NCSC-TG-009 - Computer Security Subsystems

     Library No. S230,512 
     Version 1 

FOREWORD

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

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

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

PATRICK R GALLAGHER, JR. 16 September 1988 

Director National Computer Security Center 

Computer Security Subsystems ACKNOWLEDGEMENT 

ACKNOWLEDGEMENT

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

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

1. INTRODUCTION

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

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

1.1 PURPOSE

This Interpretation has been prepared for the following purposes: 

     1. to establish a standard for manufacturers as to what security
     features and assurance levels to build into their new and planned
     computer security subsystem products to provide widely available
     products that satisfy trust requirements for sensitive applications; 
     2. to provide a metric to evaluate the degree of trust that can be
     placed in a subsystem for protecting classified and sensitive
     information; 
     3. to lend consistency to evaluations of these products by explicitly
     stating the implications that are in the TCSEC; and 
     4. to provide the security requirements for subsystems in acquisition
     specifications. 

1.2 BACKGROUND

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

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

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

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

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

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

1.3 SCOPE

This document interprets the security feature, assurance and documentation
requirements of the TCSEC for subsystem evaluations. In this
interpretation, the functional requirements of the TCSEC are divided into
four general categories: 

     1. Discretionary Access Control (DAC) 
     2. Object Reuse (OR). 
     3. Identification and Authentication (I&A) 
     4. Audit (AUD) 

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

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

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

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

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

1.4 EVALUATION OF SUBSYSTEMS

1.4.1 Basis for Evaluation

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

TABLE 1.1. Possible Subsystem Ratings 

SUBSYSTEM FUNCTION                   POSSIBLE RATINGS                      

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

Object Reuse                         OR/D,OR/D2                            

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

Audit                                AUD/D, AUD/D2, AUD/D3                 



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

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

The D3 class contains additional requirements and interpretations only for
functionality, not for assurances or documentation. So, subsystems with
this rating will adhere to the D2 assurance and documentation requirements
and interpretations. 

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

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

1.4.2 Integration Requirements

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

Most all computer security subsystems will rely on other security-relevant
functions in the enviromnent where they are implemented. Audit subsystems,
for example, depend on an identification and authentication function to
provide the unique user identities that are necessary for individual
accountability. Also, it is important to realize that some of these
functions may be dependent on each other in a cyclic fashion (e.g., I&A
depends on DAC and DAC depends on I&A). In these cases, the cyclic
dependencies should be removed either by complete integration of the
functions or by modularizing the functions in a way that allows linear
dependencies. Tl~is latter method is termed "sandwiching" and it requires
the splitting of one function and surroundmg the other dependent function
with the two functions resulting from the split. For example, in the case
of DAC and I&A cyclic dependencies, one might split I&A into two parts so
that there is a system I&A, a DAC subsystem, and a DAC module containing
its own I&A functionality. 

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

TABLE 1.2. Required Supporting Functions 

SUBSYSTEM FUNCTION                   REQUIRED SUPPORTING FUNCTIONS         

Discretionary Access Control         I&A, Audit                            

Object Reuse                         None                                  

Identification & Authentication      Audit,DAC2, Audit, I&A, DAC2          



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

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

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

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

1.4.3 WARNING

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

2. FEATURE REQUIREMENTS

2.1 DISCRETIONARY ACCESS CONTROL DAC) SUBSYSTEMS

2.1.1 Global Description of Subsystem Features

2.1.1.1 Purpose

This subsystem provides user-specified, controlled sharing of resources. 

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

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

2.1.1.2 Role Within Complete Security System

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

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

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

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

2.1.2 Evaluation of DAC Subsystems

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

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

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

2.1.3 Feature Requirements For DAC Subsystems

2.1.3.1 DAC/Dl

     TCSEC Quote: 

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

     Interpretation: 

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

2.1.3.1.1 Identified users and objects

DAC subsystems must use some mechanism to determine whether users are
authorized for each access attempted. At DAC/Dl, this mechanism must
control access by groups of users. The mechanisms that can meet this
requirement include, but are not limited to: access control lists,
capabilities, descriptors, user profiles, and protection bits. The DAC
mechanism uses the identification of subjects and objects to perform
access control decisions. This implies that the DAC subsystem must
interface with or provide some I&A mechanism. The evaluation shall show
that user identities are available to DAC. 

2.1.3.1.2 User-specified object sharing

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

2.1.3.1.3 Mediation

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

2.1.3.2 DAC/D2

     TCSEC Quote: 

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

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

     Interpretation: 

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

2.1.3.2.1 DAC/D2

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

2.1.3.2.2 Authorized user-specified object sharing

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

2.1.3.2.3 Default protection

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

2.1.3.3 DAC/D3

     ž TCSEC Quote: 

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

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

     ž Interpretation: 

The following interpretation, in addition to the interpretations and 

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

2.1.3.3.1 Access control lists for each object

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

2.1.4 Assurance Requirements for DAC Subsystems

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

Subsystems at the DAC/Dl class must comply with: 

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

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

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

2.1.5 Documentation Requirements for DAC Subsystems

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

Subsystems at the DAC/Dl class must comply with: 

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

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

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

2.2 OBJECT REUSE SUBSYSTEMS

2.2.1 Global Description of Subsystem Features

2.2.1.1 Purpose

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

2.2.1.2 Role Within the Complete Security System

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

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

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

2.2.2 Evaluation of Object Reuse Subsystems

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

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

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

2.2.3 Feature Requirements for Object Reuse Subsystems

2.2.3.1 OR/D2

     TCSEC Quote: 

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

     Interpretation: 

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

     Rationale/Discussion: 

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

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

2.2.4 Assurance Requirements for Object Reuse Subsystems

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

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

2.2.5 Documentation Requirements for Object ReuseSubsystems 

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

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

2.3 IDENTICATION & AUTHENTICATION (I&A) SUBSYSTEMS 

2.3.1 Global Description of Subsystem Features

2.3.1.1 Purpose

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

2.3.1.2 Role Within Complete Security System

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

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

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

2.3.2 Evaluation of I&A Subsystems

Subsystems which are designed to implement I&A must comply with all of the
TCSEC requirements outlined below for features, assurances, and
documentation. Compliance with these requirements will assure that the
subsystem can enforce, either wholly or in part, a specific I&A policy. As
a part of the evaluation, the subsystem vendor shall set up the subsystem
in a typical functional configuration for security testing. This will show
that the subsystem interfaces correctly with the protected system to meet
all of the feature requirements in this section and all of the assurance
and documentation requirements in Sections 3 and 4. It will also show that
the subsystem can be integrated into a larger system environment. 

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

2.3.3 Feature Requirement for I&A Subsystems

2.3.3.1 I&A/Dl

     TCSEC Quote: 

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

     Interpretation: 

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

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

     Rationale/Discussion: 

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

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

2.3.3.2 I&A/D2

     TCSEC Quote: - 

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

     Interpretation ~ 

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

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

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

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

     Rationale/Discussion: 

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

2.3.4 Assurance Requirements for I&A Subsystems

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

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

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

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

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

2.3.5 Documentation Requirements for I&A Subsystems

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

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

     ž Security Features User's Guide (Dl) 
     ž Trusted Facility Manual (Dl) 
     ž Test Documentation (Dl) 
     ž Design Documentation (Dl) 

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

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

2.4 AUDlT SUBSYSTEMS

2.4.1 Global Description of Subsystem Features

2.4.1.1 Purpose

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

2.4.1.2 Role Within Complete Security System

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

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

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

2.4.2 Evaluation of Auditing Subsystems

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

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

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

2.4.3 Feature Requirements For Auditing Subsystems

2.4.3.1 AUD/D2

     TCSEC Quote: 

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

     Interpretations: 

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

2.4.3.1.1 Creation and management of audit trail

The auditing subsystem shall create and manage the audit trail of
security-relevant " events in the system. If the other portions of the
system are unable to capture data about such events, the auditiug
subsystem shaU coutain the necessary interfaces into the system to perform
this function. Alternatively, the auditing subsystem might simply accept
and store data about events if the other portions of the system are
capable of creating such data and passing them on. 

     Rationale/Discussion: 

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

2.4.3.1.2 Protection of audit data

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

     Rationale/Discussion: 

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

2.4.3.1.3 Access control to audit

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

     Rationale/Discussion: 

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

2.4.3.1.4 Specific types of events

Data about all security relevant events must be recorded. The other
portions of the system shall be able to pass data concerning these events
to the auditing subsystem, or the auditing subsystem shall have the
necessary code integrated into the other portions of the system to pass
the data to the collection point. 

2.4.3.1.5 Specific infolmation per event

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

     Rationale/Discussion: 

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

2.4.3.1.6 Ability to selectively audit individuals

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

     Rationale/Discussion: 

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

2.4.3.2 AUD/D3

     ž TCSEC Quote: 

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

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

2.4.3.2.1 Real-time alarms

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

2.4.4 Assurance Requirements for Auditing Subsystems

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

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

2.4.5 Documentation Requirements for Auditing Subsystems

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

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

3. ASSURANCE REQUIREMENTS

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

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

3.1 SUBSYSTEM ARCHITECTURE

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

3.1.1 Arch:D1

     TCSEC Quote: 

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

     Interpretation: 

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

3.1.1.1 Execution Domain Protection

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

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

     Rationale and Discussion: 

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

3.1.1.2 Defined Subsets

I&A subsystems, when used for the system's I&A, define the subset of
subjects under the control of the system's TCB. DAC subsystems may protect
a subset of the total collection of objects on the protected system. 

3.1.2 Arch:D2

     TCSEC Quotes: 

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

     Interpretation: 

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

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

     ž Interpretation for DAC Subsystems: 

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

     ž Interpretation for Auditing Subsystems: 

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

     ž Interpretation for Object Reuse Subsystems 

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

     ž Interpretation for I&A Subsystems: 

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

3.2 SUBSYSTEM INTEGRITY

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

3.2.1 Integity:D1

     TCSEC Quote 

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

     Interpretation: 

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

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

     Rationale/Discussion 

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

3.2.2 Integrity:D2

There are no additional requirements for System Integrity at D2. 

3.3 SECURITY TESTING

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

3.3.1 Test:Dl

     TCSEC Quote: 

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

     Interpretation 

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

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

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

     Rational/Discussion: 

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

3.3.2 Test:D2

     TCSEC Quote: 

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

     Interpretation: 

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

     Rationale/Discussion 

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

4. DOCUMENTATION REQUIREMENTS

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

4.1 SECURITY FEATURES USER'S GUIDE

4.1.1 SFUG:Dl

     TCSEC Quote: 

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

     Interpretation: 

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

     Rationale/Discussion: 

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

4.1.2 SFUG:D2

     Interpretation: 

There are no additional requirements at the D2 class. 

4.2 TRUSTED FACILITY MANUAL

4.2.1 TFM:Dl

     TCSEC Quote : 

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

     Interpretation: 

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

4.2.2 TFM:D2

     TCSEC Quote: 

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

     Interpretation: 

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

4.3 TEST DOCUMENTATION

4.3.1 TD:Dl

     TCSEC Quote: 

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

     Interpretation: 

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

4.3.2 TD:D2

     Interpretation 

There are no additional requirements at the D2 class. 

4.4 DESIGN DOCUMENTATION

4.4.1 DD:Dl

     TCSEC Quote: 

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

     Interpretation: 

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

4.4.2 DD:D2

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

5- GLOSSARY

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

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

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

Authenticate - To establish the validity of a claimed identity. 

Authorization - Permission which establishes right to access information. 

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

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

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

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

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

Product evaluation - Thc technical evaluation of a product's security
features to determine the level of trust that can be placed in that
product as defined by thc NCSC. evaluation criteria for that type of
product (e.g., operating system, database management system, computer
network, computer security subsystem). Product evaluations do not consider
the application of the product in the evaluation. 

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

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

Subsystem - See "computer security subsystem." 

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

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