address the
          entire  TCB  (even  if  the  result  of  covert channel
          analyses of the individual TCB subsets were available).

          Trusted Facility Management

                    The ability to run  a trusted system facility
          properly  applies  to  the  combination  of TCB subsets
          making  up the  TCB.   This  requirement should  not be
          difficult  to meet,  provided that  the individual  TCB
          subsets  meet  the  requirement  and  the  interactions
          between  the  TCB  subsets  at  the facility management
          level are clear.

          Trusted Recovery

                    In  the case  of  "an  ADP system  failure or
          other discontinuity," each TCB subset  in a B3 or above
          system  needs   to  be  able  to   recover  "without  a
          protection compromise."  Further,  the recovery actions
          of  distinct TCB  subsets needs  to be  coordinated and
          combined  so  that  the  resulting  system  is not only
          recovered as  far as each TCB subset  is concerned, but
          is also recovered as a composite TCB.


          Security Testing

                    This  requirement applies   as stated  in the
          TCSEC  to the  entire TCB.   If a  TCB consists  of TCB
          subsets meeting the conditions for evaluation by parts,
          the satisfaction of the requirements by each TCB subset
          suffices to satisfy the requirement for the entire TCB.
          Otherwise, security testing must include testing of the
          entire  TCB  (even  if   the  results  of  testing  the
          individual TCB subsets are available).

          Design Specification and Verification

                    For  many  of  the  design  specification and
          verification   requirements,   demonstrating   that   a
          requirement is satisfied by all of the consitituent TCB
          subsets  is  sufficient  to   demonstrate  that  it  is
          satisfied for the composite  TCB.  The requirements for
          a "formal model of the security policy supported by the
          TCB" and that the DTLS at B3  and the FTLS at A1 "be an
          accurate description  of the TCB interface"  apply in a
          limited way to the entire TCB.

                    After complying security  models are provided
          for the  individual TCB subsets, a  convincing argument
          is required to explain how the set of models represents
          abstractly the policy of the entire system.

                    After   complying  top-level   specifications
          (DTLS  at  B3  and  FTLS  at  A1)  are provided for the
          individual  TCB  subsets,  an  explicit  and convincing
          description of how the  set of top-level specifications
          describe the TCB interface  with respect to exceptions,
          errors and effects must also be provided.


          8.   CONTENT-DEPENDENT  AND  CONTEXT-DEPENDENT ACCESS
          CONTROL

                    An  attractive  proposition   in  a  database
          managment system  is to implement access  controls that
          depend  in some  way on  the values  of the  data in  a


          storage object or the  context in which the information
          is  accessed.  For example,  one might desire  to limit
          access to all personnel records in a database according
          to the  salary value (content-dependent  access rules).
          On the other hand, a company's earnings report might be
          held in confidence until announced at the stockholders'
          meeting,  at  which  time   it  is  public  information
          (context-dependent access rules).

                    This document  does not encourage  or endorse
          mandatory  access control  on storage  objects that are
          based on the  content of data values or  on the context
          in which  the information is viewed.   Given that these
          are  research  topics,  it  is  prudent  to  take  this
          conservative stance.  Research and current practice are
          insufficient  to  allow  definitive  guidance  on  such
          implementations.


          9.  BULK LOADING OF A DATABASE

                    The bulk loading of  a database into (perhaps
          thousands of) database objects  must be done with care.
          If the data  to be loaded are unlabeled, it  may not be
          practical to require an  authorized user to examine the
          data  to be  loaded into  each object  and assign  it a
          sensitivity label.  Instead it may be more practical to
          assign labels to the  data according to the sensitivity
          label of the single-level device that is used to import
          it.   In  this  way,  bulk   loading  may  be  done  in
          single-level stages.

                    Even  importing labeled  multilevel data  may
          prove  difficult.   The  imported  data  records may be
          organized on the input  device in accordance with their
          logical structure,  not their sensitivity  levels.  For
          some trusted DBMS architectures, this requires that the
          TCB first  separate the data by  sensitivity levels and
          subsequently  load  the  data   into  the  database  as
          single-level structures.


                    Another problem with  bulk loading of labeled
          data  is  granularity.   It  may  be  that the labeling
          granularity  of data on  the input device  is different
          from   the  labeling   granularity  supported   by  the
          receiving trusted DBMS.  As  an example, the data being
          imported may be labeled at the file or field level, and
          the  importing DBMS may  support labeling at  the tuple
          level.  In  such instances, the  data would have  to be
          mapped into objects of  the proper labeling granularity
          as the data are being imported.


          10.  LOCAL ANALYSIS IN SYSTEM ASSESSMENT

                    The  ability  to  distinguish  and separately
          reference the results of  local analysis of TCB subsets
          is  a primary  aspect of  evaluation by  parts, and the
          benefits  which  accrue  are  apparent  in two, closely
          related,  cases  that  arise  in  evalutions  by parts.
          These may be thought of as dealing with the problems of
          "hosting" and "porting" although  they are actually two
          aspects of the same problemthat of assessing a trusted
          system constructed of previously evaluated parts.

                    For the first case (i.e., that of "hosting"),
          consider an  operating system which has  been evaluated
          against  the  TCSEC  requirements  and  has  received a
          rating.  Thus, the operating system  is a TCB for which
          both the local and global  analysis has been done.  The
          results  of  the  local  analysis  can  now  be used to
          support  the  evaluation  of  a  TCB  made  up  of  the
          operating  system (or,  the more  primitive TCB subset)
          and any proposed TCB  extension (or, less primitive TCB
          subset).  Suppose,  for example, that vendor  A chooses
          the  rated operating  system as   the host  for a  DBMS
          product, which implements an access control policy.  As
          described  in  TC-6,  it  is  now  possible,  under the
          correct conditions, to re-use  the results of the local
          analysis of  the host operating system  in developing a
          rating for the composite  system.  Even for those cases
          not meeting all the conditions for evaluation by parts,
          it  may be  possible that   some, if  not most,  of the


          previous  results are  still valid.   If vendor  B also
          chooses the rated operating system  as the host for his
          DBMS  product, it  will be  possible (again,  under the
          proper  conditions) to develop  a rating for  the (new)
          composite  system without  having to  repeat the  local
          analysis of the host operating system.  As an alternate
          case,  suppose a  site has  need of  an electronic mail
          service as well as the  DBMS utility.  The mail utility
          will operate in a peer-entity relation with the trusted
          DBMS utility (i.e., both the  mail service and the DBMS
          depend  on  the  host  operating  system,  but  neither
          depends  on the other).   Again, having the  results of
          the local  analysis of the host  operating system eases
          the burden of assessing the security characteristics of
          the user  interface to the composite system  made up of
          the  mail system  and  the  host operating  system.  In
          short,  the  ability   to  distinguish  and  separately
          reference the results of the local analysis of the host
          operating  system  makes  it  feasible  to evaluate the
          effect of  adding arbitrary trusted  applications, only
          by performing the local analyis for the application and
          any global analysis required.
                    For   the  second    case,  (i.e.,   that  of
          "porting") the question becomes that of determining the
          effect of moving a known trusted application, such as a
          DBMS,  across arbitrary  host systems.   Assume that  a
          trusted  DBMS   product  meeting  the   conditions  for
          evaluation by parts has  been evaluated on some trusted
          host, and a rating determined for the composite system.
          Clearly,  the  results  of  the  local  analysis of the
          trusted  application available  are also  applicable to
          the  analysis of  a composite   system made  up of  the
          trusted  application  and  a  different  host operating
          system.  Thus, having the local analysis of the trusted
          application will ease  the evaluation burden associated
          with  porting  of  trusted  applications  to  different
          hosts.    To  the   extent  that   the  conditions  for
          evaluations  by parts  have been  satisfied, the  local
          analysis  of  the  application  is  valid by reference.
          Hence  only the  local analysis  of the  host operating
          system and  the requisite global analysis  is needed to


          assess  the security  attributes of  the new  composite
          system.

          11.  RATING MORE COMPLEX SYSTEMS

                    The  view taken  by the  TCSEC is  that of an
          "atomic"  TCB; the TCB  is seen to  be a single  entity
          which  is, in some  sense, homogeneous.  This  allows a
          relatively  simple measure  (i.e., the  digraphs) to be
          assigned to the TCB.  However, real systems may be more
          complex, resulting in the inability of a single, simple
          rating  to convey  the full  complexity of  the system.
          This  is  implicitly  recognized  in  TCSEC  evaluation
          reports and  EPL entries, in which credit  may be given
          to a vendor for meeting TCSEC (functional) requirements
          beyond those necessary to satisfy the rating (e.g., the
          B3 discretionary  access control feature in  a C2 TCB).
          In   short,  systems   which  reflect   straightforward
          implementations   or  extensions   of  the   TCSEC  can
          accurately be described with  a single digraph.  On the
          other  hand, adding  complexity to  systems may violate
          assumptions  which underlie   the TCSEC  rating system,
          requiring a more complex  description if accuracy is to
          be achieved.
                    If a TCB made up of TCB subsets is consistent
          with  the  TCSEC  assumptions  on  homogeneity,  then a
          simple  digraph  suffices  for   a  full  and  accurate
          description of the security  properties of the product.
          However,  to the  extent that  a subsetted architecture
          introduces complexity not captured by the digraphs, the
          simple TCSEC ratings cannot be applied to the composite
          system.   More  specifically,  for  a  subsetted TCB to
          achieve a  single rating, all the  requirements of that
          class   must   be   satisfied.    For   example,  if  a
          discretionary access control-enforcing  DBMS TCB subset
          is  added onto a  previously evaluated B3  product, the
          entire system can achieve a  B3 rating if it could also
          have  achieved the B3  rating evaluated as  a monolith.
          That is, the  new TCB subset must also  satisfy all the
          assurance and architectural requirements of B3.
                    Consider   a  candidate   TCB  subset   which
          enforces a  discretionary access control policy  over a


          new type of object, targeted at a host system which has
          already been evaluated at the B3 level.  Examples are a
          database  management   system  providing  discretionary
          access  control over   tuples, a  transaction processor
          providing    discretionary    access    control    over
          transactions,   and   a    message   system   providing
          discretionary  access control   over messages.   If the
          candidate TCB subset meets all the C2 requirements, the
          problem  is  what  rating   will  be  assigned  to  the
          composite  system.  To designate  it a "C2"  is clearly
          inaccurate, as well as being  unfair to the original B3
          product  vendor.  To designate  it "B3" may  be equally
          inaccurate, and it creates  ambiguity in the meaning of
          the  metric  used  for  comparing  systems.   In  fact,
          depending on the details of the specific candidate, the
          composite  system could  legitimately be  rated at  any
          level from C2 to B3 under a TCSEC evaluation.
                    The TCSEC rating system  assumes a measure of
          homogeneity  which  the  above  example  violates  thus
          invalidating the very basis upon which a single digraph
          may  be assigned.   Hence, a  subsetted system  such as
          described above,  will have to be  characterized with a
          more  complex   description  than  a   single  digraph.
          Although this  may seem undesirable, it will  be a more
          accurate  description of  the system,  and it  provides
          sufficient  information to  allow system  designers and
          accreditors  to  make  decisions  about  sufficiency of
          security for their  specific applications.  In essence,
          such  an  approach  is  necessary  for  recognizing the
          additional  complexity  which   can  be  introduced  by
          architectures  which   allow  system  elements   to  be
          developed separately.




          GLOSSARY

          candidate  TCB  subset     The  identification  of the
          hardware,  firmware,  and  software  that  make  up the
          proposed TCB  subset, along with the  identification of


          its  subjects and  objects; one  of the  conditions for
          evaluation by parts.

          content-dependent access  control   Access  control in
          which access is determined by  the value of the data to
          be accessed.

          context-dependent access  control   Access  control in
          which   access   is    determined   by   the   specific
          circumstances under which the data is being accessed.

          database management  system   A computer  system whose
          main function is to facilitate  the sharing of a common
          set of data among many  different users.  It may or may
          not  maintain  semantic  relationships  among  the data
          items.

          DBMS   Abbreviation for "database management system."

          depends   A TCB subset A depends (for its correctness)
          on  TCB  subset  B  if  and  only  if the (engineering)
          arguments  of  the  correct  implementation  of  A with
          respect to its specification assume, wholly or in part,
          that  the  specification  of  B  has  been  implemented
          correctly.

          domain    The set  of objects that  a subject has  the
          ability to access.

          dominated  by     Security  level  A  is  dominated by
          security level B if (1) the clearance/classification in
          A is less than or equal to the clearance/classification
          in  B,  and  (2)  the  set  of  access approvals (e.g.,
          compartment designators)  in A is contained  in the set
          of  access approvals in  B (i.e., each  access approval
          appearing  in A  also  appears  in B).   This dominance
          relation is a special case of a partial order.

          dominates   "Security level B dominates security level
          A" is synonymous with "security level A is dominated by
          security level B."  See "dominated by."


          global requirements   Those  which require analysis of
          the  entire system and  for which separate  analysis of
          the  individual  TCB  subsets  does  not  suffice.  See
          Section TC-5.3.2 for a summary list.

          lattice   A partially ordered set for which every pair
          of  elements has  a greatest  lower bound  and a  least
          upper bound.

          local requirements   Those for which separate analysis
          of  the individual  TCB subsets  suffices to  determine
          compliance for the composite TCB.  See Section TC-5.3.1
          for summary list.

          metadata    (1)  Data  referring  to other  data; data
          (such as  data structures, indices, and  pointers) that
          are  used  to  instantiate   an  abstraction  (such  as
          "process," "task," "segment,"  "file," or "pipe").  (2)
          A  special  database,  also   referred  to  as  a  data
          dictionary,  containing  descriptions  of  the elements
          (e.g., relations, domains,  entities, or relationships)
          of a database.

          monolithic TCB    A TCB that consists of  a single TCB
          subset.

          object    A passive  entity that contains  or receives
          information.  Access  to an object  potentially implies
          access  to the  information it  contains.  Examples  of
          objects are:  records,  blocks, pages, segments, files,
          directories, directory trees, and  programs, as well as
          bits, bytes, words, fields, processors, video displays,
          keyboards, clocks, printers, network nodes, etc.

          partial  order    A relation  that is  symmetric (a is
          related to a),  transitive (if a is related to  b and b
          is  related  to  c,  then  a  is  related  to  c),  and
          antisymmetric (if a is related to b and b is related to
          a, then a and b are identical.)

          primitive    An ordering relation between  TCB subsets
          based  on  dependency  (see  "depends"  above).   A TCB


          subset B is  more primitive than a second  TCB subset A
          (and  A is  less primitive  than B)  if (a)  A directly
          depends on B or (b) a chain  of TCB subsets from A to B
          exists  such that  each element  of the  chain directly
          depends on its successor in the chain.

          reference monitor concept    An access control concept
          that  refers to an  abstract machine that  mediates all
          accesses to objects by subjects.

          reference validation mechanism   "An implementation of
          the reference  monitor concept .  .   .  that validates
          each  reference  to  data   or  programs  by  any  user
          (program)  against  a  list   of  authorized  types  of
          reference  for that  user."  It  must be  tamper proof,
          must always be invoked, and  must be small enough to be
          subject  to  analysis  and  tests,  the completeness of
          which can be assured.  [1]

          security  policy     The   set  of  laws,  rules,  and
          practices  that regulate  how an  organization manages,
          protects, and distributes sensitive information.

          storage object   An object that supports both read and
          write accesses.

          subject   An active entity, generally in the form of a
          person, process,  or device that causes  information to
          flow  among  objects  or   changes  the  system  state.
          Technically, a process/domain pair.

          subset-domain      A  set  of  system   domains.   For
          evaluation  by parts,  each candidate  TCB subset  must
          occupy a distinct subset domain such that modify-access
          to  a domain  within  a  TCB subset's  subset-domain is
          permitted  only to  that TCB  subset and  (possibly) to
          more primitive TCB subsets.

          TCB subset   A set of software, firmware, and hardware
          (where  any  of  these  three  could  be  absent)  that
          mediates the access  of a set S of subjects  to a set O


          of  objects on  the basis  of a  stated access  control
          policy P and satisfies the properties:

                    (1) M  mediates every access to  objects O by
          subjects in S;

                    (2) M is tamper resistant; and

                    (3)  M  is  small  enough  to  be  subject to
          analysis  and tests, the  completeness of which  can be
          assured.

          technical policy   The  set of rules regulating access
          of subjects to objects enforced by a computer system.

          Trusted  Computing  Base  (TCB)      The  totality  of
          protection   mechanisms   within   a   computer  system
          including   hardware,   firmware,   and   software  the
          combination  of which  is responsible  for enforcing  a
          security  policy.   A  TCB  consists  of  one  or  more
          components  that together   enforce a  unified security
          policy over a product or  system.  The ability of a TCB
          to correctly  enforce a security policy  depends solely
          on  the mechanisms  within the  TCB and  on the correct
          input by system  administrative personnel of parameters
          (e.g.,  a  user's  clearance)  related  to the security
          policy.

          trusted subject   A subject  that is permitted to have
          simultaneous view- and alter-access  to objects of more
          than one sensitivity level.

          user     Any  person  who  interacts  directly  with a
          computer system.

          view   That portion of the database that satisfies the
          conditions specified in a query.

          view  definition    A stored  query; sometimes loosely
          referred to as a "view."


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