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NCSC-TG-009
Library No. S230,512
Version 1

FOREWORD

This publication is issued by the National Computer Security Center
(NCSC) as part of its program to promulgate technical computer security
guidelines. This interpretation extends the Department of Defense Trusted
Computer System Evaluation Criteria (DOD 5200.28-STD) to computer
security subsystems.

This document will be used for a period of at least one year after date of
signature. During this period the NCSC will gain experience using the
Computer Security Subsystem Interpretation in several subsystem
evaluations. After this trial period, necessary changes to the document will be
made and a revised version issued.

Anyone wishing more information, or wishing to provide comments on the
usefulness or correctness of the Computer Security Subsystem Interpretation
may contact: Chief Technical Guidelines Division, National Computer
Security Center, Fort George G. Meade, MD 20755-6000, ATTN: Cll.

~

PATRICK R GALLAGHER, JR. 16 September 1988
Director ~
National Computer Security Center


_

Computer Security Subsystems ACKNOWLEDGEMENT

ACKNOWLEDGEMENT

Acknowledgment is extended to the members of the working group who produced
this Interpretation. Members were: Michael W. Hale, National Computer
Security Center (Chair); James P. Anderson; Terry Mayfie!d, Institute For
Defense Analyses; Alfred W. Arsenault, NCSC; William Geer, NCSC; John C.
Inglis, NCSC; Dennis Steinauer, National Bureau of Standards; Mario Tinto,
NCSC; Grant Wagner, NCSC; and Chris Wilcox, NCSC.

Acknowledgement is further extended to those individuals who conducted
thorough reviews and and provided constructive comments on this document.
Reviewers included: Steve Lipner, Earl Boebert, Virgil Gligor, Debbie Downs,
Len Brown, Doug Hardie, Steve Covington, Jill Sole and Bob Morris.


CONTENTS

1. INTRODUCTION.. ........................................ 1
1.1 PURPOSE ........................................ 1
1.2 BACKGROUND ..................................... 1
1.3 SCOPE .......................................... 2
1.4 EVALUATION OF SUBSYSTEMS.............. . . . . . . . . . . 3
1.4.1 Basis for Evaluation . . . . . . . . . . . . . . 3
1.4.2 Integration Requirements............. 5
1.4.3 WARNING.............................. 7
2. FEATURE REQUIREMENTS . . . . . . . . . . . . . 8
2.1 DISCRETIONARY ACCESS CONTROL (DAC)
SUBSYSTEMS ........................................ 8
2.1.1 Global Description of Subsystem Features . . . . 8
2.1.1.1 Purpose . . . . . . . . . . . . . . . . . . 8
2.1.1.2 Role Within Complete Security System . . . 8
2.1.2 Evaluation of DAC Subsystems . . . . . . . . . . 9
2.1.3 Feature Requirements For DAC Subsystems. . . . . 9
2.1.3.1 DAC/Dl............................... 9
2.1.3.1.1 Identified users and objects. . . . 9
2.1.3.1.2 User-specified object sharing. . . .. 10
2.1.3.1.3 Mediation ................... . 10
2.1.3.2 DAC/D2............................... 10
2.1.3.2.1 Single-user access granularity. . . .. 11
2.1.3.2.2 Authorized user-specified object
sharing . . . . . . . . . . . . . . 11
2.1.3.2.3 Default protection....... 11
2.1.3.3 DAC/D3........................ 11
2.1.3.3.1 Access control lists for each
object .......................... 12
2.1.4 Assurance Requirements for DAC Subsystems. . . . 12
2.1.5 Documentation Requirements for DAC Subsystems. . 12
2.2 OBJECTREUSESUBSYSTEMS ........................... 14
2.2.1 Global Description of Subsystem Features.. 14
2.2.1.1 Purpose....................... 14
2.1.2 Role Within the Complete Security System 14
2.2.2 Evaluation of Object Reuse Subsystems 14
2.2.3 Feature Requirements for Object Reuse Subsystems 15
2.2.3.1 OR/D2 ...................................... 15

iii
~
2.2.4 Assurance Requirements for Object Reuse
Subsystems........................................16
2.2.5 Docurnentation Requirements for Object Reuse
Subsystems........................................16
2.3 IDENTIFICATION & AUTHENTICATION (I&A)
SUBSYSTEMS ........... .......................................17
2.3.1 Global Description of Subsystem Features . . . . . . . .17
2.3.1.1 Purpose ........................................17
2.3.1.2 Role Within Complete Security System . . . . . . 17
2.3.2 Evaluation of I&A Subsystems . . . . . . . . . . . . 17
2.3.3 Feature Requirements for I&A Subsystems . . . .. . . 18
2.3.3.1 I&A/Dl ..........................................18
2.3.3.2 I&A/D2 ..........................................19
2.3.4 Assurance Requirements for I&A Subsystems . . . . . . 20
2.3.5 Documentation Requirements for I&A Subsystems . . . . . 20
2.4 AUDITSUBSYSTEMS .............................................21
2.4.1 Global Description of Subsystem Features . . . . . . ..21
2.4.1.1 Purpose .........................................21
2.4.1.2 Role Within Complete Security System .. . . . . 21
2.4.2 Evaluation of Auditing Subsystems . . . . . . .. . . 21
2.4.3 Feature Requirements For Auditing Subsystems . . .. . .22
2.4.3.1 AUD/D2 ....... ..................................22
2.4.3.1.1 Creation and management of audit
trail.. .................................22
2.4.3.1.2 Protection of audit data . . . 23
2.4.3.1.3 Access control to audit . . . 23
2.4.3.1.4 Specific types of events . . . 23
2.4.3.1.5 Specific information per event . . 24
2.4.3.1.6 Ability to selectively audit
individuals............................. 24
2.4.3.2 AUD/D3 ............ ............................ 24
2.4.3.2.1 Real-timealarms ....................... 25
2.4.4 Assurance Requirements for Auditing Subsystems . . . . 25
2.4.5 Documentation Requirements for Auditing
Subsystenns .......................................... 25

3. ASSURANCE REQUIREMENTS . . . . . . . . . . . . 26
3.1 SUBSYSTEMARCHITECTURE .................................... 26
3.1.1 Arch:Dl .......... ................................... 26
3.1.1.1 Execution Domain Protection . . . . . . . . . 26
3.1.1.2 Defined Subsets ............................... 27
3.1.2 Arch:D2 ............................................. 27

-iv-

3.2 SUBSYSTEM INTEGRITY. . . . . . . . . . . . . . . . . . . . . 28
3.2.1 Integrity:Dl. . . ....................................28
3.2.2 Integrity:D2 . . . ....................................29
3.3 SECURITY TESTING . . . ....................................29
3.3.1 Test:Dl ...............................................29
3.3.2 Test:D2 ............................ ............ 30

4. DOCUMENTATION REQUIREMENTS ....... ............................31
4.1 SECURITY FEATURES USER'S GUIDE . . . . . . . . . . . 31
4.1.1 SFUG:Dl ...............................................31
4.1.2 SFUG:D2 ...............................................31
4.2 TRUSTED FACILITY MANUAL . . . . . . . . . . . . . . 32
4.2.1 TFM:Dl ................................................32
4.2.2 TFM.D2 ................................................32
4.3 TEST DOCUMENTATION .........................................33
4.3.1 TD:Dl .................................................33
4.3.2 TD:D2 .................................................33
4.4 DESIGN DOCUMENTATION . . . . . . . . . . . . .. 33
4.4.1 DD:Dl .................................................33
4.4.2 DD:D2. . . . ......... 34

5. GLOSSARY . . . . .............................................35

-v-

LIST OF TABLES

TABLE 1.1. Possible Subsystem Ratings . . . . . . . . . . . . . . .4
TABLE 1.2. Required Supporting Functions . . . . . . . . . . . . . 6

-vi-
Computer Security Subsystems INTRODUCTION

1. INTRODUCTION

This document provides interpretations of the Department of Defense Trusted
Computer System Evaluation Criteria (DoD 5200.28-STD or TCSEC) for computer
security subsystems. A computer security subsystem (subsystem) is defined,
herein, as hardware, firmware and/or software which can be added to a computer
system to enhance the security of the overall system. A subsystem's primary
utility is to increase the security of a computer system. The computer system
that the subsystem is to protect is referred to as the protected system in
this Interpretation.

When incorporated into a system environment, evaluated computer security
subsystems may be very effective in reducing or eliminating certain types of
vulnerabilities whenever entire evaluated systems are unavailable or
impractical.

1.1 PURPOSE

This Interpretation has been prepared for the following purposes:

1. to establish a standard for manufacturers as to what security features and
assurance levels to build into their new and planned computer security
subsystem products to provide widely available products that satisfy trust
requirements for sensitive applications;

2. to provide a metric to evaluate the degree of trust that can be placed in a
subsystem for protecting classified and sensitive information;

3. to lend consistency to evaluations of these products by explicitly stating
the implications that are in the TCSEC; and

4. to provide the security requirements for subsystems in acquisition
specifications.

1.2 BACKGROUND

The Department of Defense Trusted Computer System Evaluation Criteria (DoD
5200.28-STD or TCSEC) was developed to establish uniform DoD policy and
security requirements for "trusted, commercially available, automatic data
processing (ADP) systems." Evaluation criteria defined in the TCSEC provides a
standard to manufacturers as to what security features to build into their
commercial products to satisfy trust requirements for sensitive applications,
and serves as a metric with which to evaluate the degree of trust that can be
placed in a computer system for the secure processing of classified or other
sensitive information.

The TCSEC specifies a variety of features that a computer system must provide
to constitute a complete security system. The security requirements specified
in the TCSEC depend on and complement one another to provide the basis for
effective
1

Computer Security Subsystems INTRODUCTION

implementation of a security policy in a trusted computer system. The
effectiveness of any one security feature present within a system is,
therefore, dependent to some degree on the presence and effectiveness of other
security features found within the same system. Because it was intended to be
used only for systems which incorporated all the security features of a
particular evaluation class, the TCSEC does not, in all cases, completely
specify these interdependencies among security features.

In addition to the class of trusted system products, there exists a recognized
need for a class of computer security products which may not individually meet
all of the security features and assurances of the TCSEC. Instead, these
products may implement some subset of the features enumerated in the TCSEC and
can potentially improve the security posture in existing systems. These
products are collectively known as computer security subsystems.

Evaluation of computer security subsystems against a subset of the
requirements given in the TCSEC has proven an extremely difficult task because
of the implied dependencies among the various features discussed in the TCSEC.
As a consequence, interpretations of these interdependencies and the relative
merits of specific subsystem implementations have been highly subjective and
given to considerable variation.

This document provides interpretations of the TCSEC for computer security
subsystems in an effort to lend consistency to evaluations of these products by
explicitly stating the implications in the TCSEC.

Evaluations can be divided into two types: (l) a product evaluation can be
perforrned on a subsystem from a perspective that excludes the application
environment, or (2) a certification evaluation can be done to assess whether
appropriate security measures have been taken to permit an entire system to be
used operationally in a specific environment. The product evaluation type is
done by the National Computer Security Center (NCSC) through the Trusted
Product Evaluation Process using this interpretation for subsystems. The
certification type of evaluation lS done in support of a formal accreditation
for a system to operate in a specific environment using the TCSEC.

1.3 SCOPE

This document interprets the security feature, assurance and documentation
requirements of the TCSEC for subsystem evaluations. In this interpretation,
the

2


Computer Security Subsystems INTRODUCTION


functional requirements of the TCSEC are divided into four general categories:

1. Discretionary Access Control (DAC)

2. Object Reuse (OR).

3. Identification and Authentication (I&A)

4. Audit (AUD)

These categories form the basis for classifying products to be evaluated as
computer security subsystems.

The document, in addition to this introductory section, is organized into
three major sections and a glossary. Section 2 contains the feature
requirements for each of the above four categories on which subsystems
evaluations are based. The requirements in this section are listed in
increments, with only new or changed requirements being added for each
subsequent class of the same feature. All requirements that are quoted from
the TCSEC are in bold print for easy identification and are clarified, in the
context of subsystems, by interpretation paragraphs.

Section 3 contains the assurance requirements for all subsystems. The
assurances that are relevant to each category are listed here in the same
format as the requirements in Section 2. Section 4 contains the requirements
and interpretations for subsystem documentation, again, in the same forrnat as
Section 2.

The TCSEC-related feature and assurance requirements described herein are
intended for the evaluation of computer security subsystems designed to
protect sensitive information. This Interpretation, like the TCSEC, assumes
that physical, administrative, and procedural protection measures adequate to
protect the inforrnation being handled are already in place.

This Interpretation can be used to support a certification evaluation. In
fact, it would be helpful whenever subsystems are a part of the overall system
being certified.

1.4 EVALUATION OF SUBSYSTEMS

1.4.1 Basis for Evaluation

Subsystems are evaluated for the specific security-relevant functions they
perforrn. This Interpretation interprets the relevant TCSEC requirements for
each function evaluated. So the function(s) for which subsystems are
evaluated will be identified within its ratings. Each function has its own
set of ratings as identified in Table 1.1. Subsystems that are evaluated for
more than one function will receive a separate


3

Computer Security Subsystems INTRODUCTION

rating for each function evaluated.

TABLE 1.1. Possible Subsystem Ratings

SUBSYSTEM FUNCTION POSSIBLE RATINGS

_ . _

Discretionary Access Control DAC/D
DAC/Dl
DAC/D2
DAC/D3

Object Reuse OR/D
OR/D2

Identification & Authentication I&A/D
I&A/Dl
I&A/D2

Audit AUD/D
AUD/D2
AUD/D3

Although the requirements for subsystems are derived from the TCSEC, the
ratings for subsystems will not directly reflect the TCSEC class they are
derived from. Since subsystems, by their very nature, do not meet all of the
requirements for a class Cl or higher computer system, it is most appropriate
to associate subsystem ratings with the D division of the TCSEC. This
Interpretation defines the Dl, D2 and D3 classes within the D division for
subsystems. The Dl class is assigned to subsystems that meet the
interpretations for requirements drawn from the Cl TCSEC class. Likewise, the
D2 class consists of requirements and interpretations that are drawn from the
C2 TCSEC class. The D3 subsystem class is reserved for DAC subsystems and
audit subsystems that meet the B3 functionality requirements for those
functions.

In addition to meeting the functionality requirements and interpretations,
subsystems must also meet the assurance and documentation requirements in
sections 3 and 4 of this document. The Dl and D2 classes have requirements
and interpretations for ~ssurances and documentation as well as functionality.
The D3

4


Computer Security Subsystems INTRODUCTION

class contains additional requirements and interpretations only for
functionality, not for assurances or documentation. So, subsystems with this
rating will adhere to the D2 assurance and documentation requirements and
interpretations.

Like the classes within the TCSEC, the Dl, D2 and D3 classes are ordered
hierarchically. Subsystems being evaluated for the Dl class must meet the
requirements and interpretations for the Dl class. Subsystems being evaluated
for the D2 class must meet the requirements and interpretations for the Dl
class plus the additional requirements and interpretations for the D2 class.
Subsystems being evaluated for the D3 class must meet the additional
requirements and interpretations associated with the functionality at D3.

Although the subsystem requirements and interpretations are derived
directly from the TCSEC, subsystems are not considered to be complete computer
security solutions. There is no general algorithm to derive a system rating
from an arbitrary collection of computer security subsystems. Any collection
of individually evaluated subsystems must be evaluated as a whole to determine
the rating of the resulting system. The ratings of the individual subsystems
in a complete system are not a factor in the rating of that system.

1.4.2 Integration Requirements

Because all of the TCSEC requirements for a given rating class were
intended to be implemented in a complete computer security system, many of the
security features are dependent upon each other for support within the system.
This poses a certain degree of difficulty with extracting only the relevant
requirements from the TCSEC for a given feature. Further, this poses a
fundamental problem for subsystems because there is an explicit dependency
between security features that restricts the "independent" incorporation of
subsystems into the system's environment. The problem has been handled in
this Interpretation by discussing the integration requirements for each type
of subsystem. The requirements for integration are discussed for each type of
subsystem in a sub-section entitled, "Role Within Complete Security System."
Furthermore, explicit requirements for integration are stated in the
interpretations at appropriate points. The developer must show, and the
evaluation shall validate, that the subsystem can be integrated into a system
to fulfill its designated role.

Most all computer security subsystems will rely on other security-relevant
functions in the enviromnent where they are implemented. Audit subsystems,
for example, depend on an identification and authentication function to
provide the unique user identities that are necessary for individual
accountability. Also, it is important to realize that some of these functions
may be dependent on each other in a cyclic fashion (e.g., I&A depends on DAC
and DAC depends on I&A). In these

5

Computer Security Subsystems INTRODUCTION

cases, the cyclic dependencies should be removed either by complete
integration of the functions or by modularizing the functions in a way that
allows linear dependencies. Tl~is latter method is termed "sandwiching" and
it requires the splitting of one function and surroundmg the other dependent
function with the two functions resulting from the split. For example, in the
case of DAC and I&A cyclic dependencies, one might split I&A into two parts so
that there is a system I&A, a DAC subsystem, and a DAC module containing its
own I&A functionality.

With the exception of object reuse, all functions implemented by subsystems
will be dependent on other functions as shown in Table 1.2. The functions
upon which any subsystem is dependent will be referred to as that subsystem's
required supporting functions. These required supporting functions must be
present in the subsystem's environment for the effective integration of the
subsystem.

TABLE 1.2. Required Supporting Functions

SUBSYSTEM FUNCTION REQUIRED SUPPORTING FUNCTIONS

~

Discretionary Access Control I&A
Audit

Object Reuse None

Identification & Authentication Audit
DAC2

Audit I&A
DAC2

Subsystems that are not self-sufficient in providing required supporting
functions must, at a minimum, provide an interface to their required
supporting functions. The

---------------------------------------------------
1 The audit supporting functions are required at D2.
2 Audit and/or authentication data must be protected through domain isolation
or DAC.

6


Computer Security Subsystems INTRODUCTION

evaluation team will perform tests to show whether the interface to the
required supporting functions is reliable and works properly. The robustness
of the required supporting functions on the other side of the interface will
not be tested, as the scope of the subsystem evaluation is bounded by the
interface.

A more integrated solution is for subsystems to be self- su~cient in
providing all of their required supporting functions. Such subsystems w_ill
be evaluated and assigned a separate rating for each function they provide.
Unlike the previous solution, where only an interface is provided, each
required supporting function is performed by the subsystem and must be a part
of the subsystem evaluation.

1.4.3 WARNING

An overan system rating, such as that provided by the TCSEC, cannot be
inferred from the application of one or more separately-rated subsystems.
Mechanisms, interfaces, and the extent of required supporting functions for
each subsystem may differ substantiany and may introduce significant
vulnerabilities that are not present in systems where security features are
designed with fun knowledge of interfaces and host system support. Therefore,
incorporation of an evaluated subsystem into any system environment does not
automaticany confer any rating to the resulting system.

7

Computer Security Subsystems FEATURE REQUIREMENTS

2. FEATURE REQUIREMENTS

2.1 DISCRETIONARY ACCESS CONTROL (DAC) SUBSYSTEMS

2.1.1 Global Description of Subsystem Features

2.1.1.1 Purpose

This subsystem provides user-specified, controlled sharing of resources.
This control is established from security policies which define, given
identified subjects and objects, the set of rules that are used by the system
to determine whether a given subject is authorized to gain access to a
specific object.

DAC features include the means for restricting access to objects; the means
for instantiating authorizations for objects; and the mechanisms for
distribution, review, and revocation of access privileges, especially during
object creation and deletion.

2.1.1.2 Role Within Complete Security System

The requirement is to give individual users the ability to restrict access
to objects created or controlled by them. Thus, given identified subjects and
objects, DAC includes the set of rules (group-oriented and/or
individually-oriented) used by the subsystem to ensure that only specified
users or groups of users may obtain access to data (e.g., based on a need-to-
know).

A DAC subsystem controls access to resowces. As such, it shall be
integrable with the operating system of the protected system and shall mediate
all accesses to the protected resources. To fully protect itself and the
resources it controls, the DAC subsystem must be interfaced to the protected
system in such a way that it is tamperproof and always invoked.

DAC subsystems use the identifiers of both subjects and DAC-controlled
objects as a basis for access control decisions. Thus, they must be supplied
with the identifiers in a reliable manner. The DAC subsystem may supply
subject identification for itself or it may rely on an I&A mechanism in the
protected system or in another subsystem. It is also essential that DAC
subsystems be implemented in an environment where the objects it protects are
well defined and uniquely identified.

At the DAC/D2 class, the DAC subsystem must interface with an auditing
mechanism. This auditing mechanism can be included within the DAC subsystem,
or it may reside elsewhere in the subsystem's environment.

8


Computer Security Subsystems FEATURE REQUIREMENTS

2.1.2 Evaluation of DAC Subsystems

Subsystems which are designed to implement discretionary access controls to
assist a host in controlling the sharing of a collection of objects must
comply with all of the TCSEC requirements as outlined below for features,
assurances and documentation. Compliance with these requirements will assure
that the subsystem can enforce a specifically defined group-oriented and/or
individually-oriented discretionary access control policy.

As a part of the evaluation, the subsystem vendor shall set up the
subsystem in a typical functional configuration for security testing. This
will show that the subsystem interfaces correctly with the protected system to
meet all of the feature requirements in this section and ali of the assurance
and documentation requirements in Sections 3 and 4. It will also show that
the subsystem can be integrated into a larger system environment.

The interpretations for applying the feature requirements to DAC subsystems
are explained in the subsequent interpretations sections. The application of
the assurances requirements and documentation requirements is explained in
Sections 3 and 4, respectively.

2.1.3 Feature Requirements For DAC Subsystems

2.1.3.1 DAC/Dl

- TCSEC Quote:

"Cl: New: The TCB shall define and control access between named users and named
objects (e.g., files and programs) in the ADP system. The enforcement
mechanism (e.g., self/group/public controls, access control lists) shall allow
users to special and control sharing of those objects by named indinduals or
defined groups or both."

- Interpretation:

In the TCSEC quote, "TCB" is interpreted to mean "DAC subsystem".

2.1.3.1.1 Identified users and objects ~.

DAC subsystems must use some mechanism to determine whether users are
authorized for each access attempted. At DAC/Dl, this mechanism must control
access by groups of users. The mechanisms that can meet this requirement
include,


9

Computer Security Subsystems FEATURE REQUIREMENTS

but are not limited to: access control lists, capabilities, descriptors, user
profiles, and protection bits. The DAC mechanism uses the identification of
subjects and objects to perform access control decisions. This implies that
the DAC subsystem must interface with or provide some I&A mechanism. The
evaluation shall show that user identities are available to DAC.

2.1.3.1.2 User-specified object sharing

The DAC subsystem must provide the capability for users to specify how
other users or groups may access the objects they control. This requires that
the user have a means to specify the set of authorizations (e.g., access
control list) of all users or groups permitted to access an object and/or the
set of all objects accessible to a user or group (e.g., capabilities).

2.1.3.1.3 Mediation

The checking of the specified authorizations of a user prior to granting
access to an object is the essential function of DAC which must be provided.
Mediation either allows or disallows the access.

2.1.3.2 DAC/D2

- TCSEC Quote:

"C2: Change: The enforcement mechanism (e.g. self/group/public controls,
access control lists) shall allow users to specify and control sharing of
those objects by named individuals, or defined groups of individuals, or by
both, and shan provide controls to limit propagation of access rights."

"C2: Add: The discretionary access control mechamsm shan, either by explicit
user action or by default, provide that objects are protected from
unauthorized access. These access controls s~ll be capable of including or
excluding access to the granularity of a single wer. Access permission to an
object by users not already possessing access pernlission shan only be
assigned by authorized users."

- Interpretation:

The following interpretations, in addition to the interpretations for the
DAC/Dl Class, shall be satisfied at the DAC/D2 Class.

10

Computer Security Subsystems FEATURE REQUIREMENTS

2.1.3.2.1 Single-user access granularity

The DAC/D2 class requires mdividual access controls; therefore, the
granularity of user identification must enable the capabili~ to discern an
individual user. That is, access control based upon group identi~ alone is
insufflcient. To comply with the requirement, the DAC subsystem must either
provide unique user identities through its own I&A mechanism or Mterface with
an I&A mechanism that provides unique user identities. The DAC subsystem must
be able to interface to an auditing mechanism that records data about access
mediation events. The evaluation shall show that audit data is created and is
available to the auditing mechanism.

2.1.3.2.2 Authorized user-specined object sharing

The ability to propagate access rights to objects must be lirnited to
authorized users. This additional feature is incorporated to limit access
rights propagation. This distribution of privileges encompasses granting,
reviewing, and revoking of access. The ability to grant the right to grant
propagation of access will itself be limited to authorized users.

2.1.3.2.3 Default protection

The DAC mechanism must deny all users access to objects when no explicit
action has been taken by the authorized user to allow access.

2.1.3.3 DAC/D3

- TCSEC Quote:

"B3: Change: The enforcement mechanism (e.g., access control lists) shall
allow users to specify and control sharing of those objects, and shall provide
controls to limit propagation of access rights. These access controls shall
be capable of specifying, for each named object, a list of named individuals
and a list of groups of named individuals with their respective modes of
access to that object."

"Add: Furtherrnore, for each such named object, it shall be possible to
specify a list of named individuals and a list of groups of named individuals
for which no access to the object is to be given."

- Interpretation:

The following interpretation, in addition to the interpretations and
requirements for

11


Computer Security Subsystems FEATURE REQUIREMENTS

the DAC/D2 class, shall be satisfied for the DACID3 class.


2.1.3.3.1 Access control lists for each object

The DAC subsystem shan anow users to specify the list of individuals or
groups of individuals who can access each object. The list shan additionally
specify the mode(s) of access that is anowed each user or group. This implies
that access control lists associated with each object is the only acceptable
mechanism to satisfy the DAC/D3 requirement.

2.1.4 Assurance Requirements for DAC Subsystems

DAC subsystems must comply with an of the assurance requirements for their
given class as indicated below. The interpretations for these assurance
requirements are contained in Section 3.

Subsystems at the DAC/Dl class must comply with:

- System Architecture (Dl)
- System Integrity (Dl)
- Security Testing (Dl)

Subsystems at the DAC/D2 and DAC/D3 classes must comply with:

- System Architecture (D2)
- System Integrity (D2)
- Security Testing (D2)

2.1.5 Documentation Requirements for DAC Subsystems

DAC subsystems must meet the documentation requirements listed below for
their target rating class. The interpretations for these documentation
requirements are contained in Section 4.

Subsystems at the DAC/Dl class must comply with:

- Security Features User's Guide (Dl)
- Trusted Facility Manual (Dl)
- Test Documentation (Dl)
- Desi~ Documentation (Dl)

12


Computer Security Subsystems FEATURE REQUIREMENTS

Subsystems at the DAC/D2 and DAC/D3 classes must comply with:

- Security Features User's Guide (D2)
- Trusted Facility Manual (D2)
- Test Documentation (D2)
- Design Documentation (D2)


13


Computer Security Subsystems FEATURE REQUIREMENTS

2.2 OBJECT REUSE SUBSYSTEMS

2.2.1 Global Description of Subsystem Features

2.2.1.1 Purpose

Object reuse subsystems clear storage objects to prevent subjects from
scavenging data from storage objects which have been previously used.

2.2.1.2 Role Within the Complete Security System

Object reuse can be used to prevent information scavenging by erasing
information residue contained in previously used storage objects that have
been released by the storage management system. Object reuse subsystems are
most effective in environments where some security policy is implemented on
the system.

To prevent scavenging of information from previously used storage objects,
object reuse subsystems must be fully integrable with the operating system of
the protected system. The object reuse subsystem must perform its function
for all reusable storage objects on the protected system (i.e., main memory,
disk storage, tape storage, I/O buffers, etc.).

Object reuse subsystems must be interfaced with the protected system in
such a way that they are tamperproof and always invoked.

2.2.2 Evaluation of Object Reuse Subsystems

Subsystems which implement object reuse must comply with all of the TCSEC
requirements as outlined below for features, assurances, and documentation.
Compliance with these requirements will show that the subsystem can enforce
object reuse adequately to receive an OR/D2 rating for object reuse.

As a part of the evaluation, the subsystem vendor shall set up the
subsystem in a typical functional connguration for security testing. This
will show that the subsystem interfaces correctly with the protected system to
meet all of the feature requirements in this section and all of the assurance
and documentation requirements in Sections 3 and 4. It will also show that
the subsystem can be integrated into a larger system environment.

The interpretations for applying the feature requirements of object reuse
subsystems are explained in the subsequent interpretations section. The
application of the assurance requirements listed below is explained in
Sections 3 and 4,


14

Computer Security Subsystems FEATURE REQUIREMENTS

respectively.

2.2.3 Feature Requirements for Object Reuse Subsystems

2.2.3.1 OR/D2

- TCSEC Quote:

"C2: New: all authorizations to the information contained within a storage
object shall be revoked prior to initial assignment, allocation or
reallocation to a subject from the TCB's pool of unused storage objects. No
information, including encrypted representations of information, produced by a
prior subject's actions is to be available to any subject that obtains access
to an object that has been released back to the system."

- Interpretation:

In the TCSEC quote, "TCB" is interpreted to mean "protected system".
Otherwise, this requirement applies as stated. The object reuse subsystem
shall perform its function for all storage objects on the protected system
that are accessible to users.

- Rationale/Discussion:

Object reuse subsystems must assure that no previously used storage objects
(e.g., message buffers, page frames, disk sectors, magnetic tape, memory
registers, etc.) can be used to scavenge residual information. Information
remaining in previously used storage objects can be destroyed by overwriting
it with meaningless or unintelligible bit patterns. An alternative way of
approaching the problem is to deny read access to previously used storage
objects until the user who has just acquired them has overwritten them with
his own data.

Object reuse subsystems do not equate to systems used to eliminate magnetic
remnance.


15

Computer Security Subsystems FEATURE REQUIREMENTS

2.2.4 Assurance Requirements for Object Reuse Subsystems

Object reuse subsystems must comply with all of the assurance requirements
shown below for the D2 class. The interpretations for these assurance
requirements for Object Reuse subsystems are contained in Section 3.

- System Architecture (D2)
- System Integrity (D2)
- Security Testing (D2)

2.2.5 Documentation Requirements for Object Reuse Subsystems

Object reuse subsystems must meet the documentation requirements shown
below for the D2 class. The interpretations for these documentation
requirements are contained in Section 4.

- Security Features User's Guide (D2)
- Trusted Facility Manual (D2)
- Test Documentation (D2) ;
- Design Documentation (D2)

16


Computer Security Subsystems FEATURE REQUIREMENTS

2.3 IDENTICATION & AUTHENTICATION (I&A) SUBSYSTEMS


2.3.1 Global Description of Subsystem Features


2.3.1.1 Purpose

This subsystem provides the authenticated identification of a user seeking
to gain access to any resources under the control of the protected system.

2.3.1.2 Role Within Complete Security System

The I&A subsystem provides an authenticated user identification needed to
provide accountability for and control access to the protected system. The
granularity of user identification is determined by the requirements in this
interpretation. The granularity increases from group identification at I&A/Dl
to individual identification at I&A/D2.

The requirement is to be able to accurately authenticate the claimed
identity of a user. The I&A subsystem must determine whether a user is
authorized to use the protected system. For all authorized users, the I&A
subsystem communicates the identity of the user to the protected system. This
identity can then be used by the protected system or other subsystems to
provide accountability for use of the system and access controls to protected
objects on the system. To be effective and to protect the authentication data
it uses, the I&A subsystem must be tamperproof and always invoked.

At I&A/D2, it is important that all uses of the I&A subsystem be recorded in
an audit trail. The auditing of these actions may be performed entirely by
the auditing mechanism on the I&A subsystem, or through an interface with an
auditing mechanism in the protected system or another subsystem.

2.3.2 Evaluation of I&A Subsystems

Subsystems which are designed to implement I&A must comply with all of the
TCSEC requirements outlined below for features, assurances, and documentation.
Compliance with these requirements will assure that the subsystem can enforce,
either wholly or in part, a specific I&A policy.

As a part of the evaluation, the subsystem vendor shall set up the subsystem
in a typical functional configuration for security testing. This will show
that the subsystem interfaces correctly with the protected system to meet all
of the feature requirements

17

Computer Security Subsystems FEATURE REQUIREMENTS

in this section and all of the assurance and documentation requirements in
Sections 3 and 4. It will also show that the subsystem can be integrated into
a larger system environment.

The interetations for applying the feature requirements to I&A subsystems
are explained in the subsequent interpretations sections. The application of
the assurance requirements and documentation requirements listed in the next
section is explained in Sections 3 and 4, respectively.

2.3.3 Feature Requirement for I&A Subsystems

2.3.3.1 I&A/Dl

- TCSEC Quote:

"Cl: New: The TCB shall require users to identify themselves to it before
beginning to perform any other actions that the TCB is expected to mediate.
Furthermore, the - TCB shall use a protected mechanism (e.g., passwords) to
authenticate the user's identity. The TCB shall protect authentication data
so that it cannot be accessed by any unauthorized user."

- Interpretation:

The I&A subsystem shall require users to identify themselves to it before
beginning to perforrn any other actions that the system is expected to
mediate. Furthermore, the I&A subsystem shall use a protected mechanism
(e.g., passwords) to authenticate the user's identity. The I&A subsystem
shall protect authentication data so that it cannot be accessed by any
unauthorized user.

The I&A subsystem shall, at a minimum, identify and authenticate system
users. At I&A/Dl, users need not be individually identified.

- Rationale/Discussion:

Identification and Authentication must be based on at least a two-step
process, which is derived from a combination of something the user possesses
(e.g., smart card, magnetic stripe card), some physical attribute about the
user (e.g., fingerprint, voiceprint), something the user knows (e.g.,
password, passphrase). The claimed identification of a user must be
authenticated by an explicit action of the user. It is not acceptable for one
step to be used as both identification and authentication. The claimed
identity can be public. The measure used for authentication must be resistant

18


Computer Secunty Subsystems FEATURE REQUIREMENTS

to forging, guessing, and fabricating.

The I&A subsystem must interface to the protected system in such a way that
it can reliably pass authenticated user identities to the protected system.
The evaluation shall show that authenticated user identities can be passed to
the protected system.

2.3.3.2 I&A/D2

- TCSEC Quote: -

"C2: Add: The TCB shan be able to enforce individual accountability by
providing the capability to uniqueb identify each individual ADP system user.
The TCB shall also ; provide the capabmty of associa~ ~is identity ~nth an
auditable actiol~ taken by ; that indindual."

- Interpretation ~

The following interpretations, in addition to those interpretations for
I&A/Dl, shall be satisfied at the I&A/D2 Class.

In the TCSEC quote, "TCB" is interpreted to mean "I&A subsystem." The I&A
subsystem shall pass to the protected system a unique identifier for each
individual.

The I&A subsystem shall be able to uniquely identify each individual user.
This includes the ability to identify individual members within an
authorized user group and the ability to identify specific system users such
as operators, system administrators, etc.

The I&A subsystem shall provide for the audit logging of security-relevant
I&A events. For I&A, the origin of the request (e.g., terminal ID, etc.), the
date and time of the event, user ID (to the extent recorded), type of
event, and the success or failure of the event shall be recorded. The I&A
subsystem may meet this requirement either through its own auditing
mechanism or by providing an interface for passing the necessary data to
another auditing mechanism. ,

- Rationale/Discussion:

The intent of this requirement is for the I&A subsystem to supply a unique
identity for each user to the protected system. The subsystem supplies a
unique user identity which may or may not be used by an auditing
mechanism. This auditing support is : required to maintain consistency with
the C2 level of trust as defined by the TCSEC.


19

Computer Security Subsystems FEATURE REQUIREMENTS

2.3.4 Assurance Requirements for I&A Subsystems

I&A subsystems must comply with all of the assurance requirements listed
below for their given class. The interpretations for these assurance
requirements to I&A subsystems are contained in Section 3.

Subsystems at the I&A/Dl class shall comply with:

- System Architecture (Dl)
- System Integrity (Dl)
-Security Testing (Dl) .

Subsystems at the I&A/D2 class shall comply with:

- System Architecture (D2)
- System Integrity (D2)
- Security Testing(D2)

2.3.5 Documentation Requirements for I&A Subsystems

I&A subsystems must meet the documentation requirements listed below for
their target rating class. The interpretations for these documentation
requirements are contained in Section 4.

Subsystems at the I&A/Dl class shall comply with:
- Security Features User's Guide (Dl)
- Trusted Facility Manual (Dl)
- Test Documentation (Dl)
- Design Documentation (Dl)

Subsystems at the I&A/D2 class shall comply with:

- Security Features User's Guide (D2)
- Trusted Facility Manual (D2)
- Test Documentation (D2)
- Design Documentation (D2)

20

Computer Security Subsystems FEATURE REQUIREMENTS

2.4 AUDlT SUBSYSTEMS

2.4.1 Global Description of Subsystem Features

2.4.1.1 Purpose

Accountability is partly achieved through auditing. That is, data from
security- relevant events is captured and passed to the audit mechanism to be
recorded for use in detecting possible security breaches and providing a trace
to the party responsible.

2.4.1.2 Role Within Complete Security System

The requirement is to be able to record security-relevant events in a
manner that will allow detection and/or after-the-fact investigations to trace
security violations to the responsible party.

An auditing subsystem must be capable of recording all security-relevant
actions -i - that take place throughout the computer system. To accomplish
this goal, it must integrate itself into the mechanisms that mediate access
and perform user identification and authentication, and capture data about the
events they control. Additionally, an audit subsystem must be interfaced with
the protected system in such a way that it is tamperproof and always invoked.

The auditing subsystem must be provided all of the necessary data
associated with actions as specified in Section 2.4.3. The necessary data
includes the unique identity of the user that is responsible for each action.
This implies that an auditing subsystem must be augmented by an identification
and authentication mechanism either within the subsystem itself or elsewhere
on the system.

2.4.2 Evaluation of Auditing Subsystems

Subsystems which are designed to implement audit data collection and
control functions for a host must comply with all of the TCSEC requirements as
outlined below for features, assurances and documentatioi. Compliance with
these features will assure that the subsystem, through its integration, can
detect or generate the relevant audit data or can record all relevant audit
data passed to it by the host or other subsystems.

As a part of the evaluation, the subsystem vendor shall set up the
subsystem in a typical functional configuration for security testing. This
will show that the subsystem interfaces correctly with the protected system to
meet all of the feature requirements

21

Computer Security Subsystems FEATURE REQUIREMENTS

in this section and all of the assurance and documentation requirements in
Sections 3 and 4. It will also show that the subsystem can be integrated
into a larger system environrnent.

The interpretations for applying the feature requirements to auditing
subsystems are explained in the subsequent interpretations sections. The
application of the assurance requirements and documentation requirements is
explained in Sections 3 and 4, respectively.

2.4.3 Feature Requirements For Auditing Subsystems


2.4.3.1 AUD/D2

- TCSEC Quote:

"C2: New: The TCB shan be able to create, maintain, and protect from
modification or unauthorized access or destruction an audit trail of accesses
to the objects it protects. The audit data shan be protected by the TCB so
that read access to it is limited to those who are authorized for audit data.
The TCB shall be able to record the following types of events: use of
identification and authentication mechanisms introduction of objects into a
user's address space (e.g., file open, program ~. initiation), deletion of
objects, actions taken by computer operators and system administrators and/or
system security officers, and other security relevant events. For each
recorded event, the audit record shall identify: date and time of the event, ~
user, type of event, and success or failure of the event. For
identincation/authentication events the origin of request (e.g., terminal ID)
shan be - included in the audit record. For events that introduce an object
into a user's address space and for object deletion events the audit record
shall include the name of the object. rne ADP system administrator shall be
able to selectively audit the actions of any one or more users based on
individual identity."

- Interpretations:

The following subsections provide interpretations of the TCSEC requirements
which shall be satisfied by auditing subsystems at AUD/D2.
~

2.4.3.1.1 Creation and management of audit trail

The auditing subsystem shall create and manage the audit trail of
security-relevant " events in the system. If the other portions of the system
are unable to capture data


22


Computer Security Subsystems FEATURE REQUIREMENT

about such events, the auditiug subsystem shaU coutain the necessary
interfaces into the system to perform this function. Alternatively, the
auditing subsystem might simply accept and store data about events if the
other portions of the system are capable of creating such data and passing
them on.

- Rationale/Discussion:

To meet this requirement, it is sufficient that the audit subsystem
provides a set of calls which permit the system to supply the needed data as
parameters that the audit subsystem puts into a data structure and routes to
audit storage (or transmits securely to an audit logger).


2.4.3.1.2 Protection of audit data

It shall be demonstrated that the audit data is protected from unauthorized
modification. This protection will be provided either by the subsystem itself
or by its integration with the protected system.

- Rationale/Discussion:

The auditing subsystem might store the audit data in a dedicated data
storage area that cannot be accessed by any subject on the system except the
auditing subsystem itself and the system security officer (or system
administrator through the auditing subsystem. Or, if the protected system
has adequate access control facilities, the audit data might be stored on the
protected system, using its access control mechanisms for protection.

2.4.3.1.3 Access control to audit

The audit mechanism, auditing parameters, and the audit data storage media
shall be protected to ensure access is allowed only to authorized individuals.
Individuals who are authorized to access the audit data shall be able to
gain access only through the auditing subsystem.

- Rationale/Discussion:

This interpretation assumes that discretionary access controls or physical
controls will be in place to keep unauthorized individuals from gaining access
to the audit data.

2.4.3.1.4 Specific types of events
~
Data about all security relevant events must be recorded. The other
portions of the system shall be able to pass data concerning these events to
the auditing

23

Computer Security Subsystems FEATURE REQUIREMENTS

subsystem, or the auditing subsystem shall have the necessary code integrated
into the other portions of the system to pass the data to the collection
point.

2.4.3.1.5 Specific infolmation per event

All of the specific information enumerated in the TCSEC quote shall be
captured for each recorded event. Of particular concern, is the recording of
the user identity with each recorded event.

-Rationale/Discussion:

This implies that the audit subsystem must be able to acquire user
identities from an I&A mechanism, which may be provided on the audit subsystem
itself, on the protected system, or in a separate I&A subsystem. Whichever is
the case, the evaluation shall show that the audit subsystem has a working
interface to an I&A mechanism.

2.4.3.1.6 Ability to selectively audit individuals

The auditing subsystem shall have the ability to perform selection of audit
data based on individual users.

- Rationale/Discussion:

This requirement can be satisfied by pre-selection of the information to be
recorded in the audit log (selective logging) and/or by post-selection of
information to be extracted from the audit log (selective reduction). The
reduction of the audit log must be able to show all of the security-relevant
actions performed by any specified individual. The intent of selective
logging is to reduce the volume of audit data to be recorded by only recording
audit data for those specific individuals that the systcm security officer (or
system administrator) specifies. The intent of selective reduction is to
reduce the large volume of audit data into a collection of intelligible
information which can be more efficiently used by the system administrator.

2.4.3.2 AUD/D3

-TCSEC Quote:

"B3: Add: The TCB shal~ contain a mechanism that is able to monitor the
occurrence or accumulation of security auditable events that may indicate an
imminent violation of security policy. This mechanism shall be able to
immediately notify the security administrator when thresholds are exceeded
and, if the occurrence or accumulation

24

Computer Security Subsystems FEATURE REQUIREMENTS

of these securib relevant events continues, the system shall take the least
disruptive action to terminate the event."

- Interpretation: The following interpretation, in addition to the
interpretation and requirement for AUD/D2, shall be satisfied for the AUD/D3
class.


2.4.3.2.1 Real-time alarms

The auditing subsystem shall provide the capability for the security
administrator to set thresholds for certain auditable events. Furthermore,
when the thresholds are exceeded, the audit subsystem shall immediately
notify the security administrator of an imminent security violation.

2.4.4 Assurance Requirements for Auditing Subsystems

Audit subsystems, whether being evaluated at AUD/D2 or AUD/D3, must comply
with the assurance requirements listed below for the D2 class. The
interpretations for these assurance requirements are contained in Section 3.


- System Architecture (D2)
- System Integrity (D2)
- Security Testing (D2)


2.4.5 Documentation Requirements for Auditing Subsystems "

Audit subsystems, whether being evaluated at AUD/D2 or AUD/D3, must meet
the documentation requirements listed below for the D2 class. The
interpretations for these documentation requirements are contained in Section
4.


- Security Features User's Guide (D2)
- Trusted Facility Manual (D2)
- Test Documentation (D2)
- Design Documentation (D2)

25

Computer Security Subsystems ASSURANCE REQUIREMENTS


3. ASSURANCE REQUIREMENTS


Rated subsystems must provide correct and accurate operations. Assurance
must be provided that correct implementation and operation of the subsystem's
function exist throughout the subsystem's life cycle. The objective in
applying these assurance requirements is to develop confidence that the
subsystem has been implemented correctly and that it is protected from
tampering and circumvention.

The requirement is that the subsystem must contain hardware/software
mechanisms that can be independently evaluated through a combination of
inspection and testing to provide sufficient assurance that the subsystem
features enforce or support the functions for which the subsystem is intended.
To receive a rating, a subsystem must meet the assurance requirements at the
same level of trust as it has I met the requirements for functionality. The
assurances must be applied to the different types of subsystems as described
in the previous sections.

3.1 SUBSYSTEM ARCHITECTURE

Subsystem architecture evaluation is designed to provide operational
assurances with regard to the design and implementation of the protection
mechanisms of the subsystem and its interfaces to the host/host TCB.

3.1.1 Arch:D1


-TCSEC Quote:

"Cl: New: The TCB shall maintain a domain for its own execution that protects
it from external interference or tampering (e.g., by modification of its code
or data structures). Resources controned by the TCB may be a defined subset
of the subjects and objects in the ADP system."

- Interpretation:

This requirement applies to all subsystems evaluated at all classes,
regardless of the function(s) they perform. There are two specific elements
of this requirement: Execution Domain Protection and Defined Subsets.


3.1.1.1 Execution Domain Protection

Protection of the subsystem's mechanism and data from external
interference or tampering must be provided. The code and data of the
subsystem may be protected' through physical protection (e.g., by the
subsystem's dedicated hardware base) or by


26

Computer Security Subsystems ASSURANCE REQUIREMENTS

logical isolation (e.g., using the protected system's domain mechanism).

- Rationale and Discussion:

The subsystem may be contained entirely on its own hardware base which must
protect the operational elements of the mechanisms. Alternatively, all or a
portion of the subsystem may be implemented on the hardware of the host, in
which case the host system's architecture must protect this portion from
external interference or tampering.

3.1.1.2 Defined Subsets

I&A subsystems, when used for the system's I&A, define the subset of
subjects under the control of the system's TCB.

DAC subsystems may protect a subset of the total collection of objects on
the protected system.

3.1.2 Arch:D2

- TCSEC Quotes:

"C2: Add: The TCB shall isolate the resources to be protected so that they
are subject to the access control and auditing requirements."


- Interpretation:

In the TCSEC quote, "TCB" is interpreted to mean "subsystem".

This requirement applies to all subsystems evaluated at the D2 class or the
D3 class. The following interpretations explain how this requirement applies
to specific functions performed by subsystems.

- Interpretation for DAC Subsystems:

All named objects which are in the defined subset of protected objects
shall be isolated such that the DAC subsystem mediates all access to those
objects.

- Interpretation for Auditing Subsystems:

The system's architecture shall ensure that the auditing mechanism cannot
be bypassed by any subjects accessing those objects under the system's
control.

27

Computer Security Subsystems ASSURANCE REQUIREMENTS

-Interpretation for Object Reuse Subsystems

The notion of subsetting objects is not applicable to object reuse
subsystems. Object reuse subsystems shall perform their function for all
storage objects on the protected system that are accessible to users.

- Interpretation for I&A Subsystems:

This requirement applies to I&A subsystems. Authentication data shall be
protected from unauthorized access. Access to the authentication data shall
also be recorded in the audit trail.

3.2 SUBSYSTEM INTEGRITY

Subsystem integrity evaluation is designed to provide operational assurances
with regard to the correct operation of the protection mechanisms of the
subsystem and its interfaces to the protected system.

3.2.1 Integity:D1

-TCSEC Quote

"Cl: New: Hardware and/or software features shan be provided that can be used
to periodicany ~aUdate the correct operation of the on site hardware and
firmware elements of the TCB."

- Interpretation:

In the TCSEC quote, "TCB" is interpreted to mean "subsystem".

This requirement applies to an subsystems evaluated at any class,
regardless of the functions they perform.

- Rationale/Discussion

The capability must exist to validate the correct operation of all hardware
and firrnware elements of the system regardless of whether they reside within
the subsystem, the protected system, or other interfacing subsystems. If the
hardware and/or firmware elements of the protected system or other interfacing
subsystems play an integral role in the protection and/or correct operation of
the subsystem, then they must comply with this requirement as though they were
part of the subsystem.

28

Computer Security Subsystems ASSURANCE REQUIREMENTS

3.2.2 Integrity:D2


There are no additional requirements for System Integrity at D2.

3.3 SECURITY TESTING

Testing, as part of the evaluation, is designed to provide life cycle
assurances with regard to the integrity of the subsystem. Further, testing
provides additional assurances regarding the correct operation of the
protection mechanisms of the subsystem and the subsystem's interfaces to the
protected system. These mechanisms and their interfaces to the protected
system, are termed the Subsystem's Security- Relevant Portion (SRP).


3.3.1 Test:Dl

- TCSEC Quote:

"Cl: New: The securib mechanisms of the ADP system shan be tested and found to
work as claimed in the system documentation. Testing shan be done to assure
that there are no ob~ious ways for an unauthorized wer to bypass or otherwise
defeat the security protection mechanisms of the TCB. (See the Security
Testing Guidelines.) "

- Interpretation

This requirement applies to all subsystems evaluated at any class,
regardless of the function(s) they perform. In the TCSEC quote, "TCB" is
interpreted to mean subsystem.

The subsystem's SRP shall be tested and found to work as claimed in the
subsystem's documentation. The addition of a subsystem to a protected system
shall not cause obvious flaws to the resulting system. _

Test results shall show that there are no obvious ways for an unauthorized
user to bypass or otherwise defeat the subsystem's SRP.

- Rational/Discussion:

Security testing is a very important part of subsystem evaluations. It is
essential that the subsystem be demonstrated to operate securely.

29

Computer Security Subsystems ASSURANCE REQUIREMENTS

3.3.2 Test:D2


- TCSEC Quote:

"C2: Add: Testing shan also include a search for obvious flaws that would anow
nolation of resource isolation, or that would permit unauthorized access to
the audit or authentication data."

- Interpretation:

This requirement applies to the testing of the SRP of any subsystem
evaluated at the D2 class or the D3 class.

- Rationale/Discussion

The requirement as written in the TCSEC quote is directly applicable. This
requirement is to ensure that subsystems at D2 cannot be circumvented or
tampered with.


30


Computer Security Subsystems DOCUMENTATION REQUIREMENTS

4. DOCUMENTATION REQUIREMENTS

Documentation shan produce evidence that the subsystem can and does provide
specified security features. The evaluation will focus on the completeness of
this evidence through inspection of documentation structure and content and
through a mapping of the documentation to the subsystem's implementation and
its operation.


4.1 SECURITY FEATURES USER'S GUIDE

4.1.1 SFUG:Dl


- TCSEC Quote:

"Cl: New: A single summaIy, chapter, or manual in user documentation shall
describe the protection mechanisms provided by the TCB, guidelines on their
use, and how they interact with one another."

- Interpretation:

All subsystems shall meet this requirement in that they shall describe the
protection mechanisms provided by the subsystem.

- Rationale/Discussion:


It is recognized that some subsystems may be partially or completely
transparent to the general user. In such cases, this requirement can be met
by documenting the functions the subsystem performs so users will be aware of
what the subsystem does. Other subsystems which have a very limited user
interface may not need to be accompanied by more than a pocketsize card
available to every user. In short, the documentation required to meet this
requirement need not be elaborate, but must be clear and comprehenslve.

4.1.2 SFUG:D2

-Interpretation:

There are no additional requirements at the D2 class.


31

Computer Security Subsystems DOCUMENTATION REQUIREMENTS

4.2 TRUSTED FACILITY MANUAL


4.2.1 TFM:Dl

- TCSEC Quote :

"Cl: New: A manual addressed to the ADP system admmistrator shan present
cautions about functions and prvileges that should be controlled when running
a secure facility."

- Interpretation:

This requirement applies to all subsystems in that the manual shall present
cautions about functions and privileges provided by the subsystem. Further,
this manual shall present specific and precise direction for effectively
integrating the subsystem into the overall system.

4.2.2 TFM:D2

- TCSEC Quote:

"C2: Add: The procedures for examining and maintaMing the audit files as well
as the detailed audit record structure for each type of audit event shall be
given."

- Interpretation:

This requirement applies directly to all auditing subsystems and to other
subsystems that maintain their own audit data concerning events that happen
under their control. For subsystems that create audit data and pass it to an
external auditing collection and maintenance facility, the audit record
structure shall be documented; however, the procedures for examination and
maintenance of audit files may be left to the external auditing facility.

32


Computer Security Subsystems DOCUMENTATION REQUIREMENTS

4.3 TEST DOCUMENTATION

4.3.1 TD:Dl

- TCSEC Quote:

"Cl: New: The system developer shall provide to the evaluators a document that
describes the test plan, test procedures that show how the securib mechanisms
were tested, and results of the security mechanisms' functional testing."

- Interpretation:

The document shall explain the exact configuration used for security
testing. All mechanisms supplying the required supporting functions shall be
identified. All interfaces between the subsystem being tested, the protected
system, and other subsystems shall be described.

4.3.2 TD:D2


-Interpretation

There are no additional requirements at the D2 class.

4.4 DESIGN DOCUMENTATION

4.4.1 DD:Dl

- TCSEC Quote:

"Cl: New: Documentation shall be available that provides a description of the
manufacturer's philosophy of protection and an explanation of how this
philosophy is translated into the TCB. If the TCB is composed of distinct
modules, the interfaces between these modules shall be described. "

- Interpretation:

This requirement applies directly to all subsystems. Specifically, the
design documentation shall state what types of threats the subsystem is
designed to protect


33


Computer Security Subsystems DOCUMENTATION REQUIREMENI~

against (e.g., casual browsing, determined attacks, accidents). This
documentation shan show how the protection philosophy is translated into the
subsystem's SRP. Design documentation shan also specify how the subsystem is
to interact with the protected system and other subsystems to provide a
complete computer security system. If the SRP is modularized, the interfaces
between these modules shall be described.

4.4.2 DD:D2

There are no additional requirements for Design Documentation at the D2 class.

34


Computer Security Subsystems GLOSSARY

5- GLOSSARY

Accreditation - The offlcial authorization that is granted to an ADP system
to process sensitive information in its operational environment, based upon ,
comprehensive security evaluation of the system's hardware, firmware, and
software . security design, configuration and implementation of the other
system procedural, administrative, physical, TEMPEST, personnel, and
comrnunications controls.

Audit - The procedure of capturing, storing, maintaining, and managing data
concerning security-relevant events that occur on a computer system. The data
recorded are intended for use in detecting security violations and tracing
thosc violations to the responsible individual.

Audit trail - A set of records that collectively provide documentary
evidence of processing users to aid in tracing from original transactions
forward to related records and reports, and/or backwards from records and
reports to their component source transactions.

Authenticate - To establish the validity of a claimed identity.

Authorization - Permission which establishes right to access information.

Certification evaluation - The technical evaluation of a system's security
features, made as part of and in support of the approval/accreditation
process, that establishes " the extent to which a particular computer system's
design and implementation meet a set of specified security requirements.

Computer security subsystem - Hardware, firmware and/or software which are
added to a computer system to enhance the security of the overall system.


Group user - A user of a computer system whose system identification is the
name of a defined group of users on that system.

Individual user - A user of a computer system whose system identification
is unique, in that no other user on that system has that same identification.

Named object - An object which is directly manipulable at the TCB
interface. Thc object must have meaning to more than one process.

Product evaluation - Thc technical evaluation of a product's security
features to determine the level of trust that can be placed in that product
as defined by thc NCSC.


35

Computer Security Subsystems GLOSSARY

evaluation criteria for that type of product (e.g., operating system, database
management system, computer network, computer security subsystem). Product
evaluations do not consider the application of the product in the evaluation.


Protected system - The system being protected. In the context of computer
security subsystems, a stand-alone computer system or a computer network to
which a subsystem is attached to pronde some computer security function.

Security Relevant Portion (SRP) - The protection-critical mechanism of the
subsystem, the subsystem's interface(s) to the protected system, and
interfaces to the mechanisms providing required supporting functions. For
most cases, the SRP encompasses the entire subsystem.

Subsystem - See "computer security subsystem."

System - The combination of the protected system and the computer security
subsystem.

*U.S. GOVERNMENT PRINTING OFFICE: 1989-225-703