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 TURNING MULTIPLE EVALUATED
PRODUCTS INTO TRUSTED SYSTEMS

NCSC TECHNICAL REPORT-003

Library No. S-241,353

July 1994

FOREWORD

This Technical Report "Turning Multiple Evaluated Products Into Trusted Systems,"
is provided to stimulate discussion on how evaluated products can be combined to
produce trusted systems. We establish the premise that the integrator/system
designer has the responsibility to retain, in as much as possible, an evaluated
product's rating while it, the product, is performing within the context of the
integrated (larger) system. In this manner, we therefore propose that a modified
evaluated product has advantage over the use of a non-evaluated product for similar
functionality.

Recommendations for revision to this publication are encouraged and will be
reviewed periodically by the NCSC. Address all proposals for revision through
appropriate channels to:

National Computer Security Center

9800 Savage Road

Fort George G. Meade, MD 20755-6000

ATTN: Standards, Criteria, and Guidelines Division

Reviewed by:_______________________________________

GLENN GOMES

Chief, INFOSEC Standards, Criteria & Guidelines Division

Released by:_______________________________________

ROBERT J. SCALZI

Chief, INFOSEC Systems Engineering Office

ACKNOWLEDGEMENTS

This document was written by Joan Fowler and Dan Gamble of Grumman
Data Systems for the Procurement Guideline Project. The project leader was MAJ
(USA) Melvin L. De Vilbiss. Besides many NSA organizations, the document was
reviewed by Department of the Army (ASIS), DISA, MITRE, and NAVELEXSECSEN.

TABLE OF CONTENTS

FORWARD

ACKNOWLEDGEMENTS

1. INTRODUCTION

2. SYSTEM DESIGN APPROACH

2.1 Classic High Level View of a System

2.2 Determine System Functions/Services

2.3 Define Functions/Services Interdependencies

2.4 Specify Dependency Lattice

2.5 Define Products and Platform

3. TRUSTED SYSTEM DESIGN APPROACH

3.1 Evaluated Products List (EPL) Product Determination

3.2 Product Conflict Resolution

3.3 Architecture Relies on External Dependencies

3.4 Trusted Computing Base (TCB) Definition

3.4.1 Product Analysis

3.4.2 System Interface Analysis

3.4.3 Application Audit Example

3.4.4 Example for An Integrated System

4. TRUSTED SYSTEM ASSURANCE

4.1 Product Assurance Documentation

4.2 System Assurance Documentation

4.3 System Documentation Standards and Analysis

5. CONCLUSION

BIBLIOGRAPHY

1 INTRODUCTION

In the past few years, more Commercial Off-The-Shelf (COTS) products have been
populating the Evaluated Products List (EPL) than in previous years. In the current
economic environment, the tendency is to use evaluated products when designing
trusted systems to meet specific procurement requirements. The process to design a
trusted system composed of evaluated products is fundamentally the same as
designing any system using COTS products. The concept that makes the process of
designing trusted systems unique is that the combination of different products
composes a totally new security environment.

A trusted system, in the context of this paper, is a system composed of multiple
products. This system, at the interface to the Trusted Computing Base (TCB),
conforms to the Department of Defense (DoD) Trusted Computer System Evaluation
Criteria (TCSEC) (DoD 5200.28-STD) [1] and the forthcoming TCSEC-derived
protection profiles to be embodied in future U.S./international criteria.

This paper discusses how evaluated products can be combined to produce trusted
systems which meet the requirements specified in a procurement document, thereby
modifying, adapting, or eliminating portions of the composing product's TCB.
Frequently, the requirements specified necessitate changes to the product TCBs.
Because the product's rating may be invalidated when the product's TCB is changed
without understanding, justification, and review; system-level assurances are
necessary to compensate for the changes. It is the responsibility of the system
integrator/system designer to do the utmost to retain and not invalidate the product
rating. However, even with this possible invalidation, the use of an evaluated
product in a system provides the knowledge that the original product was
scrutinized, and those portions of the product that are not changed continue to retain
that scrutiny for the correctness of processing. Therefore, even if a product's TCB
must be modified, adapted, or portions eliminated, the use of an evaluated product
in a system development is advantageous over the use of a non-evaluated product
for the similar functionality. The combination of unequal security qualified
components to build a system is another dilemma in the integration process which
will not be discussed in this paper.

The need for the modification, adaptation, or elimination of a TCB in evaluated
products has greatly diminished in recent years. When the modification, adaptation,
or elimination is dictated due to system requirements, these changes can take many
forms. The easiest and most trusted form is to tune the product using the product's
configuration options, "hooks", or switches. (For example, in many products it is
possible to audit all or no activity for a user.) Another form is to use the product as it
was not necessarily intended to be used. If a

product with Mandatory Access Control (MAC) labels and controls is used in a
system high environment, the MAC processing actually occurs in the execution of the
software, but it does not have any security relevancy in the system. Another form of
adapting an evaluated product's TCB is to develop an extension to overcome the
shortcomings of the combined products used in a system. A final form of eliminating
security functionality is to actually modify the code of the product. This form is the
least desirable and should only be done when the system requirements dictate that
product code modification is the only solution. No matter which form the
modification takes, great care must be taken to determine the effect on the entire
system. The time required to integrate evaluated products into a trusted system and
ascertain the effects on each facet of the product must be assessed since that time, in
some cases, may be greater than the time required to develop a trusted system, or a
portion of a trusted system, from the beginning.

2 SYSTEM DESIGN APPROACH

This section discusses an approach to designing a system to integrate COTS products.
This approach is a single method that can effectively be used for system integration,
although it is not the only approach. The approach, as it is described in this section, is
used for the integration of untrusted systems from COTS products. It is also
applicable to the integration of EPL products into trusted systems, with a few
modifications to the approach. This revised approach for trusted systems will be
discussed in following sections.

2.1 CLASSIC HIGH LEVEL VIEW OF A SYSTEM

The textbook high level view of a system is a processing box which receives inputs,
processes the inputs according to a set of requirements, and generates outputs. This
is the high level view of a system whether it is trusted or not. The list of requirements
which must be satisfied by the system processing is defined by the operational needs
and outputs required of the system. In the case of trusted systems, the security policy
of the system also determines some of the system requirements. All of these
requirements may be defined in a Request for Proposal, a System Specification, a
Statement of Work, or some other type of requirements document. Finally, these
requirements must be available to the system integrators/designers for analysis and
subsequent design of the system.

2.2 DETERMINE SYSTEM FUNCTIONS/SERVICES

When designing a system, the first step beyond this classic high level view of a
system is to determine what functions must be performed, as defined by the
requirements for the system.

A function is a "series of related activities, involving one or more entities, performed
for the direct, or indirect, purpose of fulfilling one or more missions or objectives. It
should be identifiable and definable, but may or may not be measurable." A function
may be composed of one or more subfunctions. [2] Subfunctions perform a portion of
the overall task assigned to the function.

Each function selected for the system should be internally cohesive in that it performs
a single task and requires little interaction with other functions in the system. [5]
Another objective in determining the functions is to minimize coupling between the
functions to make them as independent as possible. [4] Of course, no system can exist
without some coupling to preserve the cohesiveness of the system as a whole. By
definition, a function that is not bypassable becomes primitive within an architecture.
That function's implemented security

policy will be invoked between each domain that it invokes. Unintentional or
intentional emergent behavior can be created when integrating functions which
detract from the cohesiveness of the system functionality.

Some examples of high level functions that may be determined for a system are data
base management, man-machine interface (MMI), communications, or mail. In
trusted systems, MAC, Discretionary Access Control (DAC), Audit, and
Identification and Authentication (I&A) are all possible functions to be defined. The
definition of any or all of these functions is determined by the set of requirements for
the system. There are security requirements that are not normally characterized as
functions. Examples of these are domain isolation, integrity, and trusted path.
However, if a system or product has a trusted path available to the user for example,
some mechanism (e.g., "function") must provide this capability.

2.3 DEFINE FUNCTIONS/SERVICES INTERDEPENDENCIES

The next step toward designing a system is to determine the coupling that has to exist
between functions. This coupling forms interdependencies between the functions or
services. An example of this interdependency at a high level is a mail function that
may be dependent on the MMI to "deliver" the mail to a user's terminal. Of the
security functions, applications may be dependent on the TCB to perform security
functions. Additionally, the I&A function may need the MMI to allow the user to
input his/her logon identification sequences. Finally, the MAC and DAC functions
depends on an I&A function to authenticate and provide the correct information for
the user.

2.4 SPECIFY DEPENDENCY LATTICE

Once all of the functions have been defined and the interdependencies have been
determined, a dependency lattice can be described. Figure 1 illustrates a dependency
lattice for generic functions. This lattice defines those functions that are dependent on
other functions, as well as those functions that are independent.

2.5 DEFINE PRODUCTS AND PLATFORM

Finally, the independent functions are used to determine, from the products
available, those products that will best meet the requirements of the system. This is
done by comparing the functions required by the system with the functionality
provided by all the available products. When a close match is determined, a product
can be selected. Sometimes dependent functions have to be rearranged to better fit
the products that are available. There is never a perfect match between the
requirements for a system defined into functions and the specifics of a single product
or a group of products. The products will either not collectively contain a needed
dependent function, will contain functions that are not requirements for the system,
or will contain redundant functions among the group of products.

Once the best correlation between all the functions or services and available software
products is made, then the physical requirements are taken into account. These
physical requirements include performance, reliability, interfaces, and other
requirements [5] which further constrain the choice of available software products,
and thus determine the platform (e.g., hardware) for the system. Again, there is
never a perfect map between the software products selected, the system's physical
requirements, and the platforms available even when the platform is selected at the
end of the process. However, selecting the platform prior to determining the software
products that will satisfy the system requirements increases the differences between
the map of the platform and the products and physical requirements.

3 TRUSTED SYSTEM DESIGN APPROACH

The approach to the design of trusted systems using evaluated products must be
taken a step further than the approach described above. When designing trusted
systems, the security functionality of each individual product may not satisfy all of
the security requirements of a system. For instance, one product may have a
compliant I&A (e.g., with an automatic password generator), while another product
may have a compliant audit mechanism (e.g., with all of the reporting capabilities for
the audit log). However, the security functionality of all of the products together may
present a redundant surplus of security functionality. Redundant security
functionality is especially important to deal with when there are conflicts between
the security functions of the various products to be used for the system. A possible
example of a conflict is the case of object reuse functions in a system in which one
product clears objects before releasing the object to the user, and the other product in
the system clears the object after the user has released the object. In this case, the
potential exists for the user to receive, under the right circumstances, an object that
has not been cleared by either product; or the user may suffer performance
degradation when the object is cleared by both. In this case, a unified object reuse
policy for the system would need to be established.

3.1 EVALUATED PRODUCTS LIST (EPL) PRODUCT DETER-
MINATION

An evaluated product is selected much as any other product would be selected,
based on a set of functions that the product must satisfy. As stated above, when a
function and its dependent functions are compared to a product, there are almost
always requirements that are not satisfied by the product. Additionally, there is
functionality in the product that is not included in the list of requirements for the
system as a whole. This surplus may lead to conflict between products when each
attempts to satisfy the same single requirement in a system with a cohesive policy.

An example of this conflict is a single processor system that has requirements
translating into a need for an evaluated operating system and a trusted application,
(e.g., mail). The operating system will probably contain I&A, DAC, and audit
capabilities. The application may also have I&A, DAC, and audit capability. In all
other aspects, the two products are a perfect match for the system requirements.
However, in this case, there is a redundancy of security functionality. The application
is not an operating system and the operating system can not perform the non-security
capabilities required of the application. Therefore, neither of the products
individually satisfies both the security and non-security requirements of the system.
If two products with a reference monitor are included in a system, one of the
reference monitors is going to be bypassed at some time during operation of the
system.

The redundant features issue can be decomposed into security policies and
mechanisms to implement the policy. If both the policy and the mechanism are
identical, as in the case of a homogeneous network environment with a single policy
in which the workstation and server both use the same evaluated operating system,
then there might be user resistance (e.g., to a double logon). If the same intended
policy is implemented with different mechanisms, as in the case of a heterogeneous
network environment in which two different operating systems are used with
different labeling schemes, then there exists a conflict between the two mechanisms.
The label conflict may be resolved by a conversion function developed as an
extension to the TCBs of either or both of the products. Additionally, if the policy is
different but the same mechanism is used, a policy conflict exists even in a
homogeneous workstation and server environment with the same operating system
containing the same DAC mechanism. The workstation may be using different
"Group" definitions and Access Control Lists (ACLs) than the server. This conflict
would violate one of the policies without the knowledge of the violated processor.
Finally, if both the policy and the mechanism are different, a heterogeneous network
environment in which the label policy and the labeling mechanism are both different,
then conflicts that might not be able to be resolved may exist. In this case, something
fundamental in the policy or the mechanism would have to be changed. The simple
conversion of the label format would not suffice to integrate these two systems.

3.2 PRODUCT CONFLICT RESOLUTION

It is not efficient to have differing DAC or audit schemes when designing a cohesive
system. This is not to state that redundancy can not, in some circumstances,
strengthen the security of a system, provided that it is user friendly and not counter
to human intuition. However, there is always a concern for consistency of the global
security policy of the system where redundancy is involved. It is not advantageous to
incorporate two I&A mechanisms into a single secure operational system, without at
least some dominance of one over the other. (Most systems today require a limiting
of a single logon for a user session.) Each of the redundant security functions may
need to be modified or disabled in one of the products (through extensions to the
product TCB, switches, configuration options, if possible; or TCB code modifications,
if necessary) in order that the system may have a single I&A, DAC, or audit. This is
done by modifying, adapting, or eliminating one or the other product to disable or
limit the function. Then the other product, in which the function is not disabled or
limited, must be changed to interface with the product in which the function has been
disabled or limited.

The process of modifying or adapting an evaluated product by limiting functionality
has ramifications to the evaluation of the modified product. It may invalidate the EPL
rating of the product, if not done with review, justification and understanding. The
integration of multiple evaluated products may stay within the bounds of the
assumed parameters as stated in the individual product's evaluation report, or the
integration effort may violate those bounds. If necessary, it is the responsibility of the
system integrators/designers to compensate for any invalidation of the product
rating using system-level, as opposed to product-level, assurance. This will be
discussed later in this paper.

The product vendor is always the best choice to make modifications to products. The
vendor may make a business decision on the marketability of changes required for a
system acquisition. If the modification can be productized, the vendor will insert the
change into the standard product and perhaps take the modification through the
Rating Maintenance Program (RAMP) frequently with no charge to the acquisition.
This is the most advantageous course of action. The spectrum from the above (vendor
made changes) down to the integrator performing the changes without any vendor
support are possible scenarios. The average contract design, integration, and/or
development strategy will lie somewhere along this spectrum.

3.3 ARCHITECTURE RELIES ON EXTERNAL DEPENDEN-
CIES

Frequently, there are external dependencies which affect the architecture of a trusted
system which would not affect the architecture of an untrusted system. An example
of this is a system that receives labeled input. This system receives the labeled input
directly into the processing stream for all data. Since the input is labeled at the source
of the data outside of the system boundary, the integrity of the label must be
assumed to be trusted as far as the system is concerned. (Mechanisms are available to
ensure this to be true.) Therefore, the MAC performed using this label is solidly
based.

However, if the data received by the same system architecture is not labeled and is at
multiple classification levels, then the system does not have a basis for MAC. The
architecture could be changed to include some sort of labeling entity prior to the
unlabeled data entering the mainstream of the system. Depending on the
requirements of the system, this could be a human on a terminal reviewing and
labeling all data; it could be a front-end component labeling all data from a single
level device; or, it could be an operating system labeling all data from a single level
port. For this example, it does not matter what the architectural change would be, just
that the overall system architecture must accommodate the differences between
labeled and unlabeled input.

3.4 TRUSTED COMPUTING BASE (TCB) DEFINITION

Once the products are selected and the architecture is defined, the TCB for the system
must be established. Under the premise of this report, the system would be designed
using COTS components (both trusted and non-trusted products). A single system
TCB would, in this case, be defined using the product TCBs as the basis and
satisfying the reference monitor assumptions and the system security policy. This is
done by examining the various TCBs of the products, identifying the mechanisms
and interfaces that will remain for the resulting system, and analyzing what
additional mechanisms and interfaces may be necessary for the system.

3.4.1 Product Analysis

As stated previously, there is never a perfect match between requirements, functions,
and products. If functionality is lacking in all of the products selected, then the
integration process must include the development of that functionality or the
inclusion of a non-trusted component to handle the functionality. Occasionally
during a tradeoff analysis, a non-automated solution (e.g., a locked room) is
determined to be the preferable manner to address any missing functionality.

However, the more likely occurrence, when a collection of evaluated products are
combined, is redundant security functionality. An analysis must be made to
determine which security features will be used in each product. This analysis must be
carried a step further for evaluated products. An additional analysis must be made to
determine how the security characteristics of each individual component may affect
the composite characteristics of the system, and what the resulting effect will be to
the overall product and system when a product's security feature is not used, either
disabled or limited. It is important that this analysis be performed in the early stages
of a program to inform the program management of the correct integration options,
even if the demonstration/proof of the satisfaction of the requirements of the TCSEC
by the modified system is required for the integrated system. To rely on the later
assurance proof for this analysis will inform the program, after delivery, that the
system has already been integrated/developed incorrectly. At that point, the
information is not beneficial to the program.

3.4.2 System Interface Analysis

Beyond the analysis of the product and the selection of which product features to use
and not use, a system analysis must be performed to identify the interfaces that will
be needed within the system TCB. This analysis includes the system interfaces that
will occur between the products without modification, or as the manufacturer
delivered it. (Again, the use of the code of products as they are delivered by the
manufacturer is the preferable manner in which to use a product.) Additionally, the
analysis must take into account the interfaces which are newly created when the
products are modified to eliminate certain features, or add a system capability.

A desirable result of any trusted system integration is to minimize the overall system
TCB while minimizing the impact on the product TCBs composing the system TCB.
Using evaluated products, each will contain a TCB. When all of the product TCBs (as
well as the new TCB functions developed for the integration effort) are taken into
account for the system, the resulting overall TCB will be a certain value. To eliminate
a portion of a product's TCB is to diminish the size of the overall system TCB by the
complexity and value of the portion of the product TCB that is eliminated. This
serves to minimize the overall system TCB by the value of the excluded portions of
all the products' TCBs. However, this minimization action must be accomplished
with care. Eliminating parts of a component TCB may increase your risk because of
internal dependencies within the product. Additionally, it may increase program cost
because the impact of removing the portion of the product TCB must be determined.
Tradeoffs and compromises must be made.

3.4.3 Application Audit Example

Figure 2 is a pictorial description of the audit function of a trusted application. The
application could be anything trusted, a trusted mail application, a trusted Database
Management System, etc. This particular audit function has a security administration
subfunction which sets the criteria on which auditing will occur. The criteria are
placed in a database. The next subfunction is the audit interface in the TCB which
detects a criteria match. When a match is detected, the event recorder subfunction
records the event using the user ID, success/fail criteria, event data, and time which
are held for the application in a database, table, or global common, depending on the
implementation. The event recorder writes the audit record to the application's audit
log. There is also a real time subfunction which checks thresholds and responds to
the matching of these thresholds. An example of this functionality is a limit of three
attempts to logon using a single user ID. On the fourth attempt, the real time
subfunction may lock a user out of the system. There are also several administrative
subfunctions dealing with the application's audit log. The data reduction subfunction
handles the queries and responses to the audit log. The administrative subfunction
allows an administrator to archive and purge the audit log.

3.4.4 Example for An Integrated System

To carry on with this example, the following is a single approach to use this product
in an integrated system. (This approach is not the only approach that can be used,
neither is it meant to be a procedural description of composing systems.) The product
has been selected to perform whatever application it does. In this example, the
product will be used in a distributed architecture which has a requirement for
centralized administration of the auditing capability and a centralized system audit
log. This is not to imply that an application's audit log must be deactivated if there is
a system audit log.

Figure 3 illustrates the system with centralized audit administration and storage. The
application described in the previous subsection is in the figure as the lightly shaded
large box. In order to achieve centralized administration, an audit management
subfunction must be developed that sets the criteria for the entire system. A portion
of this subfunction must be written to interface with the security administration
subfunction of the application. To have the application's event recorder subfunction
write the audit records to the system audit log instead of the application's audit log, a
common interface must be written between the event recorder subfunction and the
system audit log. Assuming that there is more than one application in the system
which produces audit records, the common interface subfunction would translate all
of the application audit record formats and data packing schemes to a single system
audit record format. Additionally, the interface between the application event
recorder subfunction and the application's audit log must be severed.

As can be seen from Figure 3, there are two new subfunctions in this system view, the
audit management and the common interface. These subfunctions are denoted in the
boxes without shading. There are also three new interfaces. In the figure, these
interfaces are denoted by the heavy arrow lines. There is a new interface between the
new system audit management subfunction and the application security
administration subfunction. There is another new interface between the application
event recorder and the new common interface subfunction. And, finally, there is a
new interface between the new common interface subfunction and the system audit
log.

Since all of the audit records are now being processed into the system audit log, the
application's audit log is no longer used. Therefore, the interface between the
application event recorder subfunction and the application's audit log is severed.
This is designated in the figure with a heavy "X". Finally, since the application's audit
log is no longer used, the three subfunctions that support the application's audit log
are also no longer needed. These three subfunctions (data reduction, real time, and
administrative) and the application's audit log are all designated in the heavily
shaded boxes.

4 TRUSTED SYSTEM ASSURANCE

The use of evaluated products is an extremely good starting point for the certification
and accreditation efforts of systems. However, the combination of evaluated
products, with the resulting changes to the products as described above, may
invalidate the rating of the product when the changes are performed without the
proper review and understanding. The assurances developed at the system level
during the integration process must compensate for any invalidation of the product
rating.

The TCSEC is the standard used to develop the assurance of products. The TCSEC
defines the assurance documentation required for a TCB. The design documentation
requirements are a subset of the overall documentation described in the TCSEC. The
TCSEC requires that "If the TCB is composed of distinct modules, the interfaces
between these modules shall be described." [1] This is true for all classes defined in
the TCSEC above the Minimal Protection Division (D). Additionally, the TCSEC
requires that "The specific TCB protection mechanisms shall be identified..." [1] This
is a requirement for all classes in the Mandatory Protection Division (B) and Verified
Protection Division (A).

Of course, there are additional assurance documentation requirements that include: a
security policy model, a Philosophy of Protection, a Descriptive Top Level
Specification, a Formal Top Level Specification, a covert channel analysis, a TCB
verification report, a Configuration Management Plan, administrator and user
manuals, and testing documentation. The modification, adaptation, or elimination of
product TCB functionality (mechanisms and interfaces) has a ripple effect through all
of the assurance documentation for the system.

Security testing, as well as other activities such as architecture, recovery, and
verification, are also required as assurance mechanisms. Security testing of the
combined evaluated products demonstrates that the modified mechanisms and
interfaces perform as intended and that the overall level of protection has not been
diminished. Finally, this testing will serve to validate the completeness of the system
level documentation. Security testing of the system, as with all assurance activities, is
performed to support a certification and accreditation, and not an evaluation, of the
system. All the engineering efforts to assure a system are documented (e.g., security
testing is reflected in the test plan, procedures, and report required by the TCSEC for
testing). Therefore, the remainder of this paper uses the term "documentation" to
refer to all of the assurance documents required by the TCSEC for evaluation.
Included in the use of the term "documentation" are all the activities (e.g., testing,
design engineering, covert channel analysis) that are performed in order to produce
these assurance documents.

4.1 PRODUCT ASSURANCE DOCUMENTATION

In order for a product to be evaluated, TCSEC documentation requirements have to
be satisfied. But what happens to this product assurance documentation when the
product is modified for use in a system? Most of the product documentation should
still be valid. If the product changes so much that a total rewrite of the
documentation is needed, then perhaps the product is not really a match for the
requirements of the system, and another product should be selected.

4.2 SYSTEM ASSURANCE DOCUMENTATION

Assuming that most of the product is going to be utilized as evaluated in the system,
and that most of the product's documentation is therefore valid, the few
modifications, adaptations, and eliminations made to the product must be
documented. When composing evaluated products into trusted systems, new
subfunctions may be needed to couple products, new interfaces are included to these
new subfunctions, some of the mechanism of the original product may be disabled,
and original interfaces may be excluded. These four types of modifications break
down into two categories: TCB interfaces and mechanisms. The modifications are the
two sides of each of these categories: eliminated and new TCB interfaces; and
eliminated and new mechanisms.

The existing evaluation version of the product documentation should describe all
interfaces and protection mechanisms to include both the original interfaces and
mechanisms that have been eliminated during the integration of the system. The
system level documentation should describe the effect that the elimination of the
mechanisms and interfaces of the evaluated product has on the system TCB as a
whole.

The previous paragraph covers the elimination of original interfaces and mechanisms
of the evaluated product used in the system. The addition of new mechanisms and
the resulting additional interfaces to the combined product TCBs for the system must
also be documented in the system-level assurance documentation. These
mechanisms and interfaces are not described in any of the product-level
documentation since they are probably either not available in any of the individual
products, or were not required to perform in the product as they are in the system.

There are options to the system integrator/developer when the modification of
product documentation is done. The vendor may develop the code modifications and
document those modifications. Or, the integrator may buy the code and
documentation, and then modify each as required. Between these two ends of the
spectrum are a range of options to both the program and the integrator.

4.3 SYSTEM DOCUMENTATION STANDARDS AND ANALY-
SIS

The Data Item Descriptions (DIDs) which have been developed for the series "A
Guide to Procurement of Trusted Systems, Volume 3," were written to be applied to
products [3]. However, they require the definition of the TCB interfaces and the
identification of the TCB protection mechanisms. In the procurement of trusted
systems, these DIDs are applicable for system-level assurance documentation. The
orientation (e.g., system-level, product-level) of the DID must be expanded outside
the framework of the DID. The Statement of Work (SOW) or the Contract Data
Requirements List (CDRL) calling out the DID should include statements for the
system-level orientation of the resulting assurance documentation. These SOW or
CDRL statements should require the examination of the interfaces and mechanisms
between products and the analysis of the elimination of interfaces and mechanisms.

A real challenge in the replacement of invalidated product-level documentation is the
analysis of the validity of the system-level assurance documentation. The certifier
validates the assurance documentation for the system and certifies that the system
meets certain requirements. However, it is ultimately left to the accreditor of the
system to determine the validity of the assurance documentation for the system and
give the permission for the system to operate. There is no other body willing to assess
the validity of system-level assurance documentation at this time.

5 CONCLUSION

In conclusion, this paper has presented a single approach to the composition of
evaluated products into trusted systems. These evaluated products can be combined
into trusted systems with assurance. The system-level assurances must compensate
for any invalidation of the individual products' ratings. The system-level assurance
must document the same types of information that the product-level assurance has
documented, i.e. interfaces and mechanisms. The only difference is that excluded and
eliminated product mechanisms and interfaces must also be assessed in the system-
level documentation. When procuring these systems, the SOW or CDRL should
include direction to the integrator to examine the new interfaces and mechanisms
between the products and assess the elimination of interfaces and mechanisms.

BIBLIOGRAPHY

[1] Department of Defense, "Trusted Computer System Evaluation
Criteria" (TCSEC), DoD 5200.28-STD, December 1985.

[2] Modell, Martin E., A Professional's Guide to Systems Anal-
ysis, McGraw-Hill Software Engineering Series, McGraw-Hill Book
Company, New York, 1988.

[3] NCSC-TG-024, Version 1

Volume 1/4, "A Guide to Procurement of Trusted Systems:
An Introduction to Procurement Initiators on Computer
Security Requirements," December 1992

Volume 2/4, "A Guide to Procurement of Trusted Systems:
Language for RFP Specifications and Statements of Work -
An Aid to Procurement Initiators," June 30, 1993

Volume 3/4, "A Guide to Procurement of Trusted Systems:
Computer Security Contract Data Requirements List and Data
Item Descriptions Tutorial," February 28, 1994

Volume 4/4, "A Guide to Procurement of Trusted Systems:
How to Evaluate a Bidder's Proposal Document - An Aid to
Procurement Initiators and Contractors," (Draft)

[4] Page-Jones, Meilir, The Practical Guide to Structured Sys-
tems Design, Yourdon Press, Englewood Cliffs, New Jersey, 1988.

[5] Pressman, Roger S., Software Engineering, A Practitioner's
Approach, McGraw-Hill Series in Software Engineering and Tech-
nology, McGraw-Hill Book Company, New York, 1987.