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A Guide to Understanding Design Documentation




A Guide to Understanding Design Documentation

ACKNOWLEDGMENTS



Special recognition is extended to James N. Menendez, National
Computer Security Center (NCSC), as project manager and co-author
of this document.  Recognition is also extended to Don Brinkley
and Rick Dedrick, Unisys,  as co-authors of this document.


Special acknowledgment is given to Barbara Mayer, NCSC, for her
invaluable input and review of this document, which has resulted
in its current form.


Acknowledgment is also given to all those members of the computer
security community who contributed their time and expertise by
actively participating in the review of this document.


A Guide to Understanding Design Documentation


NCSC-TG-07 Version 1
1. INTRODUCTION

1.1 Purpose



The Trusted Computer System Evaluation Criteria (TCSEC) is the
standard used for evaluating the effectiveness of security controls
built into Automated Data Processing (ADP) systems.  The TCSEC
is divided into four divisions: D, C, B, and A, ordered in a hierarchical
manner, with the highest division (A) being reserved for those
systems providing the best available level of assurance.  Within
Divisions C through A are a number of subdivisions known as classes,
which are also ordered in a hierarchical manner to represent different
levels of trust in these classes.


Design Documentation is a TCSEC requirement for classes C1 and
above.  The purpose of this guideline is to provide developers
of trusted computer systems with guidance in understanding and
meeting the design documentation requirements contained in the
TCSEC.  To accomplish this, the guideline addresses two goals.
 First, the guideline increases the vendors' awareness of the
importance of design documentation to the security of their system
throughout the system life-cycle.  Second, the guideline forms
an initial basis of understanding between the vendor and evaluator
communities concerning what is expected by the evaluation team
in the review process and deliverables for design documentation.


Any examples in this document are not to be construed as the only
implementation that will satisfy the TCSEC requirement.  The examples
are merely suggested implementations.  The recommendations in
this document are also not to be construed as supplementary requirements
to the TCSEC.  The TCSEC is the only metric against which systems
will be evaluated.


This guideline is part of the Technical Guidelines Program to
provide helpful guidance on TCSEC issues and the features they
address.
1.2 Scope



Design Documentation is a TCSEC requirement for classes C1 through
A1.  It is one of the four types of documentation required by
the TCSEC.  The other three documentation requirements are for
a Trusted Facility Manual (TFM), Security Features Users Guide
(SFUG), and Test Plan Documentation.  The role of Design Documentation
is to identify and describe the Trusted Computing Base (TCB) and
its security features.  Only Design Documentation for the TCB
is required to meet the TCSEC requirements, but it is strongly
recommended that design documentation exist for the entire system.
 Throughout this document, the word system will be used as the
object of design documentation to include the TCB and the untrusted
portions of the system.  However, it should be emphasized that
the TCSEC requirements are based solely on the design documentation
of the TCB.


Design Documentation assists vendors during the system life-cycle
by thoroughly defining the policies that the system enforces.
 It also provides the material by which the evaluator can assess
whether, and to what degree, the design intent was carried into
the implementation.  The design documentation is intended to guide
the implementation of the product; it is not intended merely as
an abstract philosophical exercise completely divorced from the
"real" product.


Design documentation also increases the developer's level of understanding
of the system.  It should facilitate the correct implementation
of the intended behavior and features of the system.  This guideline
will discuss design documentation and its features as they apply
to computer systems and products that are being built with the
intention of meeting the requirements of the TCSEC.
1.3 Control Objective



Each of the TCSEC requirements serves to ensure that one of the
three basic control objectives for trusted computing - security
policy, accountability, and assurance - are satisfied.  Throughout
the system life-cycle, design documentation aids in attaining
the third objective, assurance, by helping to "substantiate
claims for the completeness of access mediation and degree of
tamper resistance." [5]

 • The TCSEC gives the following as the Assurance Control Objective:







 • "Systems that are used to process or handle classified
or other sensitive information must be designed to guarantee correct
and accurate interpretation of the security policy and must not
distort the intent of that policy.  Assurance must be provided
that correct implementation and operation of the policy exists
throughout the system's life-cycle."[5]







Design documentation plays an important role in providing this
life-cycle assurance.  It demonstrates that correct implementation
and enforcement of the system's security policy exists throughout
the system's life-cycle.  As it relates to this control objective,
design documentation facilitates the efforts of vendors and system
developers in modifying and maintaining the system throughout
its life-cycle, without compromising the trustworthiness of the
system.


In addition, design documentation serves as a useful training
tool.  Design documentation presents a technical history of the
system, containing documentation on past changes to the system
as well as the current system.  It can be used in the training
of new systems programmers and hardware engineers to familiarize
them with the system.
2. OVERVIEW OF DESIGN DOCUMENTATION PRINCIPLES



Design documentation is a requirement for TCSEC classes C1 and
above.  It provides a means of communicating the design of a system
to developers that enables them to comprehend the design principles
of the system and to make changes or upgrades to the system without
compromising the trustworthiness of the system.  The information
contained in the design documentation provides a rationale as
to why a system is designed as it is and whether changes to the
system will alter the intent of the design.


Design documentation plays an important role in the life-cycle
maintenance of a system and should not be viewed as a burden to
system development.  This document should help developers understand
the importance of design documentation in the life-cycle of computer
systems, as well as to the maintenance of trust in these systems.
 Developers should recognize the importance of meeting the purpose
and intent of the TCSEC design documentation requirements as opposed
to meeting them in a strictly mechanical fashion.
2.1 Purpose of Design Documentation



The primary purpose of design documentation is to define and describe
the properties of a system.  As it relates to the TCSEC, design
documentation provides an explanation of how the security policy
of a system is translated into a technical solution through the
TCB hardware, software, and firmware.


Design documentation explains the system's protection mechanisms
so that the effect a change may have on the security of the system
can be evaluated prior to a change being performed.  It relates
the TCSEC requirements to the architecture of a system and guides
the implementation of the system under development.  Complete
documentation ensures that the vendor has an understanding of
what elements of the system are protection critical.  Design documentation
explains the system design to the vendor's development team and
enables the developers to understand the design of the system
well enough to maintain the system and to perform any necessary
changes to it without adversely affecting the trustworthiness
of the system.   In addition, the design documentation assists
the evaluators by providing them with a vehicle by which the completeness
and correctness of the implementation can be assessed.
2.2 Design Documentation Development for Evaluation



Developers should incorporate the design documentation requirements
into the system development process.  A plan that addresses each
design documentation requirement should be developed early in
the development phase and shared with the National Computer Security
Center evaluators to ensure the thoroughness of the documentation.


Iterative development of the design documentation is the key to
minimizing vendor and evaluator efforts during the evaluation
process.  Vendors should precede their design documentation with
the submittal of an outline of the design documentation to the
evaluators.  This outline should contain, among other things,
a statement of purpose and the intended audience of the design
documentation.  Then, through a process of draft submittal, evaluator
comments and requests for additional information, and draft revision
the design documentation requirements will be met.  This guideline
should expedite this process by bringing the vendor's first drafts
closer to evaluator expectations and by facilitating convergence
between vendor product and evaluator expectations.  If vendors
establish a dialogue with evaluators early in the design documentation
development and solicit their comments on early and subsequent
drafts of the design documentation, both vendors and evaluators
will save a great deal of time and effort in completing the evaluation
process.
2.3 Level of Detail of Design Documentation



The level of detail of design documentation will determine its
usefulness and adequacy in meeting the TCSEC requirements, as
well as its usefulness to the vendor in the development and maintenance
of the system.  For evaluators, the level of detail of the design
documentation is reviewed to ensure that the system developer
understands the design of the system well enough to make changes
or upgrades to the system without compromising the trustworthiness
of the system.  Design documentation also ensures that the developer
understands the overall security concepts that are required to
be part of the system design.  How well the security properties
of the system are documented, and how this information is integrated
into the design documentation will determine whether or not the
level of detail of the design documentation is sufficient in meeting
the TCSEC requirements.


The design documentation shall be detailed enough to serve as
a useful tool for vendor maintenance of the system and shall clearly
indicate what elements of the design impact the trustworthiness
of the system.  One purpose behind design documentation is to
assist vendors in maintaining the system and should not present
a burden to the vendor in terms of quantity or detail.  A good
rule of thumb is that the level of detail of design documentation
should be sufficient to permit an individual with a degree in
Computer Science, Electrical Engineering, or the equivalent with
knowledge and skills in programming, hardware, or firmware development
to understand the system design and be able to design system modifications
without adversely affecting trustworthiness of the system.
2.4 Level of Effort for Meeting the Requirements



The level of effort necessary for developing satisfactory design
documentation has historically been underestimated because the
intent and implications of the TCSEC requirements for design documentation
have seldom been completely understood.  An important factor to
consider when deciding on an appropriate level of effort is the
importance of the design documentation throughout the system life-cycle.
 Well structured design documentation that is carefully planned
and developed will make it easier to understand the design of
the system.


The level of effort necessary for a vendor to meet the design
documentation requirements varies from system to system.  The
level of effort generally will depend upon the necessary level
of detail, which depends upon the class of evaluation and the
complexity of the system being evaluated.  The requirements for
TCSEC classes C1 and C2 may be met by simply following good engineering
documentation practices, but as the TCSEC class level increases,
so does the level of detail and effort necessary for meeting the
TCSEC requirements.


In terms of quantity, the length of design documentation at the
higher classes has been found to be roughly comparable in bulk
to the source listings of the overall system.  In general, producing
the design documentation may require several man months to a man
year of system development time at Classes C1 and C2, and up to
several man years at the higher classes.  Although developing
design documentation for a system may be time consuming, this
time will be amply rewarded by the ease of system maintainability
during its life-cycle.
2.5 Format of Design Documentation



The format and style for each vendor's design documentation is
specific to that vendor, and to suggest a specific format would
restrict vendors in developing their design documentation.  Although
this guideline addresses distinct requirements for design documentation,
it should not be assumed that separate documents are necessary
to meet each requirement.  Indeed, the design documentation shall
address each of the requirements, but it is acceptable for evaluators
to be pointed to a number of documents to address a specific requirement.
 Also, graphics serve as a useful adjunct to design documentation,
although not sufficient alone to meet the TCSEC requirement for
design documentation.  Developers may choose to use graphics to
describe a system in addition to other design documentation.


Differences among computer system architectures, designs, and
implementation approaches make developing a standard format for
design documentation inadvisable.  In addition, the format of
design documentation for one system may be totally inappropriate
for meeting another system's needs.  The format chosen by the
vendor for presenting the design documentation may be influenced
by business concerns other than expeditious security evaluation.
 Different design documentation formats present different advantages
and challenges to evaluators and different advantages and costs
to vendors that should be weighed.


A system's design may be evolutionary, resulting from improvements
that build upon an initial version.  Maintaining documentation
on a system in a release/update form may be convenient for a developer.
  However, it is difficult for  new developers and life-cycle
personnel to gain an understanding of the overall system architecture
from documentation that describes the system in chronological
terms through system releases and updates.  To be useful, these
updates shall be incorporated into the design documentation, and
the design documentation shall be presented as a complete description
of the system, and not the initial description plus supplemental
sections describing changes.
3. MEETING THE CRITERIA REQUIREMENTS



This section lists the TCSEC requirements for design documentation
at each class.  All of these requirements have been extracted
from the TCSEC design documentation requirements and include explicit
and implicit design documentation requirements, where necessary.
 Each numbered requirement is referenced in the discussions that
follow in Sections 6 and 7 of this document.  This section serves
as a quick reference for TCSEC class requirements.


As the TCSEC evaluation class level increases, it is implicitly
required that the design documentation be more detailed.  This
is due to an increase in assurance required at the higher classes,
as well as the introduction of new features at the higher classes
that need to be documented, for example, labeling, auditing.
3.1 The C1 Design Documentation Requirements



Requirement 1 - Describe the philosophy of protection.


Requirement 2 - Describe how the philosophy of protection is translated
into the TCB.


Requirement 3 - Describe how the TCB is modularized (if modular).


Requirement 4 - Describe all interfaces between the TCB modules
(if modular).


Requirement 5 - Describe how the TCB protects itself.


Requirement 6 - Provide a statement of the system security policy.
3.2 The C2 Design Documentation Requirements



No new requirements have been added at the C2 class.
3.3 The B1 Design Documentation Requirements



Requirement 7 - Provide an informal or a formal description of
the security policy model enforced by the TCB.


Requirement 8 - Explain the sufficiency of the security policy
model to enforce the security policy.


Requirement 9 - Identify and describe the TCB protection mechanisms.


Requirement 10 - Explain how the TCB mechanisms satisfy the security
policy model.
3.4 The B2 Design Documentation Requirements



Requirement 11 - Describe how the TCB is modularized.


Requirement 12 - Describe all of the interfaces between the TCB
modules.


Requirement 13 - Provide a formal description of the security
policy model.


Requirement 14 - Prove the sufficiency of the security policy
model to enforce the security policy.


Requirement 15 - Show that the Descriptive Top Level Specification
(DTLS) is an accurate description of the TCB interface.


Requirement 16 - Describe how the TCB implements the Reference
Monitor Concept.


Requirement 17 - Describe why the reference monitor is tamper
resistant.


Requirement 18 - Describe why the reference monitor cannot be
bypassed.


Requirement 19 - Describe why the reference monitor is correctly
implemented.


Requirement 20 - Describe how the TCB is structured to facilitate
testing.


Requirement 21 - Describe how the TCB is structured to enforce
least privilege.


Requirement 22 - Present the results and methodology of the covert
channel analysis.


Requirement 23 - Describe the tradeoffs involved in restricting
covert channels.


Requirement 24 - Identify all auditable events that may be used
in exploitation of known covert storage channels.


Requirement 25 - Provide the bandwidths of known covert storage
channels whose use is not detectable by auditing mechanisms.
3.5 The B3 Design Documentation Requirements



Requirement 26 - Identify all auditable events that may be used
in exploitation of known covert timing channels.


Requirement 27 - Provide the bandwidths of known covert timing
channels whose use is not detectable by auditing mechanisms.


Requirement 28 - Describe how the system complies with additional
B3 system architecture requirements, for example, minimal TCB
and layering.


Requirement 29 - Informally show consistency of the TCB implementation
(in hardware, firmware, and software) with the DTLS.


Requirement 30 - Informally show correspondence between elements
of the DTLS and elements of the TCB.


Requirement 31 - Informally show consistency of the DTLS with
the model.
3.6 The A1 Design Documentation Requirements



Requirement 32 - Informally show consistency of the TCB implementation
with the Formal Top Level Specification (FTLS).


Requirement 33 - Informally show correspondence between elements
of the FTLS and elements of the TCB.


Requirement 34 - Clearly describe hardware, software, and firmware
internal to the TCB that is not dealt with in the FTLS.


Requirement 35 - Informally or formally show consistency of the
FTLS with the model.


Requirement 36 - Informally show correspondence between the FTLS
and the DTLS.
4. COMPONENTS OF DESIGN DOCUMENTATION



Design documentation describes why a system is trusted, how this
trust is achieved, the mechanisms which provide the trust, and
the relevant information that makes proper maintenance of a system
possible.  Design documentation at TCSEC class C1 lays the foundation
for trusted systems by defining the philosophy of protection of
a system.  As the TCSEC classes increase, the level of detail
and the quantity of information contained in the design documentation
shall also increase.  The following sections discuss design documentation
and its role in describing the security policy of the system,
the protection mechanisms of the system, and the specific requirements
concerning covert channels.
4.1 Documenting The Security Policy



The design and development of any trusted system, from TCSEC class
C1 to A1, is based upon a philosophy of protection that shall
be described in the design documentation (Requirement 1).  Design
documentation explains and defines the philosophy of protection
by describing how a system provides trust.  Trust in computer
systems is provided by the protection mechanisms contained within
the TCB, such as discretionary access controls and identification
and authentication mechanisms.  These and all of the TCB mechanisms
and their functions shall be described in the design documentation.
 In addition, the system security policy, i.e., what is being
accessed by whom or from what, shall also be described in the
design documentation (Requirement 6).


In order to describe how a system is trustworthy, the design documentation
shall describe how the philosophy of protection is translated
into the TCB (Requirement 2) and how it is supported by the TCB
protection mechanisms.  The design documentation shall first define
the boundaries of the system and shall describe the parts of the
system that are security relevant and the parts that are not.
 Rationale shall be presented that those portions of the system
which are claimed to be outside of the TCB, are really outside.
 The proper identification of these parts is important to the
maintenance of security in the system because it is necessary
to know when a change to the system will affect the TCB implementation,
and possibly violate the security policy of the system.


At the higher TCSEC classes, the description of the philosophy
of protection evolves into a more structured description of how
a system provides trust.  At TCSEC class B1, this philosophy of
protection shall be presented as an informal or formal security
policy model in the design documentation (Requirement 7).  This
security policy model shall informally or formally define the
subjects, objects, modes of access, and the security properties
of the system.  In addition, the model shall define the initial
state of the system, a secure state of the system, and the way
in which the system progresses from one state to the next.  An
informal security policy model may be presented in a natural language,
for example, English.  An explanation shall be provided demonstrating
that the informal model is sufficient to enforce the security
policy (Requirement 8).


At TCSEC class B2, a formal security policy model shall exist
(Requirement 13).  In addition to the B1 requirements, the formal
security policy model shall contain: a set of security properties
that captures the security policy, an abstract description of
the operations performed by the TCB, and a rigorous argument through
the use of predicate calculus that the description is consistent
- internally consistent, that is, is not self-contradictory.
The model shall include a proof that if the initial state of the
system satisfies the definition of a "secure" state
and if all assumptions of the model are satisfied, then all future
states of the system will be secure.


A security policy model provides assurance that the system has
been designed to enforce the security policy and provides a basis
for the TCB implementation.  As a means of increasing assurance,
the design documentation shall show that the security policy model
is sufficient to enforce the security policy of the system (Requirement
8).  At TCSEC class B1, it shall be sufficient to show this in
a natural language, e.g., English, but at class B2, this sufficiency
of the security policy model shall be shown through a formal proof
(Requirement 14).  The design documentation shall provide a mapping
of the security properties to the security policy.  This sufficiency
shall be demonstrated by describing how all aspects of the security
policy are addressed by the security policy model.


An example of a formal security policy that enforces the DoD security
policy is the Bell-La Padula model [1].  Although the Bell-La
Padula security policy model supports the DoD security policy,
it is important to realize that this does not mean that the Bell-La
Padula model model can be directly used for all systems.  The
Bell-La Padula model, when used with different systems, will need
to be representative of the system.


At TCSEC class B2, the TCSEC design specification and verification
requirement calls for a descriptive top level specification (DTLS)
of the TCB to be maintained to provide documentary evidence of
how the formal security policy model is implemented through the
TCB interface.  The DTLS provides evaluators with a better understanding
of the implementation of the reference monitor and provides maintenance
personnel with the necessary documentation to correct, modify,
or augment the TCB without destroying the TCB's cohesiveness and
internal consistency.  The description of the TCB should contain
a description of the services and functions provided by the TCB
and how they are invoked.  For example, for UNIX1-based systems,
the DTLS may be based upon enhanced manual pages.  The manual
pages shall include enough information to satisfy the design specification
and verification requirement that the TCB be described in "terms
of exceptions, error messages, and effects."[5] These individual
manual sections should be accompanied by detailed section headers
which clearly explain the security concepts and entities referenced
within each section.  The design documentation shall demonstrate
that the DTLS is an accurate description of the TCB interface
(Requirement 15).  It should do this by accurately and completely
describing the DTLS in relation to the TCB interface.


The design documentation shall be used to test against the TCB
to, "demonstrate that the TCB implementation is consistent
with the descriptive top level specification." [5]  The design
documentation shall describe how the TCB is structured to facilitate
this testing (Requirement 20).


At TCSEC class B3, the design documentation shall informally show
consistency of the TCB hardware, firmware, and software implementation
with the DTLS as well as showing correspondence between elements
of the DTLS and elements of the TCB (Requirements 29, 30).  The
goal of these two requirements is to ensure that the mapping between
the TCB and DTLS is complete, easily understandable, unambiguous,
and one-to-one between elements of the TCB implementation and
elements of the DTLS.  The level of detail of this mapping should
be sufficient for any inconsistency to be obvious to members of
the vendor's development team throughout the system life-cycle.


Also at TCSEC class B3, the design documentation shall provide
a mapping from the DTLS to the TCB implementation (Requirement
30).  This mapping should demonstrate that all elements that were
specified were, in fact, implemented, and that any code that appears
which is not specified directly merely reflects implementation
detail; that there are no new, unspecified, user interfaces implemented.
 With this mapping, the design documentation shall describe all
areas of correspondence between elements of the -


-----------------------------------


1 UNIX is a trademark of AT&T Bell Labs


DTLS and elements of the TCB.  For example, the mapping may be
pointers in the DTLS description to source code modules of the
TCB implementation.


The addition of the design specification and verification requirement
of a mapping between the DTLS and the formal security policy model
completes the evidence from the security policy to implementation.
 The entities of the model shall be shown to correspond to the
elements of the DTLS (Requirement 31).  This correspondence provides
assurance that the security properties that are proven in the
formal model are accurately reflected in the implementation.


At TCSEC class A1, the mapping shall be from the formal top level
specification (FTLS) to the TCB (Requirements 32, 33).  In addition,
the mapping to the model shall be from the FTLS (Requirement 35).
 These changes reflect the introduction of an FTLS requirement
in the design specification and verification requirements at A1.
 The design documentation shall describe how the FTLS accurately
represents the TCB interface.  The hardware/firmware components
of the TCB, such as mapping registers and direct memory access
input/output (I/O) components, that are directly or indirectly
visible at the TCB interface shall be described in the design
documentation.  As stated previously, the goal of these requirements
is to ensure that the mapping between the elements of the TCB
implementation and the FTLS are complete, easily understandable,
unambiguous, and one-to-one.


Although the TCSEC design documentation requirement changes at
class A1 to require a mapping from the FTLS to the TCB, a DTLS
is still required for class A1 systems.  At TCSEC class A1, the
DTLS serves to augment the FTLS by completing the description
of the TCB in an informal language and by providing the conceptual
glue to the specification of the reference monitor mechanism and
the other TCB components.  Since there is an explicit requirement
at class A1 that both the FTLS and DTLS correspond to the formal
security policy model, the DTLS and the FTLS must correspond (Requirement
36).  At TCSEC class A1, "the FTLS and DTLS may be two separate
documents, or they may be combined into a Complete Top Level Specification
(CTLS).  In a CTLS, the FTLS and DTLS portions (shall) be separately
identifiable.  The CTLS (shall) be a complete and accurate description
of the TCB, and it (shall) be sufficiently well commented/annotated
so that it can be easily understood with little or no knowledge
of formal specifications."[8]


It is recognized that not all of the TCB internals are able to
be specified within the FTLS.  For the hardware, firmware, and
software internal to the TCB, but not dealt with in the FTLS,
the design documentation shall describe them in complete, clear,
and careful detail (Requirement 34).
4.2 Documenting TCB Protection Mechanisms



As part of the description of the philosophy of protection and
how it translates into the TCB, the design documentation shall
include explanations of the security services offered by the TCB
software, hardware, and firmware mechanisms from a system level
view (Requirement 2).  At TCSEC class C1, the design documentation
for these protection mechanisms shall include how the mechanisms
protect the TCB from tampering (Requirements 5).  The description
of why the TCB is tamper resistant is an important requirement
for all of the TCSEC classes.  This design documentation requirement
supports the TCSEC class C1 system architecture requirement which
calls for the TCB to maintain a domain that implements the reference
monitor concept that, "protects it from external interference
or tampering"[5]. The mechanisms described in this section
of the design documentation include things such as Discretionary
Access Control (DAC) and identification and authentication (I&A)
mechanisms.  For example, the design documentation shall describe
the DAC enforcement mechanism and how it controls discretionary
access between named users or groups and named objects within
the ADP system.  As it relates to identification and authentication,
the design documentation shall describe how users are identified
to the TCB and the mechanism that authenticates the user's identity.
 Furthermore, the design documentation shall describe how the
TCB protects the authentication data.  To ensure that these mechanisms
have not failed in any way, hardware and software mechanisms shall
exist to periodically validate the correct operation of the on-site
hardware and firmware elements of the TCB.  These system integrity
mechanisms shall also be described in the design documentation.


At TCSEC class B1, the design documentation shall identify and
provide descriptions of the TCB protection mechanisms (Requirement
9).  This  documentation is required to provide the additional
assurance required at TCSEC class B1.  In most cases, these TCB
protection mechanisms at TCSEC class B1 may be the same protection
mechanisms that were described in TCSEC class C1, but at class
B1, the description of these mechanisms shall describe how they
support the additional system architecture requirement for process
isolation.  Process isolation mechanisms that prevent untrusted
subjects from directly accessing separate address spaces are introduced
at TCSEC class B1 and shall be described in the design documentation.
 The design documentation shall also show that all of the security
services required by the security policy model are provided by
the TCB mechanisms (Requirement 10).


At TCSEC class B2, the design documentation shall describe how
the TCB protection mechanisms implement the reference monitor
concept, i.e., is nonbypassable, always invoked, and small enough
to be analyzed (Requirement 16).  The design documentation requirement
should demonstrate how the reference validation mechanism is tamper
resistant, cannot be bypassed, is correctly implemented, and is
structured to enforce least privilege (Requirements 17, 18, 19,
21).  Although a reference monitor has been in place since TCSEC
class C1, the system architecture requirements at TCSEC class
B2 require that the TCB be protected, "from external interference
and tampering," maintain process isolation, and be "internally
structured into well defined largely independent modules."[5]
These additional requirements shall be reflected in the design
documentation.


One position for how the reference monitor hardware should be
documented is presented in the following paragraphs:


"For microprograms (firmware), design documentation is needed
for common routines, that is, documentation which fully describes
the functionality and what is done to implement that functionality.
 At the least, a high level view of major operations, e.g., interrupts,
I/O instruction interpretations is needed if the microcode is
not modular enough to be described in terms of microroutines.


For items inside the TCB, but outside of the reference monitor
(such as most disk controllers, printers, and other peripherals),
the interface must be described, but not the internals.


In the case of systems that do not use microcode, convincing arguments
must be provided as to what elements of the hardware are security
critical and why."[2]


The design documentation for the TCB firmware should parallel
the documentation that is written for the TCB software, that is,
it should fully describe the functionality and what is done to
implement the functionality of the security kernel.


Assurance needs to be provided that the TCB is protected from
modification.  At TCSEC class B2, the design documentation shall
provide this assurance through a description of why the reference
monitor is tamper resistant (Requirement 17).  This description
shall include the methods and mechanisms by which the TCB protects
itself from illicit modification.  Any hardware mechanisms used
by the TCB to separate protection critical elements from those
that are not protection critical shall be described.  The mechanisms
used by the TCB to help support logically distinct storage objects
with separate attributes shall also be described.  The mechanisms
used to protect against illicit modification may include some
of the same mechanisms used to mediate accesses of objects by
subjects that were introduced at TCSEC class C1.  These mechanisms
shall be described again at TCSEC class B2, but in greater detail
as to how they apply to the reference validation mechanism.


The previous paragraph explained how the design documentation
describes protection mechanisms, but more importantly, at TCSEC
class B2, the design documentation shall show that all of the
TCB software, firmware, and hardware mechanisms have been implemented
as described and that the implementation functions correctly (Requirement
19).  The design documentation shall justify the correctness of
the entire TCB.


Also, at TCSEC class B2, the design documentation shall describe
how the TCB is structured to enforce least privilege (Requirement
21).  This description shall relate to the hardware, firmware,
and software modules of the TCB, as well as to the enforcement
of least privilege both within the TCB and upon trusted subjects.
 Least privilege ensures that any TCB module or trusted process
has only those privileges and capabilities needed for it to perform
the specific function for which it was designed.  For example,
if the hardware architecture implements protection rings, a description
shall be given of the ring mechanisms.  This description shall
show how access to the innermost ring provides a means of running
highly privileged processes, while the outermost ring provides
a means of running unprivileged processes.  Likewise, the description
shall justify placement of functions within the higher privileged
rings and the conferring of special privileges to trusted processes.
 Thus, the hardware is shown as a means of enforcing least privilege.



Similarly, firmware and software mechanisms may provide a means
of enforcing least privilege.  For example, a labeling mechanism
may be implemented in software or firmware.  Because labels may
be used to enforce least privilege, the software or firmware modules
enforcing the labeling and label based access control shall be
shown as a means of enforcing least privilege.


The separation of administrative roles in the system is one more
way in which least privilege may be exercised.  In this case,
the roles of system administrator, security administrator, and/or
system auditor may be performed by separate individuals.  This
is to ensure that the security functions of the system are not
able to be performed by a single person.  The way these roles
are carried out in the system shall be described in the design
documentation.


At TCSEC class B3, the system architecture requirements call for
the TCB to be minimized, i.e., only security relevant functions
appear within the TCB.  The TCB at this class, "shall incorporate
significant use of layering, abstraction, and data hiding,"
and shall have minimal complexity.  The design documentation shall
describe how the system complies with these additional architectural
requirements in (Requirement 28).  As stated previously, as the
TCSEC classes increase and the implementation of the reference
monitor concept becomes more defined, the amount of design documentation
shall also increase.
4.3 Documentation of Covert Channels



A portion of the B2 requirements for design documentation addresses
covert channels.  The results of all covert channel analysis need
to be in the design documentation to aid in the design and development
of TCB mechanisms.  For this reason, the design documentation
shall present the results of the covert channel analysis and the
methodology used (Requirement 22).  The design documentation shall
provide an overview of the covert storage channel analysis and
testing procedures.  It shall document the results of these tests
and all of the covert channels identified.  All auditable events
shall be identified and described for all covert storage channels
that are not removed from the system (Requirement 24).


When covert channels are identified, actions are sometimes taken
to restrict the bandwidth of those channels.  The design documentation
shall describe and discuss these actions and the resulting degree
of covert channel restriction in light of performance degradation,
operational utility, or other considerations (Requirement 23).
 Processing delays resulting from reducing the number and bandwidth
of covert channels shall be identified and characterized.  The
design documentation shall also note whether the exploitation
of known covert channels is auditable.  There will be some covert
storage channels whose use will not be detectable by auditing
mechanisms.  The design documentation shall document the worst
case and expected case bandwidths of these storage channels whose
exploitation is not auditable (Requirement 25).


At TCSEC class B3, the design documentation shall recognize the
introduction of covert timing channels into the requirements and
shall consider them in all covert channel related descriptions
as stated above (Requirements 26, 27).  The covert timing channel
analysis and testing procedures, and the results obtained from
the tests shall be described in the design documentation.  Additionally
at TCSEC class A1, formal methods shall be used in the covert
channel analysis and shall be described in the design documentation.
5. OTHER TOPICS

5.1 Modularity



An important architectural feature of trusted systems for TCSEC
class B2 and above is that the TCB be modular.   The modularity
of the TCB is important for ease of understanding, ease of analysis,
and ease of maintenance.  Modularity ensures that interfaces are
well defined and errors are contained.  It also provides a basis
for enforcing least privilege.  The content of hardware and software
modules should be selected based on the following criteria: a
module performs exactly one well defined action, a module has
a well defined interface, a module interacts with other modules
only in well defined ways, and a module is called upon to perform
a function whenever that function is required.  Although TCB modularity
is not a requirement until class B2 (Requirements 11, 12), it
is possible that vendors would want to build systems with modular
TCBs at the lower classes.  Regardless of class, if the TCB is
modular, the design documentation shall describe how the TCB is
modular and the interfaces between the TCB modules (Requirement
3, 4).  As with all design documentation, the level of detail
shall permit the description of the interfaces between the modules
to be a useful description.  Specifically, the design documentation
shall include identification of the TCB hardware, software, and
firmware modules, why the modules are considered as such, the
interfaces between them, and the implementation of the modules.
 A mapping of the security services and mechanisms to the modules
should also be described.


The level of detail of the design documentation increases the
amount of assurance to be gained by the developers and evaluators.
 The description of the interfaces between the modules, whether
hardware, firmware, or software, shall describe the types and
sources of information passing between them (Requirement 4).
In addition, the interfaces between these TCB modules and other
system modules external to the TCB shall be described.  This description
is necessary to show that no breach of security can occur through
the interfaces.


In some cases, software modules may depend upon hardware or firmware
modules to perform correctly and these dependencies should also
be included in the design documentation.
5.2 Hardware Design Documentation



Hardware design documentation for a system shall be provided at
all levels of trust.  Specifically, at TCSEC classes B2 and above,
it is felt that the hardware design documentation is critical
to the security of a system.  At this class, systems "shall
make effective use of available hardware to separate those elements
that are protection critical from those that are not."[5]
To meet this TCSEC requirement, developers should know the hardware
base they are building on top of.  Also, evaluators will need
the hardware design documentation in order to evaluate that the
vendor is making "effective use" of the hardware.


The hardware design documentation includes descriptive information
about the system's Central Processing Unit(s) (CPU), Memory Management
Unit(s) (MMU), and all other additional processors, for example,
I/O Processors, channel devices.  The hardware design documentation
is intended to discuss what the hardware is meant to do, but does
not need to include details of implementation, such as the flow
of control to perform a specific action.  The hardware design
documentation for every logical module in the hardware base should
include a functional name, functional description, and a functional
interface of that module.


The hardware design documentation defines the hardware portion
of the TCB interface.  The information on the hardware interface
is important to correctly develop TCB routines and device drivers.
 Additionally, the hardware design documentation provides sufficient
information that the TCB meets the System Architecture requirements
of the TCSEC.


The information contained in the hardware design documentation
shall be complete, specifying all possible interfaces to the system
hardware, including the user-to-hardware interface as well as
the TCB software-to-hardware interface.  The hardware design documentation
should include unprivileged instructions, privileged instructions,
unpublished instructions, and all CPU-to-MMU, CPU-to-Channel,
CPU-to-I/O bus, and additional processor interactions.  Also,
the software interface visible registers that exist on the CPU,
MMU, and other processors shall be described in the hardware design
documentation.


The design documentation for some hardware modules may require
internal detail down to a bit level functional description of
the module.  The modules that fall into this category are those
that make up thereference monitor, such as the address translation
module, process isolation support module, fault handling module,
I/O control module, and the diagnostic module.  These hardware
modules of the reference monitor directly support security and
will require an explanation of why the reference monitor is "tamper
resistant, cannot be bypassed, and is correctly implemented."[5]
5.3 Configuration Management



The design documentation for the system shall be under configuration
management for the entire life-cycle of the system.  Design documentation
is only useful if it is complete and accurate.  This means that
any change to the system should also result in a change to the
design documentation for the system.


The design documentation for a system should be treated as a configuration
item for the system and should be subject to the identification,
control, accounting, and audit functions of configuration management.
 "Initial phases of configuration control are directed towards
control of the system configuration as defined primarily in design
documents.


Often a change to one area of a system may necessitate a change
to another area.  It is not acceptable to only write documentation
for new code or newly modified code, but rather documentation
for all parts of the TCB that were affected by the addition or
change shall be updated accordingly.  Although documentation may
be available, unless it is kept under configuration management
and updated properly it will be of little, if any use.  In the
event that the system is found to be deficient in documentation,
efforts should be made to create new documentation for areas of
the system where it is presently inadequate or nonexistent."[7]


The TCSEC requirements for configuration management do not begin
until TCSEC class B2, but this should not mean that the design
documentation for TCSEC class C1 to B1 systems not be under some
type of control.  At these lower classes, the control process
for the design documentation may be less formal than that required
by the configuration management requirements, but it should still
provide assurance that the design documentation accurately describes
the current system.


The National Computer Security Center has recently developed the
Ratings Maintenance Program (RAMP) which requires configuration
management at these lower classes of trust.  "By training
vendor personnel to recognize which changes may adversely affect
the implementation of the security policy of the system, and to
track these changes to the evaluated product through the use of
configuration management, RAMP will permit a vendor to maintain
the rating of the evaluated product without having to reevaluate
the new version." [7]


For further information about the RAMP program and about the configuration
management requirements for RAMP, contact:


National Computer Security Center


9800 Savage Road


Fort George G. Meade, MD  20755 6000


       Attention: C12
6. SUMMARY OF DESIGN DOCUMENTATION



Design documentation is responsible for describing systems at
all levels of trust.  During the life-cycle of a system, it describes
the system to facilitate changes and maintenance of the system.
 As it relates to the security of a system, design documentation
provides assurance by describing how a system provides trust and
shows that all of the protection mechanisms of a system are correctly
implemented and sufficiently provide the needed trust.  At the
lower classes, design documentation begins to describe how security
is provided in a system by stating the philosophy of protection
of the system.  At TCSEC class B1, the design documentation describes
the security policy model of a system, and at TCSEC class B2 the
security policy model is required to be formal.


Many of the other requirements in the TCSEC are related to design
documentation.  Design documentation shall describe how these
requirements are satisfied.  Covert channels are specifically
addressed in the design documentation requirement.  The assurance
provided by design documentation is dependent upon its thoroughness
and accuracy.  When design documentation is written, the role
that it plays in the system life-cycle should be kept in mind.
 A new employee should be able to look at the design documentation
and get an understanding of what the current system is and how
it works.  The key word in design documentation is current.  When
a system changes, the design documentation shall change accordingly.
 By accurately describing a system, design documentation provides
assurance that there is an understanding of how and why the system
provides trust.  In addition, it provides information that will
enable developers to analyze changes to the system to ensure that
they do not adversely affect the trustworthiness of the system.
APPENDIX B

EXCERPTS FROM FINAL EVALUATION REPORTS



This appendix reproduces excerpts from Final Evaluation Reports
for products currently on the Evaluated Products List.  The excerpts
are reproduced from the "Applicable Features" portion
of the section describing how the product met the requirements
for Design Documentation.


The Final Evaluation Reports are available from the National Computer
Security Center.  However, most of the vendor documents mentioned
in this appendix contain proprietary information, and therefore
are not publicly available.  Please do not request copies of the
vendor documents from the National Computer Security Center.
B.1 CLASS C2

B.1.1 UTX/32S[6]



The following documents were provided to the evaluation team in
fulfillment of the Design Documentation criterion:


"Security Policy Model"


"Program Maintenance Manual UTX/32S, Release 1.0 (DRAFT)".


"System Calls" and "Maintenance" in the "System
Administrator's Reference Manual".


"4.2BSD and UTX-32 Differences Study for Gould


UTX/32S"


"Memory Management for Gould UTX/32S"


"Object Reuse Study for Gould UTX/32S"


The "Gould UTX/32S 1.0 Security Policy Model" describes
Gould's philosophy of protection and explains how this philosophy
is translated into the TCB.  It identifies all elements comprising
 the TCB, including the kernel, programs, data files, and processes.
 Subjects and objects are identified, and the mediation of accesses
between them is described.  A mapping from the TCB to the security
philosophy is provided, and the discretionary access control,
identification and authentication, and audit features and mechanisms
are described.  Additionally,  the document discusses the role
of secure sockets in  interprocess communications.  The "Gould
UTX/32S 1.0 Security Policy Model" identifies all programs
comprising the TCB.


The kernel interface is described by the "System Calls"
section of the "System Administrator's Reference Manual".
 The "Maintenance" section of the reference manual comprises
manual pages useful for systems programmers in maintaining UTX/32S.
"4.2BSD and UTX-32 Differences Study for Gould UTX/32S"
describes differences between 4.2BSD UNIX and Gould UTX/32
1.2. Using "4.2BSD and 4.3BSD as Examples of the UNIX
System," by J.S. Quarterman, A. Silberschatz, and J.L. Peterson
(Computing  Surveys, Vol. 17, No. 4, December 1985, pp. 379-418),
as a  baseline, the document identifies all instances where Gould
UTX/32 differs from the described UNIX system.



The "UTX/32S Program Maintenance Manual" describes code
modifications made to UTX/32 to meet the requirements of the "Gould
UTX/32S 1.0 Security Policy Model".  The document includes
an overview of the mechanisms implemented in UTX/32S to strengthen
security and to correct problems found in UTX/32 and  other UNIX
systems, and detailed descriptions for: the implementation of
trusted servers to replace the functionality of the eliminated
setuid and setgid bits; kernel modifications; auditing mechanisms;
and additions, deletions, and modifications  to utilities and
libraries.  Each module description includes an overview, a functional
specification, and a design specification.  Pointers to source
code, which Gould made  available to the evaluation team are provided.


Security critical features of the Gould PowerNode hardware used
 by UTX/32S are described in "UTX/32S Traps and Interrupts
and Memory Management for Gould UTX/32S."  "UTX/32S
Traps and  Interrupts" describes how UTX/32S makes use of
the trap and interrupt facilities to interface with the hardware
and process  environments.  "Memory Management for Gould
UTX/32S" describes how UTX/32S uses the memory management
facilities of the  PowerNode hardware to provide the process environment.
 Both  documents include applicable material from "Gould
SS 6 (Virtual  Mode), V6, and V9 Central Processing Unit Reference
Manual".


"Object Reuse Study for Gould UTX/32S" provides details
regarding how UTX/32S hardware and software manage system objects.
 This  study identifies the system resources which can be allocated
and deallocated, and details the strategies used to ensure that
one process cannot gain access to the resources or data previously
allocated to another process.  This study, along with "Memory
Management for Gould UTX/32S", provides a good description
of UTX/32S design features which are used to meet the Object Reuse
criterion.
B.2 CLASS B2

B.2.1 Multics3]



The following documents satisfy the Design Documentation  requirement:


Applicable Features


The "Computer Security Model: Unified Exposition and MULTICS
Interpretation" provides a description of Honeywell's philosophy
for protection and how this is translated into the TCB.  The security
model enforced by the TCB is the Bell-La Padula model.


Multics has a set of Multics Design Documents (MDDs) that describe
the TCB.  (These documents are Honeywell Internal Documentation
and are available only through the vendor by request.  Honeywell
reserves the right to deny such requests.)  The MDDs are organized
by major TCB service or function.  These design documents describe
the interfaces between TCB modules, how the TCB implements the
reference monitor, and how the TCB is structured to facilitate
testing and enforce least privilege.


These documents coupled with the Honeywell produced "Multics
Interpretation," referenced in the previous paragraph identify
the security protection mechanisms and explain how they satisfy
the model.


The DTLS is an accurate description of the TCB interface.


The "Covert Channel Analysis" describes all identified
covert channels, how they can and cannot be restricted, how they
are audited, and their bandwidths.
B.3 CLASS A1

B.3.1 SCOMP [4]



The following documents satisfy the Design Documentation requirement:
The manufacturer's philosophy of protection is documented in
"SCOMP:  A Solution to the Multilevel Security Problem"
and its translation into the TCB given in "SCOMP Trusted
Computing Base."  The interfaces between the TCB modules
are described in the several Part II specifications,


"Detail Specification for SCOMP Kernel Part I,


Release 2.1"


"Detail Specification for SCOMP Kernel Part II,


Release  2.1"


A formal description of the security policy model (Bell-La Padula)
that is enforced by the TCB is given in "Secure Computer
Systems", for the general case and Multics in particular
in "Computer Security Model:


Unified Exposition and MULTICS Interpretation".  The Bell-La
Padula Model has been accepted by the National Computer Security
Center to model security policy "Security Requirements for
Automatic Data Processing Systems" and to be consistent with
its axioms.  An interpretation of the model for the SCOMP system
is given in "SCOMP Interpretation of the Bell-La Padula Model."


The specific TCB protection mechanisms are 1) protection rings,
2) SPM  mediation of per user virtual memory, 3) minimum privilege
for each TCB function, 4) integrity levels  for users, operators,
administrators, and security administrators, 5) individual trusted
software processes for  separate functions, and 6) ring gates
and checks on parameter passing.  The Part II Specifications previously
referenced provide the necessary documentation for satisfaction
of this requirement.  The explanation given to show that the TCB
protection mechanisms satisfy the model appears in "SCOMP
 Interpretation of the Bell-La Padula Model."


Section 3 of "SCOMP Trusted Computing Base" describes
the SCOMP TCB reference monitor implementation.  An analysis of
 the Reference Monitor appears in Appendix C and concludes that
the informal proofs that the SCOMP system implements the reference
monitor concept are adequate.


The TCB implementation was shown to be consistent with the FTLS
by specification to source code mappings


"FTLS to Code Mapping for the SCOMP Kernel Software"


"FTLS to Code Mapping for SCOMP Trusted Software"


"Justification for Unspecified Code for the SCOMP Kernel
Software, Release 2.1"


"Justification for Unspecified Code for SCOMP Trusted Software"


TCB testing is documented in:


"SCOMP Kernel Test Procedures"


"SCOMP Kernel Functional Test Summary"


"Kernel Software Test Report for the SCOMP,
Release 2.1"



"Trusted Software Test Plan for the SCOMP"


"Trusted Software Test Report for the SCOMP,
STOP Release 2.0"



"Trusted Software Test Report for the SCOMP,


Appendix A:Test Programs, Appendix B: Test Results"


"SCOMP Test and Verification Software


Description, Rev.  3"


The TCB structure provided added assurance of the validity of
the testing and helped to demonstrate the implementation of least
privilege.  The results of the covert channel analysis including
conservative bandwidth estimates are presented in "Covert
Channels in the SCOMP Kernel" and "Flow and Covert Channel
Analysis for SCOMP Trusted Software, Release 2.1." 61 Auditable
events, identified in Section 13 of "SCOMP Trusted Facility
Manual, STOP Release 2.1", and the scheme of randomly selected
delays on exception returns appear to satisfactorily limit the
utility of the identified covert channels.


Finally, the internal TCB mechanisms that are not security related
and hence not dealt with in the FTLS are described in the commercial
Honeywell Level 6 documentation ("Honeywell Level 6 Minicomputer
Systems Handbook, CC71" and "Honeywell Level 6 Communications
Handbook, AT97-02D"), and the SCOMP system unique specifications:


"Detail Specification for SCOMP Kernel Part I,Release 2.1"


"Detail Specification for SCOMP Kernel Part II, Release 2.1".
GLOSSARY

Access



A specific type of interaction between a subject and an object
that results in the flow of information from one to the other.[9]
Access Attribute



Characteristic of an access of an object that specifies possible
results of the access.  Four example access attributes follow:
execute (processing based upon  the object accessed, but neither
altering nor viewing  capability); read (viewing but not altering
capability); append (altering but not viewing capability); and
write  (both altering and viewing capabilities).[1]
Audit Trail



A chronological record of system activities that is sufficient
to enable the reconstruction, reviewing, and examination of the
sequence of environments and activities surrounding or leading
to an operation, a procedure, or an event transaction from its
inception to final results.[9]
Covert Channel



A communication channel that allows two cooperating processes
to transfer information in a manner that violates the system's
security policy.  Also called confinement channel.[9]
Covert Storage Channel



A covert channel that involves the direct or indirect writing
of a storage location by one process and the direct or indirect
reading of the storage location by another process.  Covert storage
channels typically involve a finite resource (e.g., sectors on
a disk) that is shared by two subjects at different security levels.[5]
Covert Timing Channel



A covert channel in which one process signals information to another
by modulating its own use of system resources (e.g., CPU time)
in such a way that this manipulation affects the real response
time observed by the second process.[5]
Descriptive Top Level Specification (DTLS)



A top level specification that is written in a natural language
(e.g.,  English), an informal program design notation, or a combination
of the two.[5]
Formal Security Policy Model



A mathematically precise statement of a security policy.  To be
adequately precise, such a model must represent the initial state
of a system, the way in which the system progresses from one state
to another,  and a definition of a "secure" state of
the system.  To be  acceptable as a basis for a TCB, the model
must be supported  by a formal proof that if the initial state
of the system satisfies the definition of a "secure"
state and if all  assumptions required by the model hold, then
all future states of the system will be secure.  Some formal modeling
techniques include: state transition models, temporal logic models,
denotational semantics models, algebraic  specification models.
 An example is the model described by Bell-La Padula.[9]
Formal Top Level Specification (FTLS)



A top level specification that is written in a formal mathematical
language to allow theorems showing the correspondence of the system
specification to its formal requirements to be hypothesized and
formally proven.[9]
Least Privilege



This principle requires that each subject in a  system be granted
the most restrictive set of privileges needed for the performance
of authorized tasks.  The application of this principle limits
the damage that can result from accident, error, or unauthorized
use.[9]
Object



A passive entity that contains or receives information.  Access
to an object potentially implies access to the  information it
contains.  Examples of objects are: records, blocks, pages, segments,
files, directories, directory trees, and programs, as well as
bits, bytes, words, fields, processors, video displays, keyboards,
clocks, printers, and  network nodes.[9]
Reference Monitor Concept



An access control concept that refers to an abstract machine that
mediates all accesses to objects by subjects.[9]
Security Kernel



The hardware, firmware, and software elements of the Trusted Computing
Base that implement the reference monitor concept.  It must mediate
all accesses, be protected from modification, and be verifiable
as correct.[9]
Security Level



The combination of hierarchical classification and a set of nonhierarchical
categories that represents the sensitivity of information.[9]
Security Mechanism



A system or means of implementing a security service within a
system.
Security Policy



The set of laws, rules, and practices that regulate how an organization
manages, protects, and distributes sensitive information.[9]
Security Policy Model



A formal presentation of the security policy enforced by the system.
 It must identify the set of rules and practices that regulate
how a system manages, protects, and distributes sensitive information.[9]
Security Service



A system or method of providing a security relevant feature in
the system.
Sensitivity Label



A piece of information that represents the security level of an
object.  Sensitivity labels are used by the TCB as the basis for
mandatory access control decisions.[9]
Subject



An active entity, generally in the form of a person, process,
or device that causes information to flow among objects or changes
the system state.  Technically, a process/domain pair.[9]


Trusted Computing Base (TCB)


The totality of protection mechanisms within a computer system-including
hardware, firmware, and software-the combination of which is responsible
for enforcing a security policy.  It creates a basic protection
environment and provides additional user services required for
a trusted computer system.  The ability of a trusted computing
base to correctly enforce a security policy depends solely on
the mechanisms within the Trusted Computing Base and on the correct
input by system administrative personnel of parameters (e.g.,
a user's clearance level) related to the security policy.[9]
REFERENCES


 • 1. Bell, D.E., and L.J.  La Padula, "Secure Computer
System: Unified Exposition and Multics Interpretation," MTR-2997,
Mitre Corp., Bedford, MA, July 1975.
 • 2. Gabriele, Mark D., Excerpts from the Criteria Discussions
Forum on the Dockmaster computer system at the National Computer
Security Center, Forum entry #0680,  Hardware Design Documentation
at B2 and Above, May 9,  1987.
 • 3. National Computer Security Center, Final Evaluation Report
of Honeywell Multics MR11.0, CSC-EPL-85/003, June 1,  1986.
 • 4. Department of Defense Computer Security Center, Final Evaluation
of SCOMP, Secure Communications Processor, STOP  Release 2.1,
CSC-EPL-85/001, September 23, 1985.
 • 5. Department of Defense Standard, Department of Defense Trusted
Computer System Evaluation Criteria, DoD 5200.28- STD, December
1985.
 • 6. National Computer Security Center, Final Evaluation Report
of Gould, Inc., Computer Systems Division, UTX/32S, Release 1.0,
CSC-EPL-86/007, December 31, 1986.
 • 7. National Computer Security Center, A Guide to Understanding
Configuration Management in Trusted Systems, NCSC-TG-006, March
28, 1988.
 • 8. National Computer Security Center, Criterion Interpretation,
Report No. C1-CI-01-87, 1987.
 • 9. National Computer Security Center, Glossary of Computer
Security Terms, NCSC-TG-004-88, October 1988.





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