







NCSC-TG-009 - Computer Security Subsystems





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


 •  TCSEC Quote:







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


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

 •  Interpretation:







The following interpretation, in addition to the interpretations
and


requirements for the DAC/D2 class, shall be satisfied for the
DACID3 class.
2.1.3.3.1 Access control lists for each object



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



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


Subsystems at the DAC/Dl class must comply with:

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







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

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





2.1.5 Documentation Requirements for DAC Subsystems



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


Subsystems at the DAC/Dl class must comply with:

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







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

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





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.

 •  System Architecture (D2)
 •  System Integrity (D2)
 •  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.

 •  Security Features User's Guide (D2)
 •  Trusted Facility Manual (D2)
 •  Test Documentation (D2)
 •  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: 

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







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

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





2.3.5 Documentation Requirements for I&A Subsystems



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


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

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







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

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


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


 •  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."

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


2.4.3.2.1 Real-time alarms

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

2.4.4 Assurance Requirements for Auditing Subsystems



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

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





2.4.5 Documentation Requirements for Auditing Subsystems



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

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





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.

 •  Interpretation for DAC Subsystems:







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

 •  Interpretation for Auditing Subsystems:







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

 •  Interpretation for Object Reuse Subsystems







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

 •  Interpretation for I&A Subsystems:







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

3.2 SUBSYSTEM INTEGRITY



Subsystem integrity evaluation is designed to provide operational
assurances with regard to the correct operation of the protection
mechanisms of the subsystem and its interfaces to the protected
system.
3.2.1 Integity:D1


 • TCSEC Quote 







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

 • Interpretation:







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


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

 • Rationale/Discussion 







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



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