CSC-STD-004-85: Yellow book

by The Editor [Published on 16 Oct. 2002 / Last Updated on 24 Jan. 2013]

Technical Rationale Behind CSC-STD-003-85: Computer Security Requirements - Guidance for Applying the DoD TCSEC in Specific Environments


 
                                                 CSC-STD-004-85







              TECHNICAL RATIONAL BEHIND CSC-STD-003-85:
                   COMPUTER SECURITY REQUIREMENTS


              GUIDANCE FOR APPLYING THE DEPARTMENT OF DEFENSE
                TRUSTED COMPUTER SYSTEM EVALUATION CRITERIA
                         IN SPECIFIC ENVIRONMENTS








              Approved for public release;
              distribution unlimited.


              25 June 1985





                                            CSC-STD-004-85
                                        Library No. S-226,728


                        FOREWORD

This publication, Technical Rationale Behind CSC-STD-003-85: Computer Security
Requirements--Guidance for Applying the Department of Defense Trusted Computer
System Evaluation Criteria in Specific Environments, is being issued by the DoD
Computer Security Center (DoDCSC) under the authority of and in accordance with
DoD Directive 5215.1, "Computer Security Evaluation Center." This document
presents background discussion and rationale for CSC-STD-003-85, Computer
Security Requirements--Guidance for Applying the Department of Defense Trusted
Computer System Evaluation Criteria in Specific Environments. The computer
security requirements identify the minimum class of system required for a given
risk index. System classes are those defined by CSC-STD-001-83, Department of
Defense Trusted Computer System Evaluation Criteria, 15 August 1983. Risk index
is defined as the disparity between the minimum clearance or authorization of
system users and the maximum sensitivity of data processed by the system. This
guidance is intended to be used in establishing minimum computer security
requirements for the processing an-or storage and retrieval of sensitive or
classified information by the Department of Defense whenever automatic data
processing systems are employed. Point of contact concerning this publication is
the Office of Standards and Products, Attention: Chief, Computer Security
Standards.


                                 25 June 1985
Robert L. Brotzman
Director
DoD Computer Security Center



                ACKNOWLEDGMENTS

Special recognition is extended to H.  William Neugent and Ingrid M.  Olson of
the MITRE Corporation for performing in-depth analysis of DoD policies and
procedures and for preparation of this document.

Acknowledgment is given to the following for formulating the computer security
requirements and the supporting technical and procedural rationale behind these
requirements: Col Roger R. Schell, formerly DoDCSC, George F. Jelen, formerly
DoDCSC, Daniel J. Edwards, Sheila L. Brand, and Stephen F. Barnett, DoDCSC.

Acknowledgment is also given to the following for giving generously of their
time and expertise in the review and critique of this document: CDR Robert
Emery, OJCS, Dan Mechelke, 902nd Ml Gp, Mary Taylor, DAMI-CIC, Maj.  Freeman,
DAMI- CIC, Ralph Neeper, DAMI-CIC, Duane Fagg, NAVDAC, H.  O.  Lubbes, NAVELEX,
Sue Berg, OPNAV, Susan Tominack, NAVDAC, Lt Linda Fischer, OPNAV, Eugene
Epperly, ODUSD(P), Maj Grace Culver, USAF-SITT, Capt Mike Weidner, ASPO, Alfred
W.  Arsenault, DoDCSC, James P.  Anderson, James P.  Anderson & Co., and Dr.
John Vasak, MITRE Corporation.





























                                    ii


               TABLE OF CONTENTS
FOREWORD.............................................................   i
ACKNOWLEDGMENTS......................................................  ii
LIST OF TABLES.......................................................  iv
1.0 INTRODUCTION.....................................................   1
2.0 RISE INDEX.......................................................   5
3.0 COMPUTER SECURITY REQUIREMENTS FOR OPEN
    SECURITY ENVIRONMENTS............................................   11
4.0 COMPUTER SECURITY REQUIREMENTS FOR CLOSED
    SECURITY ENVIRONMENTS............................................   19
APPENDIX A: SUMMARY OF CRITERIA......................................   23
APPENDIX B: DETAILED DESCRIPTION OF CLEARANCES
     AND DATA SENSITIVITIES..........................................   27
APPENDIX C: ENVIRONMENTAL TYPES......................................   31
GLOSSARY.............................................................   33
ACRONYMS.............................................................   37
REFERENCES...........................................................   39






















                                   iii


                 LIST OF TABLES
Table
1: Rating Scale for Minimum User Clearance.........................    6
2: Rating Scale for Maximum Data Sensitivity.......................    7
3: Security Risk Index Matrix......................................    8
4: Computer Security Requirements for Open Security Environments...   12
5: Security Index Matrix for Open Security Environments............   13
6: Computer Security Requirements for Closed Security Environments.   20
7: Security Index Matrix for Closed Security Environments..........   21






























                                    iv

 1.0 INTRODUCTION
The purpose of this technical report is to present background discussion and
rationale for Computer Security Requirements--Guidance for Applying the DoD
Trusted Computer System Evaluation Criteria in Specific Environments(1)
(henceforth referred to as the Computer Security Requirements).  The
requirements were prepared in compliance with responsibilities assigned to the
Department of Defense (DoD) Computer Security Center (DoDCSC) under DoD
Directive 5215.1, which tasks the DoDCSC to "establish and maintain technical
standards and criteria for the evaluation of trusted computer systems."(2)

DoD computer systems have stringent requirements for security. In the past,
these requirements have been satisfied primarily through physical, personnel,
and information security safeguards.(3) Recent advances in technology make it
possible to place increasing trust in the computer system itself, thereby
increasing security effectiveness and efficiency. In turn, the need has arisen
for guidance on how this new technology should be used. There are two facets to
this required guidance:

     a.  Establishment of a metric for categorizing systems according to the
         security protection they provide.

     b.  Identification of the minimum security protection required in
         different environments.

The DoD Trusted Computer System Evaluation Criteria (henceforth referred to
as the Criteria), developed by the DoDCSC, satisfy the first of these two
requirements by categorizing computer systems into hierarchical security
classes.(4) The Computer Security Requirements satisfy the second requirement
by identifying the minimum classes appropriate for systems in different risk
environments. They are to be used by system managers in applying the Criteria
and thereby in selecting and specifying systems that have sufficient security
protection for specific operational environments.

Section 2 of this document discusses the risk index.  Section 3 presents a
discussion of the Computer Security Requirements for open security
environments.  Section 4 presents a discussion of the Computer Security
Requirements for closed security environments. A summary of the Criteria is
contained in Appendix A.  Appendix B contains a detailed description of
clearances and data sensitivities, and Appendix C describes the environmental
types.  A glossary provides definitions of many of the terms used in this
document.

1.1 Scope and Applicability

This section describes the scope and applicability for both this report and the
Computer Security Requirements. The primary focus of both documents is on the
technical aspects (e.g., hardware, software, configuration control) of computer
security, although the two documents also address the relationship between
computer security and physical, personnel, and information security.  While

                                    2


communications and emanations security are important elements of system
security, they are outside the scope of the two documents.

Both documents apply to DoD computer systems that are entrusted with the
protection of information, regardless of whether or not that information is
classified, sensitive, national security-related, or any combination thereof.
Furthermore, both documents can be applied throughout the DoD.(5,6,7,8,9)

The two documents are concerned with protection against both disclosure and
integrity violations. Integrity violations are of particular concern for
sensitive unclassified information (e.g., financial data) as well as for some
classified applications (e.g., missile guidance data).

The recommendations of both this report and the Computer Security
Requirements are stated in terms of classes from the Criteria. Embodied in each
class and therefore encompassed within the scope of both documents are two
types of requirements:  assurance and feature requirements.  Assurance
requirements are those that contribute to confidence that the required features
are present and that the system is functioning as intended.  Examples of
assurance requirements include modular design, penetration testing, formal
verification, and trusted configuration management.  Feature requirements
encompass capabilities such as labeling, authentication, and auditing.

1.2 Security Operating Modes

DoD computer security policy identifies several security operating modes, for
which the following definitions are adapted:(10,11,12,13)

     a.  Dedicated Security Mode--The mode of operation in which the system is
         specifically and exclusively dedicated to and controlled for the
         processing of one particular type or classification of information,
         either for fulltime operation or for a specified period of time.

     b.  System High Security Mode--The mode of operation in which system
         hardware/software is only trusted to provide need-to-know protection
         between users.  In this mode, the entire system, to include all
         components electrically and/or physically connected, must operate with
         security measures commensurate with the highest classification and
         sensitivity of the information being processed and/or stored.  All
         system users in this environment must possess clearances and
         authorizations for all information contained in the system, and all
         system output must be clearly marked with the highest classification
         and all system caveats, until the information has been reviewed
         manually by an authorized individual to ensure appropriate
         classifications and caveats have been affixed.

     c.  Multilevel Security Mode--The mode of operation which allows two or
         more classification levels of information to be processed
         simultaneously within the same system when some users are not cleared
         for all levels of information present.
                                      3


     d.  Controlled Mode--The mode of operation that is a type of multilevel
         security in which a more limited amount of trust is placed in the
         hardware/software base of the system, with resultant restrictions on
         the classification levels and clearance levels that may be supported.

     e.  Compartmented Security Mode--The mode of operation which allows
         the system to process two or more types of compartmented information
         (information requiring a special authorization) or any one type of
         compartmented information with other than compartmented information.
         In this mode, system access is secured to at least the Top Secret (TS)
         level, but all system users need not necessarily be formally
         authorized access to all types of compartmented information being
         processed and/or stored in the system.

In addition to these security operating modes, Service policies may define
other modes of operation.  For example, Office of the Chief of Naval Operations
(OPNAV) Instruction 5239.  IA defines Limited Access Mode for those systems in
which the minimum user clearance is uncleared and the maximum data sensitivity
is not classified but sensitive (6)

                                 5


2.0 RISK INDEX

The evaluation class appropriate for a system is dependent on the level of
security risk inherent to that system.  This inherent risk is referred to as
that systems risk index.  Risk index is defined as follows:
     The disparity between the minimum clearance or authorization of system
     users and the maximum sensitivity of data processed by a system.
 
The Computer Security Requirements are based upon this risk index.  Although
there are other factors that can influence security risk, such as mission
criticality, required denial of service protection, and threat severity, only
the risk index is used to determine the minimum class of trusted systems to be
employed, since it can be uniformly applied in the determination of security
risk.  The risk index for a system depends on the rating associated with the
system's mimimum user clearance (Rmin) taken from Table 1 and the rating
associated with the system's maximum data sensitivity (Rmax) taken from Table

2.  The risk index is computed as follows:

Case a. If Rmin is less than Rmax, then the risk index is determined by
subtracting Rmin from Rmax.2
                        Risk Index  Rmax   Rmin

Case b. If Rmin is greater than or equal to Rmax, then
                1, if there are categories on the system to which some users
                   are not authorized access;
Risk Index
                0, otherwise (i.e., if there are no categories on the system or
                   if all users are authorized access to all categories)

     Example: For a system with a minimum user clearance of Confidential and
     maximum data sensititivy of Secret (without categories), Rmin 2 and
     Rmax 3.


1 Since a clearance implicitly encompasses lower clearance levels (e.g., a
Secret- cleared user has an implicit Confidential clearance), the phrase
"minimum clearance...of system users" is more accurately stated as "maximum
clearance of the least cleared system user." For simplicity, this document uses
the former phrase.

2 There is one anomalous case in which this formula gives an incorrect result
This is the case where the minimum clearance is Top Secret/Background
Investigation and the maximum data sensitivity is Top Secret. According to
the formula, this gives a risk index of l. In actuality, the risk index in this
case is zero. The anomaly results because there are two "levels" of Top Secret
clearance and only one level of Top Secret data.

                                     6




                        TABLE 1

       RATING SCALE FOR MINIMUM USER CLEARANCE1





            MINIMUM USER CLEARANCE                                RATING

                      Uncleared (U)                                  0

   Not Cleared but Authorized Access to Sensitive Unclassified       1
                     Information (N)
                     Confidential (C)                                2
                        Secret(S)                                    3
     Top Secret (TS)/Current Background Investigation (BI)           4
 Top Secret (TS)/Current Special Background Investigation (SBI)      5
                One Category (1C)                                    6
              Multiple Categories (MC)                               7



















1 See Appendix B for a detailed description of the terms listed

                             7


                       TABLE 2

      RATING SCALE FOR MAXIMUM DATA SENSITIVITY

 MAXIMUM DATA
  SENSITIVITY
  RATINGS 2         RATING   MAXIMUM DATA SENSITIVITY WITH
    WITHOUT         (Rmax)        CATEGORIES1
  CATEGORIES
     (Rmax)

 Unclassified (U)      0          Not Applicable3
Not Classified but     1     N With One or More Categories            2
     Sensitives4
 Confidential (C)      2     C With One or More Categories            3
   Secret(S)           3     S With One or More Categories With No    4
                               More Than One Category Containing
                               Secret Data
                             S With Two or More Categories Containing 5
                               Secret Data
 Top Secret (TS)       55    TS With One or More Categories With No   6
                             More Than One Category Containing
                             Secret or Top Secret Data
                             TS With Two or More Categories           7
                             Containing Secret or Top Secret Data



1 The only categories of concern are those for which some users are not
authorized access to the category.  When counting the number of categories,
count all categories regardless of the sensitivity level associated with the
data.  If a category is associated with more than one sensitivity level, it is
only counted at the highest level.

2 Where the number of categories is large or where a highly sensitive category
is involved, a higher rating might be warranted.

3 Since categories imply sensitivity of data and unclassified data is not
sensitive, unclassified data by definition cannot contain categories.

4 N data includes financial, proprietary, privacy, and mission sensitive data.
Some situations (e.g., those involving extremely large financial sums or
critical mission sensitive data), may warrant a higher rating.  The table
prescribes minimum ratings

5 The rating increment between the Secret and Top Secret data sensitivity
levels is greater than the increment between other adjacent levels.  This
difference derives from the fact that the loss of Top Secret data causes
exceptionally grave damage to the national security, whereas the loss of Secret
data causes only serious damage.  (4)
                                     
                                 8


                            TABLE 3
                  SECURITY RISK INDEX MATRIX

                         Maximum Data Sensitivity

                             U     N     C    S    TS   1C   MC

                      U      0     1     2    3    4    5    6
                      N      0     0     1    2    4    5    6
   Minimum            C      0     0     0    1    3    4    5
   Clearance          S      0     0     0    0    2    3    4
   or
   Authorization    TS(BI)   0     0     0    0    0    2    3
   of
   System Users     TS(SBI)  0     0     0    0    0    1    2
                      1C     0     0     0    0    0    0    1
                      MC     0     0     0    0    0    0    0




U = Uncleared or Unclassified
N = Not Cleared but Authorized Access to Sensitive Unclassified Information or
Not Classified but Sensitive
C = Confidential
S = Secret
TS = Top Secret
TS(BI) = Top Secret (Background Investigation)
TS(SBI) = Top Secret (Special Background Investigation)
1C = One Category
MC = Multiple Categories
                                 9


In situations where the local environment indicates that additional risk
factors are present, a larger risk index may be warranted.  Table 2 and the
above discussion show how the presence of nonhierarchical sensitivity
categories such as NOFORN (Not Releasable to Foreign Nationals) and PROPIN
(Caution- Proprietary Information Involved) influences the ratings.(14)
Compartmented information is also encompassed by the term sensitivity
categories as is information revealing sensitive intelligence sources and
methods.  A' subcategory (and a subcompartment) is considered to be independent
from the category to which it is subsidiary.

Table 3 presents a matrix summarizing the risk' indices corresponding to the
various clearance/sensitivity pairings. For simplicity no categories are
associated with the maximum data sensitivity levels below Top Secret.
                                     11


3.0 COMPUTER SECURITY REQUIREMENTS FOR OPEN
    SECURITY ENVIRONMENTS


This section discusses the application of the Computer Security Requirements to
systems in open security environments. An open security environment is one in
which system applications are not adequately protected against the insertion of
malicious logic.  Appendix C describes malicious logic and the open security
environment in more detail.

3.1 Recommended Classes

Table 4 presents the minimim evaluation class identified in the Computer
Security Requirements for different risk indices in an open security
environment.  Table 5 illustrates the impact of the requirements on individual
minimum clearance/maximum data sensitivity pairings, where no categories are
associated with maximum data sensitivity below Top Secret.  The minimum
evaluation class is determined by finding the matrix entry corresponding to the
minimum clearance or authorization of system users and the maximum sensitivity
of data processed by the system.

     Example: If the minimum clearance of system users is Secret and the
     maximum sensitivity of data processed is Top Secret (with no categories),
     then the risk index is 2 and a class B2 system is required.

The classes identified are minimum values.  Environmental characteristics must
be examined to determine whether a higher class is warranted.  Factors that
might argue for a higher evaluation class include the following:

     a. High volume of information at the maximum data sensitivity.

     b. Large number of users with minimum clearance.

Both of these factors are often present in networks.

The guidance embodied in the Computer Security Requirements is best used during
system requirements definition to determine which class of trusted system is
required given the risk index envisioned for a specific environment. They are
also of use in determining which choices are feasible given either the maximum
sensitivity of data to be processed or minimum user clearance or authorization
requirements. The Computer Security Requirements can also be used in a security
evaluation to determine whether system safeguards are sufficient.

3.2 Risk index and Operational Modes

Situations with a risk index of zero encompass systems operating in system high
or dedicated mode.  Systems operating in dedicated mode--in which all users
have both the clearance and the need-to-know for all information in the
system--do not need to rely on hardware and software protection measures for
security.(10) Therefore, no minimum level of trust is prescribed.  However,
because of the integrity and denial of service requirements of many systems,
additional protective features may be warranted.

                                 12




                             TABLE 4

  COMPUTER SECURITY      REQUIREMENTS FOR OPEN SECURITY
                         ENVIRONMENTS


    RISK INDEX         SECURITY OPERATING        MINIMUM CRITERIA
                               MODE                  CLASS1

         0              Dedicated                  No Prescribed
                                                    Minimum2
         0                  System High                C23
         1            Limited Access, Controlled,      B14
                      Compartmented, Multilevel
         2            Limited Access, Controlled,       B2
                      Compartmented, Multilevel
         3               Controlled, Multilevel         B3
         4              Multilevel                      A1
         5              Multilevel                      *
         6              Multilevel                      *
         7              Multilevel                      *




1 The asterisk (*) indicates that computer protection for environments with
that risk index are considered to be beyond the state of current technology.
Such environments must augment technical protection with personnel or
administrative security safeguards.

2 Although there is no prescribed minimum, the integrity and denial of service
requirements of many systems warrant at least class C1 protection.

3 If the system processes sensitive or classified data, at least a class C2
system is required.  If the system does not process sensitive or classified
data, a class C1 system is sufficient.

4 Where a system processes classified or compartmented data and some users do
not have at least a Confidential clearance, or when there are more than two
types of compartmented information being processed, at least a class B2 system
is required.
                              13


                            TABLE 5

 SECURITY INDEX MATRIX FOR OPEN SECURITY ENVIRONMENTS1


                Maximum Data Sensitivity

                          U     N     C     S     TS    1C    1M

                    U     C1    B1    B2    B3    *     *     *
     Minimum        N     C1    C2    B2    B2    A1    *     *
     Clearance or   C     C1    C2    C2    B1    B3    A1    *
     Author-
     ization        S     C1    C2    C2    C2    B2    B3    A1
     of System
     Users        TS(BI)  C1    C2    C2    C2    C2    B2    B3

                 TS(SBI)  C1    C2    C2    C2    C2    B1    B2
                   1C     C1    C2    C2    C2    C2    C22   B13
                   MC     C1    C2    C2    C2    C2    C22   C22


1 Environments for which either C1 or C2 is given are for systems that operate
in system high mode.  No minimum level of trust is prescribed for systems that
operate in dedicated mode.  Categories are ignored in the matrix, except for
their inclusion at the TS level.

2 It is assumed that all users are authorized access to all categories present
in the system.  If some users are not authorized for all categories, then a
class B1 system or higher is required.

3 Where there are more than two categories, at least a class B2 system is
required.

U = Uncleared or Unclassified
N = Not Cleared but Authorized Access to Sensitive Unclassified Information or
Not Classified but Sensitive
C = Confidential
S = Secret
TS = Top Secret
TS(BI) = Top Secret (Background Investigation)
TS(SBI) = Top Secret (Special Background Investigation)
1C = One Category
MC = Multiple Category
                                           14





In system high mode, all users have sufficent security clearances and category
authorizations for all data, but some users do not have a need-to-know for all
information in the system.(10) Systems that operate in system high mode thus
are relied on to protect information from users who do not have the appropriate
need-to-know. Where classified or sensitive unclassified data is involved, no
less than a class C2 system is allowable due to the need for individual
accountability.

In accordance with policy, individual accountability requires that individual
system users be uniquely identified and an automated audit trail kept of their
actions.  Class C2 systems are the lowest in the hierarchy of trusted systems
to provide individual accountability and are therefore required where sensitive
or classified data is involved.  The only case where no sensitive or classified
data is involved is the case in which the maximum sensitivity of data is
unclassified.  In this case, hardware and software controls are still required
to allow users to protect project or private information and to keep other
users from accidentally reading or destroying their data.  However, since there
is no officially sensitive data involved, individual accountability is not
required and a class C1 system suffices.  In system high mode sensitivity
labels are not required for making access control decisions.  In this mode
access is based on the need-to-know, which is based on permissions (e.g., group
A has access to file A), not on sensitivity labels.  The type of access control
used to provide need-to-know protection is called discretionary access control.
It is defined as a means of restricting access to objects based on the identity
of subjects and/or groups to which the subjects belong.  All systems above
Division D provide discretionary access control mechanisms.  These mechanisms
are more finely grained in class C2 systems than in Class C1 systems in that
they provide the capability of including or excluding access to the granularity
of a single user.  Division C systems (C1 and C2) do not possess the capability
to provide trusted labels on output.  Therefore, output from these systems must
be labeled at the system high level and manually reviewed by a responsible
individual to determine the correct sensitivity prior to release beyond the
perimeter of the system high protections of the system.(10)

Environments with a risk index of 1 or higher encompass systems operating in
controlled, compartmented, and multilevel modes.  These environments require
mandatory access control, which is the type of access control used to provide
protection based on sensitivity labels.  It is defined as a means of
restricting access to objects based on the sensitivity (as represented by a
label) of the information contained in the objects and the formal clearance or
authorization of subjects to access information of such sensitivity.  Division
B and A systems provide mandatory access control, and are therefore required
for all environments with risk indices of 1 or greater.

The need for internal labeling has a basis in policy, in that DoD Regulation
5200.1-R requires computer systems that process sensitive or classified data to
provide internal classification markings.(3) Other requirements also exist.

     Example: The DCID entitled "Security Controls on the Dissemination of
     Intelligence Information" requires that security control markings be
                                 15


     "associated (in full or abbreviated form) with data stored or processed in
     automatic data processing systems."(14)

Sensitivity labeling is also required for sensitive unclassified data.(15,16)

     Example: Data protected by Freedom of Information (FOI) Act exemptions
     must be labeled as being "exempt from mandatory disclosure under the FOI
     Act."(15)

This example illustrates not only the need for labeling but also the fact that
the purpose of FOI Act exemptions is to provide access control protection for
sensitive data.  In summary, it is a required administrative security practice
that classified and unclassified sensitive information be labeled and
controlled based on the labels.  It follows that prudent computer security
practice requires similar labeling and mandatory access control.

The minimum class recommended for environments requiring mandatory access
control is class B1, since class B1 systems are the lowest in the hierarchy of
trusted systems to provide mandatory access control.

     Example:  Where no categories are involved, systems with minimum
     clearance/maximum data sensitivity pairings of U/N and C/S have a risk
     index of 1 and thus require at least a class B1 system.

Some systems that operate in system high mode use mandatory access control for
added protection within the system high environment, even though the controls
are not relied upon to properly label and protect data passing out of the
system high environment.  There has also been a recommendation that mandatory
access controls (i.e., class B1 or higher systems) be used whenever data at two
or more sensitivity levels is being processed, even if everyone is fully
cleared, in order to reduce the likelihood of mixing data from files of higher
sensitivity with data of files of lower sensitivity and releasing the data at
the lower sensitivity.(17) These points reaffirm the fact that the classes
identified in the requirements are minimum values.

This report emphasizes that output from a system operating in system high mode
must be stamped with the sensitivity and category labels of the most sensitive
data in the system until the data is examined by a responsible individual and
its true sensitivity level and category are determined.  If a system can only
be trusted for system high operation, its labels cannot be assumed to
accurately reflect data sensitivity.  The use of division B or A systems does
not necessarily solve this problem.

     Example: Take the case of a system in an open security environment that
     processes data classified up to Secret and supports some users who have
     only Confidential clearances.  According to the requirements, such a
     situation represents a risk index of 1 and thus requires a class B1
     system.  Some of the reports produced by the system might be unclassified.
     Nevertheless, such a report cannot be forwarded to uncleared people until
     the report is examined and its contents determined to be unclassified.
     Without the existence of such a review, the recipient becomes an indirect
     user and the risk index becomes 3. A class B1 system no longer provides
                                      16


     adequate data protection. Therefore, even though the system is trusted to
     properly label and segregate Confidential and Secret data, it is not
     simultaneously trusted to properly label and segregate unclassified data.

Systems with a risk index of 2 require more trust than can be placed in a class
B1 system.  Where no categories are involved, class B2 systems are the minimum
required for minimum clearance/maximum data sensitivity pairings such as U/C,
N/S and S/TS, all of which have a risk index of 2.  Class B2 systems have
several characteristics that justify this increased trust:

     a.  The Trusted Computing Base (TCB) is carefully structured into
         protection-critical and nonprotection-critical elements.  The TCB
         interface is well defined, and the TCB design and implementation
         enable it to be subjected to more thorough testing and more complete
         review.

     b.  The TCB is based on a clearly defined and documented formal security
         policy model that requires the discretionary and mandatory access
         control enforcement found in class B1 systems to be extended to all
         subjects and objects in the system. That is, security rules are more
         rigorously defined and have a greater influence on system design.

     c.  Authentication mechanisms are strengthened, making it more difficult
         for a malicious user or malicious software to improperly intervene in
         the login process.

     d.  Stringent configuration management controls are imposed for life-cycle
         assurance.

     e.  Covert channels are addressed to defend against their exploitation by
         malicious software.(18) A covert channel is a communication channel
         that violates the system's security policy.

Because of these and other characteristics, class B2 systems are relatively
resistant to penetration.  A risk index of 3, however, requires greater
resistance to penetration.  Class B3 systems are highly resistant to
penetration and are the minimum required for situations with a risk index of 3
such as those with minimum clearance/maximum data sensitivity pairings of U/S,
C/TS, S/TS with one category, and TS(BI)/TS with multiple categories.
Characteristics that distinguish class B3 from class B2 systems include the
following:

     a.  The TCB must satisfy the reference monitor requirements that it
         mediate all accesses of subjects to objects, be tamperproof, and be
         small enough to be subjected to analysis and tests.  Much effort is
         thus spent on minimizing TCB complexity.

     b.  Enhancements are made to system audit mechanisms and system
         recovery procedures.

     c.  Security management functions are performed by a security
         administrator rather than a system administrator.
                              17


While several new features have been added to class B3 systems, the major
distinction between class B2 and class B3 systems is the increased trust that
can be placed in the TCB of a class B3 system.  The most trustworthy systems
defined by the Criteria are class Al systems.  Class Al systems can be used for
situations with a risk index of 4, such as the following minimum
clearance/maximum data sensitivity pairings: N/TS, C/TS with one category, and
S/TS with multiple categories.  Class Al systems are functionally equivalent to
those in class B3 in that no additional architectural features or policy
requirements are added.  The distinguishing characteristic of systems in this
class is the analysis derived from formal design specification and verification
techniques and the resulting high degree of assurance that the TCB is correctly
implemented.  In addition, more stringent configuration management is required
and procedures are established for securely distributing the system to sites.

The capability to support systems in open security environments with a risk
index of 5 or greater is considered to be beyond the state-of-the-art.  For
example, technology today does not provide adequate security protection for an
open environment with uncleared users and Top Secret data.  Such environments
must rely on physical, personnel, or information security solutions or on such
technical approaches as periods processing.
                             19




4.0 COMPUTER SECURITY REQUIREMENTS FOR CLOSED
    SECURITY ENVIRONMENTS


This section discusses the application of the Computer Security Requirements to
systems in closed security environments.  A closed security environment is one
in which system applications are adequately protected against the insertion of
malicious logic.  Appendix C describes the closed security environment in more
detail.  The main threat to the TCB from applications in this environment is
not malicious logic, but logic containing unintentional errors that might be
exploited for malicious purposes.  As system quality reaches class B2, the
threat from logic containing unintentional errors is substantially reduced.
This reduction permits the placement of increased trust in class B2 systems due
to (1) the increased attention that B2 systems give to the interface between
the application programs and the operating system, (2) the formation of a more
centralized TCB, and (3) the elimination of penetration flaws.  Nevertheless,
the evaluation class of B1 assigned for open security environments cannot be
reduced to a class C1 or C2 in closed security environments because of the
requirement for mandatory access controls.

Table 6 presents the minimum evaluation class identified in the Computer
Security Requirements for different risk indices in a closed security
environment.  The principal difference between the requirements for the open
and closed environments is that in closed environments class B2 systems are
trusted to provide sufficient protection for a greater risk index.  As a
result, environments are supportable that were not supportable in open
situations (e.g., uncleared user on a system processing Top Secret data).
Table 7 illustrates the requirements' impact on individual minimum
clearance/maximum data sensitivity pairings.
                                    20




                             TABLE 6

COMPUTER SECURITY      REQUIREMENTS FOR CLOSED SECURITY
                       ENVIRONMENTS





  RISK INDEX         SECURITY OPERATING       MINIMUM CRITERIA
                            MODE                   CLASS1

        0                  Dedicated             No Prescribed
                                                  Minimum 2
        0                 System High                C23
        1        Limited Access, Controlled,         B14
                 Compartmented, Multilevel
        2        Limited Access, Controlled          B2
                 Compartmented, Multilevel
        3              Controlled, Multilevel        B2
        4                  Multilevel                B3
        5                  Multilevel                A1
        6                  Multilevel                *
        7                  Multilevel                *





1 The asterisk (*) indicates that computer protection for environments with
that risk index are considered to be beyond the state of current technology.
Such environments must augment technical protection with physical, personnel,
and/or administrative safeguards.

2 Although there is no prescribed minimum, the integrity and denial of service
requirements of many systems warrant at least class C1 protection.

3 If the system processes sensitive or classified data, at least a class C2
system is required.  If the system does not process sensitive or classified
data, a class C1 system is sufficient.

-Where a system processes classified or compartmented data and some users do
not have at least a Confidential clearance, at least a class B2 system is
required.   
                                21


                             TABLE 7
      SECURITY INDEX MATRIX FOR CLOSED SECURITY ENVIRONMENTS1

                            Maximum Data Sensitivity

                        U     N     C     S     TS    1C    MC

                  U     C1    B1    B2    B2    A1    *     *
   Minimum        N     C1    C2    B1    B2    B3    A1    *
   Clearance or   C     C1    C2    C2    B1    B2    B3    A1
   Author-        S     C1    C2    C2    C2    B2    B2    B3
   ization      TS(BI)  C1    C2    C2    C2    C2    B2    B2
   of System   TS(SBI)  C1    C2    C2    C2    C2    B1    B2
   Users         1C     C1    C2    C2    C2    C2    C22   B13
                 MC     C1    C2    C2    C2    C2    C22   C22




1 Environments for which either C1 or C2 is given are for systems that operate
in system high mode.  There is no prescribed minimum level of trust for systems
that operate in dedicated mode.  Categories are ignored in the matrix, except
for their inclusion at the TS level.

2 It is assumed that all users are authorized access to all categories on the
system. If some users are not authorized for all categories, then a class B1
system or higher is required.

3 Where there are more than two categories, at least a class B2 system is
required.

U = Uncleared or Unclassified
N = Not Cleared but Authorized Access to Sensitive UnclassiFied Information or
Not Classified but Sensitive
C = Confidential
S = Secret
TS = Top Secret
TS(BI) = Top Secret (Background Investigation)
TS (SBI) = Top Secret (Special Background Investigation)
1C = One Category
MC = Multiple Categories
                                       23



                   APPENDIX A

          SUMMARY OF CRITERIA The DoD Trusted Computer System Evaluation
Criteria(4) provides a basis for specifying security requirements and a metric
with which to evaluate the degree of trust that can be placed in a computer
system.  These criteria are hierarchically ordered into a series of evaluation
classes where each class embodies an increasing amount of trust.  A summary of
each evaluation class is presented in this appendix.  This summary should not
be used in place of the Criteria.  The evaluation criteria are based on six
fundamental security requirements that deal with controlling access to
information.  These requirements can be summarized as follows:

   a. Security policy--There must be an explicit and well-defined security
      policy enforced by the system.

   b. Marking--Access control labels must be associated with objects.

   c. Identification--Individual subjects must be identified.

   d. Accountability--Audit information must be selectively kept and
      protected so that actions affecting security can be traced to the
      responsible party.

   e. Assurance--The computer system must contain hardware and software
      mechanisms that can be evaluated independently to provide sufficient
      assurance that the system enforces the security policy.

   f. Continuous protection--The trusted mechanisms that enforce the
      security policy must be protected continuously against tampering and
      unauthorized changes.

The evaluation criteria are divided into four divisions--D, C, B, and A;
divisions C, B, and A are further subdivided into classes.  Division D
represents minimal protection, and class A1 is the most trustworthy and
desirable from a computer security point of view.

The following overviews are excerpts from the Criteria:

   Division D: Minimal Protection. This division contains only one class. It is
reserved for those systems that have been evaluated but fail to meet the
requirements for a higher evaluation class.

   Division C: Discretionary Protection. Classes in this division provide for
discretionary (need-to-know) protection and accountability of subjects and the
actions they initiate, through inclusion of audit capabilities.
                               24


     Class C1: Discretionary Security Protection.  The TCB of class C1 systems
nominally satisfies the discretionary security requirements by providing
separation of users and data.  It incorporates some form of, credible controls
capable of enforcing access limitations on an individual basis, i.e.,
ostensibly suitable for allowing users to be able to protect project or private
information and to keep other users from accidentally reading or destroying
their data.  The class C I environment is expected to be one of cooperating
users processing data at the same level(s) of sensitivity.

     Class C2: Controlled Access Protection. Systems in this class enforce a
more finely grained discretionary access control than class C1 systems, making
users individually accountable for their actions through logic procedures,
auditing of security-relevant events, and resources encapsulation.

     Division B: Mandatory Protection.  The notion of a TCB that preserves the
integrity of sensitivity labels and uses them to enforce a set of mandatory
access control rules is a major requirement in this division.  Systems in this
division must carry the sensitivity labels with major data structures in the
system.  The system developer also provides the security policy model on which
the TCB is based and furnishes a specification of the TCB.  Evidence must be
provided to demonstrate that the reference monitor concept has been
implemented.

     Class B1: Labeled Security Protection. Class B1 systems require all the
features required for class C2. In addition, an informal statement of the
security policy model, data labeling, and mandatory access control over named
subjects and objects must be present. The capability must exist for accurately
labeling exported information. Any flaws identified by testing must be removed.

     Class B2: Structured Protection.  In class B2 systems, the TCB is based on
a clearly defined and documented formal security policy model that requires the
discretionary and mandatory access control enforcement found in B1 systems be
extended to all subjects and objects in the system.  In addition, covert
channels are addressed.  The TCB must be carefully structured into
protection-critical and nonprotection-critical elements.  The TCB interface is
well defined and the TCB design and implementation enable it to be subjected to
more thorough testing and more complete review.  Authentication mechanisms are
strengthened, trusted facility management is provided in the form of support
for systems administrator and operator functions, and stringent configuration
management controls are imposed.  The system is relatively resistant to
penetration.

     Class B3: Security Domains. The class B3 TCB must satisfy the reference
monitor requirements that it mediate all accesses of subjects to objects, be
tamperproof, and be small enough to be subjected to analysis and tests. To this
end, the TCB is structured to exclude code not essential to security policy
enforcement, with significant software engineering during TCB design and
implementation directed toward minimizing its complexity.  A security
administrator is supported, audit mechanisms are expanded to signal security-
relevant events, and system recovery procedures are required. The system is
highly resistant to penetration.

     Division A: Verified Protection. This division is characterized by the use
of formal security verification methods to assure that the mandatory and
                               25


discretionary security controls employed in the system can effectively protect
the classified and other sensitive information stored or processed by the
system.  Extensive documentation is required to demonstrate that the TCB meets
the security requirements in all aspects of design, development, and
implementation.

   Class A1: Verified Design.  Systems in class A1 are functionally equivalent
to those in class B3 in that no additional architectural features or policy
requirements have been added.  The distinguishing feature of systems in this
class is the analysis derived from formal design specification and verification
techniques and the resulting high degree of assurance that the TCB is correctly
implemented.  This assurance is developmental in nature starting with a formal
model of security policy and a formal top-level specification (FTLS) of the
design.  In keeping with the extensive design and development analysis of the
TCB required of systems in class A1, more stringent configuration management is
required and procedures are established for securely distributing the system to
sites.  A system security administrator is supported.
                                  27



                        APPENDIX B

DETAILED DESCRIPTION OF CLEARANCES AND DATA
               SENSITIVITIES
This appendix describes in detail the clearances and data sensitivities (e.g.,
classification) introduced in the body of the report.

B.1 Clearances

This section defines increasing levels of clearance or authorization of system
users. System users include not only those users with direct connections to the
system but also those users without direct connections who might receive output
or generate input that is not reliably reviewed for classification by a
responsible
individual.

   a. Uncleared (U)--Personnel with no clearance or authorization.
      Permitted access to any information for which there are no specified
      controls, such as openly published information.

   b. Unclassified Information (N)--Personnel who are authorized access to
      sensitive unclassified (e.g., For Official Use Only (FOUO)) information,
      either by an explicit official authorization or by an implicit
      authorization derived from official assignments or responsibilities.(15)

   c. Confidential Clearance (C)--Requires U.S. citizenship and typically
      some limited records checking.(19) In some cases, a National Agency
      Check (NAC) is required (e.g., for U.S. citizens employed by colleges or
      universities).(20)

   d. Secret Clearance (S)--Typically requires a NAC, which consists of
      searching the Federal Bureau of Investigation fingerprint and
      investigative files and the Defense Central Index of Investigations.(19)
      In some cases, further investigation is required.

   e. Top Secret Clearance based on a current Background Investigation
      (TS(BI))--Requires an investigation that consists of a NAC, personal
      contacts, record searches, and written inquiries. A B1 typically
      includes an investigation extending back 5 years, often with a spot
      check investigation extending back 15 years.(19)

   f. Top Secret Clearance based on a current Special Background
      Investigation (TS(SBI))--Requires an investigation that, in addition to
      the investigation for a B1, includes additional checks on the subject's
      immediate family (if foreign born) and spouse and neighborhood
      investigations to verify each of the subject's former residences in the
      United States where he resided six months or more. An SBI typically
      includes an investigation extending back 15 years.(19)
                                 28


     g. One category (1C)1 - In addition to a TS(SBI) clearance, written
        authorization for access to one category of information is required.
        Authorizations are the access rights granted to a user by a responsible
        individual (e.g., security officer).

    h.  Multiple categories (MC)' - In addition to TS(SBI) clearance, written
        authorization for access to multiple categories of information is
        required.

The extent of investigation required for a particular clearance varies based
both on the background of the individual under investigation and on derogatory
or questionable information disclosed during the investigation.  Identical
clearances are assumed to be equivalent, however, despite differences in the
amount of investigation peformed.

Individuals from non-DoD agencies might be issued DoD clearances if the
clearance obtained in their agency can be equated to a DoD clearance.  For
example, the "Q" and "L" clearances granted by both the Department of Energy
and the Nuclear Regulatory Commission are considered acceptable for issuance of
a DoD industrial personnel security clearance.(20) The "Q" clearance is
considered an authoritative basis for a DoD Top Secret clearance (based on a
B1) and the "L" clearance is considered an authoritative basis for a DoD Secret
clearance.(20)

Foreign individuals might be granted access to classified U.S.  information
although they do not have a U.S.  clearance.  Access to classified information
by foreign nationals, foreign governments, international organizations, and
immigrant aliens is addressed by National Disclosure Policy, DoD Directive
5230.11, and DoD Regulation 5200.I-R.(3,21,22) The minimum user clearance
rating table applies in such cases if the foreign clearance can be equated to
one of the clearance or authorization levels in the table.

B.2 Data Sensitivities

Increasing levels of data sensitivity are defined as follows:

     a. Unclassified (U)--Data that is not sensitive or classified:  publicly
        releasable information within a computer system. Note that such data
        might still require discretionary access controls to protect it from
        accidental destruction.

     b. Not Classified but Sensitive (N)--Unclassified but sensitive data. Much
        of this is FOUO data, which is that unclassified data that is exempt
        from release under the Freedom of Information Act.(15) This includes
        data such as the following:

        I.  Manuals for DoD investigators or auditors.


1 These are actually authorizations rather than clearance levels, but they are
included here to emphasize their importance.
                               29


     2. Examination questions and answers used in determination of the
        qualification of candidates for employment or promotion.

     3. Data that a statute specifically exempts from disclosure, such as
        Patent Secrecy data.(23)

     4. Data containing trade secrets or commercial or financial
        information.

     5. Data containing internal advice or recommendations that reflect
        the decision-making process of an agency.(24)

     6. Data in personnel, medical, or other files that, if disclosed, would
        result in an invasion of personal privacy.(25)

     7. Investigative records.

        DoD Directive 5400.7 prohibits any material other than that cited
        in FOI Act exemptions from being considered or marked
        FOUO.(15) One other form of unclassified sensitive data is that
        pertaining to unclassified technology with military application.(16)
        This refers primarily to documents that are controlled under the
        Scientific and Technical Information Program or acquired under
        the Defense Technical Data Management Program.(26,27) In
        addition to specific requirements for protection of particular forms
        of unclassified sensitive data, there are two general mandates. The
        first is Title 18, U.S. Code 1905, which makes it unlawful for any
        office or employee of the U.S. Government to disclose information
        of an official nature except as provided by law, including when such
        information is in the form of data handled by computer
        systems.(28) Official data is data that is owned by, produced by or
        for, or is under the control of the DoD. The second is Office of
        Management and Budget (OMB) Circular A-71, Transmittal
        Memorandum Number I, which establishes requirements for
        Federal agencies to protect sensitive data.(30)

c.   Confidential (C)--Applied to information, the unauthorized disclosure of
     which reasonably could be expected to cause damage to the national
     security.(3)

d.   Secret (S)--Applied to information, the unauthorized disclosure of which
     reasonably could be expected to cause serious damage to the national
     security.(3)

e.   Top Secret (TS)--Applied to information, the unauthorized disclosure of
     which reasonably could be expected to cause exceptionally grave
     damage to the national security.(3)
                                     30


     f.  One Category (1C)2--Applied to Top Secret Special Intelligence
        information (e.g., Sensitive Compartmented Information (SCI) or
        operational information (e.g., Single Integrated Operational
        Plan/Extremely Sensitive Information (SIOP/ESI)) that requires
        special controls for restrictive handling.(3) Access to such
        information requires authorization by the office responsible for the
        particular compartment.  Compartments also exist at the C and 5 levels
        (see the discussion below).

     g.  Multiple Categories (MC)2--Applied to Top Secret Special Intelligence
        or operational information that requires special controls for
        restrictive handling.  This sensitivity level differs from the 1C level
        only in that there are multiple compartments involved.  The number can
        vary from two to many, with corresponding increases in the risk
        involved.

Data sensitivity groupings are not limited to the hierarchical levels discussed
in Section B.2.  Nonhierarchical sensitivity categories such as NOFORN and
PROPIN are also used.(14) Compartmented information is also included under the
term sensitivity categories, as is information revealing sensitive intelligence
sources and methods.  Other sources of sensitivity categories include (a) the
Atomic Energy Act of 1954, (b) procedures based on International Treaty
requirements, and (c) programs for the collection of foreign intelligence or
under the jurisdiction of the National Foreign Intelligence Advisory Board or
the National Communications Security Subcommittee.(11,32,33,34,35) Such
nonhierarchical sensitivity categories can occur at each hierarchical
sensitivity level.

















2 These are actually categories rather than classification levels.  They are
included here to emphasize their importance.
                                     31



                   APPENDIX C

          ENVIRONMENTAL TYPES The amount of computer security required in a
system depends not only on the risk index (Section 2) but also on the nature of
the environment.  The two environmental types of systems defined in this
document are based on whether the applications that are processed by the TCB
are adequately protected against the insertion of malicious logic.  A system
whose applications are not adequately protected is referred to as being in an
open environment.  If the applications are adequately protected, the system is
in a closed environment.  The presumption is that systems in open environments
are more likely to have malicious application than systems in closed
environments.  Most systems are in open environments.

Before defining the two environmental categories in more detail, it is
necessary to define several terms.

   a. Environment. The aggregate of external circumstances, conditions,
      and objects that affect the development, operation, and maintenance of
      a system.

   b. Application. Those portions of a system, including portions of the
      operating system, that are not responsible for enforcing the systems
      security policy.

   c. Malicious Logic. Hardware, software, or firmware that is intentionally
      included for the purpose of causing loss or harm (e.g., Trojan horses).

   d. Configuration Control. Management of changes made to a system's
      hardware, software, firmware, and documentation throughout the
      development and operational life of the system.

C.1 Open Security Environment

Based on these definitions, an open security environment includes those systems
in which either of the following conditions holds true:

   a. Application developers (including maintainers) do not have sufficient
      clearance (or authorization) to provide an acceptable presumption that
      they have not introduced malicious logic. Sufficient clearance is
      defined as follows: where the maximum classification of data to be
      processed is Confidential or below, developers are cleared and
      authorized to the same level as the most sensitive data; where the
      maximum classification of data to be processed is Secret or above,
      developers have at least a Secret clearance.

   b. Configuration control does not provide sufficient assurance that
      applications are protected against the introduction of malicious logic
      prior to or during the operation of system applications.
                                    32


Configuration control, by the broad definition above, encompasses all factors
associated with the management of changes to a system.  For example, it
includes the factor that the application's user interface might present a
sufficiently extensive set of user capabilities such that the user cannot be
prevented from entering malicious logic through the interface itself.

In an open security environment, the malicious application logic that is
assumed to be present can attack the TCB in two ways.  First, it can attempt to
thwart TCB controls and thereby "penetrate" the system.  Secondly, it can
exploit covert channels that might exist in the TCB.  This distinction is
important in understanding the threat and how it is addressed by the features
and assurances in the Criteria.

C.2 Closed Security Environment

A closed security environment includes those systems in which both of the
following conditions hold true:

     a. Applications developers (including maintainers) have sufficient
        clearances and authorizations to provide an acceptable presumption
        that they have not introduced malicious logic.

     b. Configuration control provides sufficient assurance that applications
        are protected against the introduction of malicious logic prior to and
        during the operation of system applications.

Clearances are required for assurance against malicious applications logic
because there are few other tools for assessing the security-relevant behavior
of application hardware and software.  On the other hand, several assurance
requirements from the Criteria help to provide confidence that the TCB does not
contain malicious logic.  These assurance requirements include extensive
functional testing, penetration testing, and correspondence mapping between a
security model and the design.  Application logic typically does not have such
stringent assurance requirements.  Indeed, typically it is not practical to
build all application software to the same standards of quality required for
security software.

The configuration control condition implicitly includes the requirement that
users be provided a sufficiently limited set of capabilities to pose an
acceptably low risk of entering malicious logic.  Examples of systems with such
restricted interfaces might include those that offer no data sharing services
and permit the user only to execute predefined processes that run on his
behalf, such as message handlers, transaction processors, and security
"filters" or "guards."
                                  33



                    GLOSSARY
For additional definitions, refer to the Glossary in the DoD Trusted Computer
System Evaluation Criteria.(4)

Application
   Those portions of a system, including portions of the operating system, that
   are not responsible for enforcing the security policy.

Category
   A grouping of classified or unclassified but sensitive information, to which
   an additional restrictive label is applied (e.g., proprietary, compartmented
   information).

Classification
   A determination that information requires, in the interest of national
   security, a specific degree of protection against unauthorized disclosure
   together with a designation signifying that such a determination has been
   made. (Adapted from DoD Regulation 5200.I-R.)(3) Data classification is
   used along with categories in the calculation of risk index.

Closed Security Environment
   An environment that includes those systems in which both of the following
   conditions hold true:

   a. Application developers (including maintainers) have sufficient
      clearances and authorizations to provide an acceptable presumption
      that they have not introduced malicious logic. Sufficient clearance is
      defined as follows: where the maximum classification of data to be
      processed is Confidential or below, developers are cleared and
      authorized to the same level as the most sensitive data; where the
      maximum classification of data to be processed is Secret or above,
      developers have at least a Secret clearance.

   b. Configuration control provides sufficient assurance that applications
      are protected against the introduction of malicious logic prior to and
      during operation of system applications.

Compartmented Information
   Any information for which the responsible Office of Primary Interest (OPI)
   requires an individual needing access to that information to possess a
   special authorization.

Configuration Control
   Management of changes made to a system's hardware, software, firmware,
   and documentation throughout the developmental and operational life of
   the system.

Covert Channel
   A communications channel that allows a process to transfer information in
   a manner that violates the system's security policy.(4)
                                34


Discretionary Access Control
   A means of restricting access to objects based on the identity of subjects
   and/or groups to which they belong. The controls are discretionary in the
   sense that a subject with a certain access permission is capable of passing
   that permission (perhaps indirectly) on to any other subject.(4)

Environment
   The aggregate of external circumstances, conditions, and objects that affect
   the development, operation, and maintenance of a system. (See Open
   Security Environment and Closed Security Environment.)

Label
   Apiece of information that represents the security level of an object and
   that describes the sensitivity of the information in the object.

Malicious Logic
   Hardware, software, or firmware that is intentionally included in a system
   for the purpose of causing loss or harm.

Mandatory Access Control
   A means of restricting access to objects based on the sensitivity (as
   represented by a label) of the information contained in the objects and the
   formal authorization (i.e., clearance) of subjects to access information of
   such sensitivity.(4)

Need-To-Know
   A determination made by the processor of sensitive information that a
   prospective recipient, in the interest of national security, has a
   requirement for access to, knowledge of, or possession of the sensitive
   information in order to perform official tasks or services.  (Adapted from
   DoD Regulation 5220.22-R.)(20)

Open Security Environment
   An environment that includes those systems in which one of the following
   conditions holds true:

   a. Application developers (including maintainers) do not have sufficient
      clearance or authorization to provide an acceptable presumption that
      they have not introduced malicious logic. (See the definition of Closed
      Security Environment for an explanation of sufficient clearance.)
   b. Configuration control does not provide sufficient assurance that
      applications are protected against the introduction of malicious logic
      prior to and during the operation of system applications.

Risk Index
   The disparity between the minimum clearance or authorization of system
   users and the maximum classification of data processed by the system.

Sensitive Information
   Information that, as determined by a competent authority, must be
   protected because its unauthorized disclosure, alteration, loss, or
                                 35


     destruction will at least cause perceivable damage to someone or
     something.(4)

System
     An assembly of computer hardware, software, and firmware configured for
     the purpose of classifying, sorting, calculating, computing, summarizing,
     transmitting and receiving, storing and retrieving data with a minimum of
     human intervention.

System Users
     Users with direct connections to the system and also those individuals
     without direct connections who receive output or generate input that is
     not reliably reviewed for classification by a responsible individual.  The
     clearance of system users is used in the calculation of the risk index.
                                   37



                     ACRONYMS
A1       An evaluation class requiring a verified design
ADP      Automated Data Processing
ADPS     Automated Data Processing System
AFSC     Air Force Systems Command

B1       An Evaluation class requiring labeled security protection
B2       An Evaluation class requiring structured protection
B3       An evaluation class requiring security domains
BI       Background Investigation

C        Confidential
C1       An evaluation class requiring discretionary access protection
C2       An evaluation class requiring controlled access protection
CI       Compartmented Information
CSC      Computer Security Center
COMINT   Communications Intelligence

DCI      Director of Central Intelligence
DCID     Director of Central Intelligence Directive
DIAM     Defense Intelligence Agency Manual
DIS      Defense Investigative Service
DoD      Department of Defense
DoDCSC   Department of Defense Computer Security Center

ESD      Electronic Systems Division

FOI      Freedom of Information
FOUO     For Official Use Only
FTLS     Formal Top-Level Specification

IEEE     Institute of Electrical and Electronics Engineers

L        A personnel security clearance granted by the Department of Energy
         and the Nuclear Regulatory Commission

MC       Multiple Compartments

N        Not Cleared but Authorized Access to Sensitive Unclassified
         Information or Not Classified but Sensitive
NAC      National Agency Check
NATO     North Atlantic Treaty Organization
NOFORN   Not Releasable to Foreign Nationals
NSA      National Security Agency
NSA/CSS  National Security Agency/Central Security Service
NTIS     National Technical Information Service

OMB      Office of Management and Budget
OPI      Office of Primary Interest
OPNAV    Office of the Chief of Naval Operations
OSD      Office of the Secretary of Defense

PRO PIN  Caution--Proprietary Information Involved
                              38



Q         A personnel security clearance granted by the Department of Energy
                   and the Nuclear Regulatory Commission

S         Secret
SBI       Special Background Investigation
SCI       Sensitive Compartmented Information
SIOP      Single Integrated Operational Plan
SIOP-ESI  Single Integrated Operational Plan--Extremely Sensitive Information
SM        Staff Memorandum
STD       Standard

TCB       Trusted Computing Base
TS        Top Secret

U         Uncleared or Unclassified
U.S.      United States

IC        One Compartment
                                  39



                   REFERENCES
1. DoD Computer Security Center, Computer Security Requirements --
   Guidance for Applying the Department of Defense Trusted Computer
   System Evaluation Criteria in Specific Environments, CSC-STD-003-85, 25
   June 1985.

2. DoD Directive 5215.1, "Computer Security Evaluation Center," 25 October
   1982.

3. DoD Regulation 5200.1-R, Information Security Program Regulation,
   August 1982.

4. DoD Computer Security Center, DoD Trusted Computer System Evaluation
   Criteria, CSC-STD-001-83, IS August 1983.

5.  Army Regulation 380-380, Automated Systems Security, IS June 1979.

6. Office of the Chief of Naval Operations (OPNAV) Instruction 5239. IA
   "Department of the Navy Automatic Data Processing Security Program," 3'
   August 1982.

7. Air Force Regulation 205-16, Automated Data Processing System (ADPS)
   Security Policy, Procedures, and Responsibilities, I August 1984.

8. Marine Corps Order P5510.14, Marine Corps Automatic Data Processing
   (ADP) Security Manual, 4 November 1982.

9. DoD Directive 5220.22, "DoD Industrial Security Program," 8 December
   1980.

10. DoD Directive 5200.28, "Security Requirements for Automatic Data
   Processing Systems," 29 April 1978.

11. DoD Manual 5200.28-M, ADP Security Manual - Techniques and
   Procedures for Implementing, Deactivating, Testing, and Evaluating
   Secure Resource-Sharing ADP Systems, 25 June 1979.

12. Defense Intelligence Agency Manual (DIAM) 50-4, "Security of
   Compartmented Computer Operations (U)," 24 June 1980,
   CONFIDENTIAL.

13. National Security Agency/Central Security Service (NSA/CSS) Directive
   10-27, "Security Requirements for Automatic Data Processing (ADP)
   Systems," 29 March 1984.

14.  Director of Central Intelligence Directive (DCID), "Security Controls on
   the Dissemination of Intelligence Information (U)," 7 January 1984,
   CONFIDENTIAL.
                                   40


15.  DoD Directive 5400.7, "DoD Freedom of Information Act Program," 24
     April 1980.

16.  Office of the Secretary of Defense (OSD) Memorandum, "Control of
     Unclassified Technology with Military Application," 18 October 1983.

17.  Anderson, James P., "An Approach to Identification of Minimum TCB
     Requirements for Various Threat/Risk Environments," Proceedings of the
     1983 IEEE Symposium on Security and Privacy, 24-27 April 1983.

18.  Schell, Roger R., "Evaluating Security Properties of Systems," Proceedings
     of the IEEE Symposium on Security and Privacy, 24-27 April 1983.

19.  Defense Investigative Service (DIS) Manual 20-1, Manual for Personnel
     Security Investigations, 30 January 1981.

20.  DoD Regulation 5220.22-R, Industrial Security Regulation, January 1983.

21.  National Disclosure Policy - I, 9 September 1981.

22.  DoD Directive 5230.11, "Disclosure of Classified Military Information to
     Foreign Governments and International Organizations," 31 December
     1976.

23.  Title 35, United States Code, Section 181-188, "Patent Secrecy."

24.  Title 5, United States Code, Section 551, "Administrative Procedures Act."

25.  DoD Directive 5400.11, "Department of Defense Privacy Program," 9 June
     1982.

26.  DoD Directive 5100.36, "Defense Scientific and Technical Information
     Program," 2 October 1981.

27.  DoD Directive 5010.12, "Management of Technical Data," 5 December
     1968.

28.  Title 18, United States Code, Section 1905, "Disclosure of Confidential
     Information Generally."

29.  DoD Directive 5200.1, "DoD Information Security Program," 7 June 1982.

30.  Office of Management and Budget (OMB) Circular No. A-71, Transmittal
     Memorandum No. I, "Security of Federal Automated Information Systems,
     27 July 1978.

31.  Joint Chiefs of Staff (JCS) Staff Memorandum (SM) 313-83, Safeguarding
     the Single Integrated Operational Plan (SIOP) (U), 10 May 1983, SECRET.


                                  41

32.  "Security Policy on Intelligence Information in Automated Systems and
     Networks (U)," Promulgated by the DCI, 4 January 1983,  CONFIDENTIAL.

33.  Director of Central Intelligence Computer Security Manual (U), Prepared
     for the DCI by the Security Committee, 4 January 1983, CONFIDENTIAL.

34.  DoD Directive 5210.2, "Access to and Dissemination of Restricted Data," 12
     January 1978.

35.  DoD Instruction C-5210.21, "Implementation of NATO Security Procedure
     (U)," 17 December 1973, CONFIDENTIAL.
   





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