DOD 5200.28-STD
Supersedes
CSC-STD-00l-83, dtd l5 Aug 83
Library No. S225,7ll

DEPARTMENT OF DEFENSE STANDARD

DEPARTMENT OF
DEFENSE
TRUSTED COMPUTER
SYSTEM EVALUATION
CRITERIA

DECEMBER 1985

ASSISTANT SECURITY OF DEFENSE
COMMAND, CONTROL, COMMUNICATIONS AND INTELLIGENCE

DoD 5200.28-STD

December 26, 1985

FOREWORD

This publication, DoD 5200.28-STD, "Department of Defense Trusted Computer System Evaluation Criteria," is issued under the authority of an in accordance with DoD Directive 5200.28, "Security Requirements for Automatic Data Processing (ADP) Systems," and in furtherance of responsibilities assigned by DoD Directive 52l5.l, "Computer Security Evaluation Center." Its purpose is to provide technical hardware/firmware/software security criteria and associated technical evaluation methodologies in support of the overall ADP system security policy, evaluation and approval/accreditation responsibilities promulgated by DoD Directive 5200.28.

The provisions of this document apply to the Office of the Secretary of Defense (ASD), the Military Departments, the Organization of the Joint Chiefs of Staff, the Unified and Specified Commands, the Defense Agencies and activities administratively supported by OSD (hereafter called "DoD Components").

This publication is effective immediately and is mandatory for use by all DoD Components in carrying out ADP system technical security evaluation activities applicable to the processing and storage of classified and other sensitive DoD information and applications as set forth herein.

Recommendations for revisions to this publication are encouraged and will be reviewed biannually by the National Computer Security Center through a formal review process. Address all proposals for revision through appropriate channels to: National Computer Security Center, Attention: Chief, Computer Security Standards.

DoD Components may obtain copies of this publication through their own publications channels. Other federal agencies and the public may obtain copies from: Office of Standards and Products, National Computer Security Center, Fort Meade, MD 20755-6000, Attention: Chief, Computer Security Standards.

Donald C. Latham

Assistant Secretary of Defense

(Command, Control, Communications, and Intelligence)

ACKNOWLEDGEMENTS

Special recognition is extended to Sheila L. Brand, National Computer Security Center (NCSC), who integrated theory, policy, and practice into and directed the production of this document.

Acknowledgment is also given for the contributions of: Grace Hammonds and Peter S. Tasker, the MITRE Corp., Daniel J. Edwards, NCSC, Roger R. Schell, former Deputy Director of NCSC, Marvin Schaefer, NCSC, and Theodore M. P. Lee, Sperry Corp., who as original architects formulated and articulated the technical issues and solutions presented in this document; Jeff Makey, formerly NCSC, Warren F. Shadle, NCSC, and Carole S. Jordan, NCSC, who assisted in the preparation of this document; James P. Anderson, James P. Anderson & Co., Steven B. Lipner, Digital Equipment Corp., Clark Weissman, System Development Corp., LTC Lawrence A. Noble, formerly U.S. Air Force, Stephen T. Walker, formerly DoD, Eugene V. Epperly, DoD, and James E. Studer, formerly Dept. of the Army, who gave generously of their time and expertise in the review and critique of this document; and finally, thanks are given to the computer industry and others interested in trusted computing for their enthusiastic advice and assistance throughout this effort.

FOREWORD
ACKNOWLEDGEMENTS
PREFACE
INTRODUCTION

PART I: CRITERIA

1.0 DIVISION D: MINIMAL PROTECTION

2.0 DIVISION C: DISCRETIONARY PROTECTION

2.1 CLASS (C1): DISCRETIONARY SECURITY PROTECTION

2.2 CLASS (C2): CONTROLLED ACCESS PROTECTION

3.0 DIVISION B: MANDATORY PROTECTION

3.1 CLASS (B1): LABELED SECURITY PROTECTION

3.2 CLASS (B2): STRUCTURED PROTECTION

3.3 CLASS (B3): SECURITY DOMAINS

4.0 DIVISION A: VERIFIED PROTECTION

4.1 CLASS (A1): VERIFIED DESIGN

4.2 BEYOND CLASS (A1)

PART II: RATIONAL AND GUIDELINES

5.0 CONTROL OBJECTIVES FOR TRUSTED COMPUTER SYSTEMS

5.1 A NEED FOR CONSENSUS

5.2 DEFINITION AND USEFULNESS

5.3 CRITERIA CONTROL OBJECTIVES

6.0 RATIONALE BEHIND THE EVALUATION CLASSES

6.1 THE REFERENCE MONITOR CONCEPT

6.2 A FORMAL SECURITY POLICY MODEL

6.3 THE TRUSTED COMPUTING BASE

6.4 ASSURANCE

6.5 THE CLASSES

7.0 THE RELATIONSHIP BETWEEN POLICY AND THE CRITERIA

7.1 ESTABLISHED FEDERAL POLICIES

7.2 DOD POLICIES

7.3 CRITERIA CONTROL OBJECTIVE FOR SECURITY POLICY

7.4 CRITERIA CONTROL OBJECTIVE FOR ACCOUNTABILITY

7.5 CRITERIA CONTROL OBJECTIVE FOR ASSURANCE

8.0 A GUIDELINE ON COVERT CHANNELS

9.0 A GUIDELINE ON CONFIGURING MANDATORY ACCESS CONTROL FEATURES

10.0 A GUIDELINE ON SECURITY TESTING

10.1 TESTING FOR DIVISION C

10.2 TESTING FOR DIVISION B

10.3 TESTING FOR DIVISION A

APPENDIX A: Commercial Product Evaluation Process
APPENDIX B: Summary of Evaluation Criteria Divisions
APPENDIX C: Summary of Evaluation Criteria Classes
APPENDIX D: Requirement Directory
GLOSSARY
REFERENCES

PREFACE

The trusted computer system evaluation criteria defined in this document classify systems into four broad hierarchical divisions of enhanced security protection. They provide a basis for the evaluation of effectiveness of security controls built into automatic data processing system products. The criteria were developed with three objectives in mind: (a) to provide users with a yardstick with which to assess the degree of trust that can be placed in computer systems for the secure processing of classified or other sensitive information; (b) to provide guidance to manufacturers as to what to build into their new, widely-available trusted commercial products in order to satisfy trust requirements for sensitive applications; and (c) to provide a basis for specifying security requirements in acquisition specifications. Two types of requirements are delineated for secure processing: (a) specific security feature requirements and (b) assurance requirements. Some of the latter requirements enable evaluation personnel to determine if the required features are present and functioning as intended. The scope of these criteria is to be applied to the set of components comprising a trusted system, and is not necessarily to be applied to each system component individually. Hence, some components of a system may be completely untrusted, while others may be individually evaluated to a lower or higher evaluation class than the trusted product considered as a whole system. In trusted products at the high end of the range, the strength of the reference monitor is such that most of the components can be completely untrusted. Though the criteria are intended to be application-independent, the specific security feature requirements may have to be interpreted when applying the criteria to specific systems with their own functional requirements, applications or special environments (e.g., communications processors, process control computers, and embedded systems in general). The underlying assurance requirements can be applied across the entire spectrum of ADP system or application processing environments without special interpretation.

INTRODUCTION

Historical Perspective

In October 1967, a task force was assembled under the auspices of the Defense Science Board to address computer security safeguards that would protect classified information in remote-access, resource-sharing computer systems. The Task Force report, "Security Controls for Computer Systems," published in February 1970, made a number of policy and technical recommendations on actions to be taken to reduce the threat of compromise of classified information processed on remote-access computer systems.[34] Department of Defense Directive 5200.28 and its accompanying manual DoD 5200.28-M, published in 1972 and 1973 respectively, responded to one of these recommendations by establishing uniform DoD policy, security requirements, administrative controls, and technical measures to protect classified information processed by DoD computer systems.[8;9] Research and development work undertaken by the Air Force, Advanced Research Projects Agency, and other defense agencies in the early and mid 70's developed and demonstrated solution approaches for the technical problems associated with controlling the flow of information in resource and information sharing computer systems.[1] The DoD Computer Security Initiative was started in 1977 under the auspices of the Under Secretary of Defense for Research and Engineering to focus DoD efforts addressing computer security issues.[33]

Concurrent with DoD efforts to address computer security issues, work was begun under the leadership of the National Bureau of Standards (NBS) to define problems and solutions for building, evaluating, and auditing secure computer systems.[17] As part of this work NBS held two invitational workshops on the subject of audit and evaluation of computer security.[20;28] The first was held in March 1977, and the second in November of 1978. One of the products of the second workshop was a definitive paper on the problems related to providing criteria for the evaluation of technical computer security effectiveness.[20] As an outgrowth of recommendations from this report, and in support of the DoD Computer Security Initiative, the MITRE Corporation began work on a set of computer security evaluation criteria that could be used to assess the degree of trust one could place in a computer system to protect classified data.[24;25;31] The preliminary concepts for computer security evaluation were defined and expanded upon at invitational workshops and symposia whose participants represented computer security expertise drawn from industry and academia in addition to the government. Their work has since been subjected to much peer review and constructive technical criticism from the DoD, industrial research and development organizations, universities, and computer manufacturers.

The DoD Computer Security Center (the Center) was formed in January 1981 to staff and expand on the work started by the DoD Computer Security Initiative.[15] A major goal of the Center as given in its DoD Charter is to encourage the widespread availability of trusted computer systems for use by those who process classified or other sensitive information.[10] The criteria presented in this document have evolved from the earlier NBS and MITRE evaluation material.

Scope

The trusted computer system evaluation criteria defined in this document apply primarily to trusted commercially available automatic data processing (ADP) systems. They are also applicable, as amplified below, the the evaluation of existing systems and to the specification of security requirements for ADP systems acquisition. Included are two distinct sets of requirements: 1) specific security feature requirements; and 2) assurance requirements. The specific feature requirements encompass the capabilities typically found in information processing systems employing general-purpose operating systems that are distinct from the applications programs being supported. However, specific security feature requirements may also apply to specific systems with their own functional requirements, applications or special environments (e.g., communications processors, process control computers, and embedded systems in general). The assurance requirements, on the other hand, apply to systems that cover the full range of computing environments from dedicated controllers to full range multilevel secure resource sharing systems.

Purpose

As outlined in the Preface, the criteria have been developedto serve a number of intended purposes:

To provide a standard to manufacturers as to what security features to build into their new and planned, commercial products in order to provide widely available systems that satisfy trust requirements (with particular emphasis on preventing the disclosure of data) for sensitive applications.
To provide DoD components with a metric with which to evaluate the degree of trust that can be placed in computer systems for the secure processing of classified and other sensitive information.
To provide a basis for specifying security requirements in acquisition specifications.

With respect to the second purpose for development of the criteria, i.e., providing DoD components with a security evaluation metric, evaluations can be delineated into two types: (a) an evaluation can be performed on a computer product from a perspective that excludes the application environment; or, (b) it can be done to assess whether appropriate security measures have been taken to permit the system to be used operationally in a specific environment. The former type of evaluation is done by the Computer Security Center through the Commercial Product Evaluation Process. That process is described in Appendix A.

The latter type of evaluation, i.e., those done for the purpose of assessing a system's security attributes with respect to a specific operational mission, is known as a certification evaluation. It must be understood that the completion of a formal product evaluation does not constitute certification or accreditation for the system to be used in any specific application environment. On the contrary, the evaluation report only provides a trusted computer system's evaluation rating along with supporting data describing the product system's strengths and weaknesses from a computer security point of view. The system security certification and the formal approval/accreditation procedure, done in accordance with the applicable policies of the issuing agencies, must still be followed-before a system can be approved for use in processing or handling classified information.[8;9] Designated Approving Authorities (DAAs) remain ultimately responsible for specifying security of systems they accredit.

The trusted computer system evaluation criteria will be used directly and indirectly in the certification process. Along with applicable policy, it will be used directly as technical guidance for evaluation of the total system and for specifying system security and certification requirements for new acquisitions. Where a system being evaluated for certification employs a product that has undergone a Commercial Product Evaluation, reports from that process will be used as input to the certification evaluation. Technical data will be furnished to designers, evaluators and the Designated Approving Authorities to support their needs for making decisions.

Fundamental Computer Security Requirements

Any discussion of computer security necessarily starts from a statement of requirements, i.e., what it really means to call a computer system "secure." In general, secure systems will control, through use of specific security features, access to information such that only properly authorized individuals, or processes operating on their behalf, will have access to read, write, create, or delete information. Six fundamental requirements are derived from this basic statement of objective: four deal with what needs to be provided to control access to information; and two deal with how one can obtain credible assurances that this is accomplished in a trusted computer system.

Policy

Requirement 1 - SECURITY POLICY - There must be an explicit and well-defined security policy enforced by the system. Given identified subjects and objects, there must be a set of rules that are used by the system to determine whether a given subject can be permitted to gain access to a specific object. Computer systems of interest must enforce a mandatory security policy that can effectively implement access rules for handling sensitive (e.g., classified) information.[7] These rules include requirements such as: No person lacking proper personnel security clearance shall obtain access to classified information. In addition, discretionary security controls are required to ensure that only selected users or groups of users may obtain access to data (e.g., based on a need-to-know).

Requirement 2 - MARKING - Access control labels must be associated with objects. In order to control access to information stored in a computer, according to the rules of a mandatory security policy, it must be possible to mark every object with a label that reliably identifies the object's sensitivity level (e.g., classification), and/or the modes of access accorded those subjects who may potentially access the object.

Accountability

Requirement 3 - IDENTIFICATION - Individual subjects must be identified. Each access to information must be mediated based on who is accessing the information and what classes of information they are authorized to deal with. This identification and authorization information must be securely maintained by the computer system and be associated with every active element that performs some security-relevant action in the system.

Requirement 4 - ACCOUNTABILITY - Audit information must be selectively kept and protected so that actions affecting security can be traced to the responsible party. A trusted system must be able to record the occurrences of security-relevant events in an audit log. The capability to select the audit events to be recorded is necessary to minimize the expense of auditing and to allow efficient analysis. Audit data must be protected from modification and unauthorized destruction to permit detection and after-the-fact investigations of security violations.

Assurance

Requirement 5 - ASSURANCE - The computer system must contain hardware/software mechanisms that can be independently evaluated to provide sufficient assurance that the system enforces requirements 1 through 4 above. In order to assure that the four requirements of Security Policy, Marking, Identification, and Accountability are enforced by a computer system, there must be some identified and unified collection of hardware and software controls that perform those functions. These mechanisms are typically embedded in the operating system and are designed to carry out the assigned tasks in a secure manner. The basis for trusting such system mechanisms in their operational setting must be clearly documented such that it is possible to independently examine the evidence to evaluate their sufficiency.

Requirement 6 - CONTINUOUS PROTECTION - The trusted mechanisms that enforce these basic requirements must be continuously protected against tampering and/or unauthorized changes. No computer system can be considered truly secure if the basic hardware and software mechanisms that enforce the security policy are themselves subject to unauthorized modification or subversion. The continuous protection requirement has direct implications throughout the computer system's life-cycle.

These fundamental requirements form the basis for the individual evaluation criteria applicable for each evaluation division and class. The interested reader is referred to Section 5 of this document, "Control Objectives for Trusted Computer Systems," for a more complete discussion and further amplification of these fundamental requirements as they apply to general-purpose information processing systems and to Section 7 for amplification of the relationship between Policy and these requirements.

Structure of the Document

The remainder of this document is divided into two parts, four appendices, and a glossary. Part I (Sections 1 through 4) presents the detailed criteria derived from the fundamental requirements described above and relevant to the rationale and policy excerpts contained in Part II.

Part II (Sections 5 through 10) provides a discussion of basic objectives, rationale, and national policy behind the development of the criteria, and guidelines for developers pertaining to: mandatory access control rules implementation, the covert channel problem, and security testing. It is divided into six sections. Section 5 discusses the use of control objectives in general and presents the three basic control objectives of the criteria. Section 6 provides the theoretical basis behind the criteria. Section 7 gives excerpts from pertinent regulations, directives, OMB Circulars, and Executive Orders which provide the basis for many trust requirements for processing nationally sensitive and classified information with computer systems. Section 8 provides guidance to system developers on expectations in dealing with the covert channel problem. Section 9 provides guidelines dealing with mandatory security. Section 10 provides guidelines for security testing. There are four appendices, including a description of the Trusted Computer System Commercial Products Evaluation Process (Appendix A), summaries of the evaluation divisions (Appendix B) and classes (Appendix C), and finally a directory of requirements ordered alphabetically. In addition, there is a glossary.

Structure of the Criteria

The criteria are divided into four divisions: D, C, B, and A ordered in a hierarchical manner with the highest division (A) being reserved for systems providing the most comprehensive security. Each division represents a major improvement in the overall confidence one can place in the system for the protection of sensitive information. Within divisions C and B there are a number of subdivisions known as classes. The classes are also ordered in a hierarchical manner with systems representative of division C and lower classes of division B being characterized by the set of computer security mechanisms that they possess. Assurance of correct and complete design and implementation for these systems is gained mostly through testing of the security- relevant portions of the system. The security-relevant portions of a system are referred to throughout this document as the Trusted Computing Base (TCB). Systems representative of higher classes in division B and division A derive their security attributes more from their design and implementation structure. Increased assurance that the required features are operative, correct, and tamperproof under all circumstances is gained through progressively more rigorous analysis during the design process.

Within each class, four major sets of criteria are addressed. The first three represent features necessary to satisfy the broad control objectives of Security Policy, Accountability, and Assurance that are discussed in Part II, Section 5. The fourth set, Documentation, describes the type of written evidence in the form of user guides, manuals, and the test and design documentation required for each class.

A reader using this publication for the first time may find it helpful to first read Part II, before continuing on with Part I.

PART I: THE CRITERIA

Highlighting is used in Part I to indicate criteria not contained in a lower class or changes and additions to already defined criteria. Where there is no highlighting, requirements have been carried over from lower classes without addition or modification.

1.0 DIVISION D: MINIMAL PROTECTION

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

2.0 DIVISION C: DISCRETIONARY PROTECTION

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

2.1 CLASS (C1): DISCRETIONARY SECURITY PROTECTION

The Trusted Computing Base (TCB) of a class (C1) system 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 (C1) environment is expected to be one of cooperating users processing data at the same level(s) of sensitivity. The following are minimal requirements for systems assigned a class (C1) rating:

2.1.1 Security Policy

2.1.1.1 Discretionary Access Control

2.1.2 Accountability

2.1.2.1 Identification and Authentication

2.1.3 Assurance

2.1.3.1 Operational Assurance

2.1.3.1.1 System Architecture

2.1.3.1.2 System Integrity

2.1.3.2 Life-Cycle Assurance

2.1.3.2.1 Security Testing

2.1.4 Documentation

2.1.4.1 Security Features User's Guide

2.1.4.2 Trusted Facility Manual

2.1.4.3 Test Documentation

2.1.4.4 Design Documentation

2.1.1 Security Policy

2.1.1.1 Discretionary Access Control

2.1.2 Accountability

2.1.2.1 Identification and Authentication

2.1.3 Assurance

2.1.3.1 Operational Assurance

2.1.3.1.1 System Architecture

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 strucutres). Resources controlled by the TCB may be a defined subset of the subjects and objects in the ADP system.

2.1.3.1.2 System Integrity

Hardware and/or software features shall be provided that can be used to periodically validate the correct operation of the on-site hardware and firmware elements of the TCB.

2.1.3.2 Life-Cycle Assurance

2.1.3.2.1 Security Testing

2.1.4 Documentation

2.1.4.1 Security Features User's Guide

A single summary, 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.

2.1.4.2 Trusted Facility Manual

A manual addressed to the ADP System Administrator shall present cautions about functions and privileges that should be controlled when running a secure facility.

2.1.4.3 Test Documentation

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

2.1.4.4 Design Documentation

2.2 CLASS (C2): CONTROLLED ACCESS PROTECTION

Systems in this class enforce a more finely grained discretionary access control than (C1) systems, making users individually accountable for their actions through login procedures, auditing of security-relevant events, and resource isolation. The following are minimal requirements for systems assigned a class (C2) rating:

2.2.1 Security Policy

2.2.1.1 Discretionary Access Control

2.2.1.2 Object Reuse

2.2.2 Accountability

2.2.2.1 Identification and Authentication

2.2.2.2 Audit

2.2.3 Assurance

2.2.3.1 Operational Assurance

2.2.3.1.1 System Architecture

2.2.3.1.2 System Integrity

2.2.3.2 Life-Cycle Assurance

2.2.3.2.1 Security Testing

2.2.4 Documentation

2.2.4.1 Security Features User's Guide
2.2.4.2 Trusted Facility Manual
2.2.4.3 Test Documentation
2.2.4.4 Design Documentation

2.2.1 Security Policy

2.2.1.1 Discretionary Access Control

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 specify and control sharing of those objects by named individuals, or defined groups of individuals, or by both, and shall provide controls to limit propagation of access rights. The discretionary access control mechanism shall, either by explicit user action or by default, provide that objects are protected from unauthorized access. These access controls shall be capable of including or excluding access to the granularity of a single user. Access permission to an object by users not already possessing access permission shall only be assigned by authorized users.

2.2.1.2 Object Reuse

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.

2.2.2 Accountability

2.2.2.1 Identification and Authentication

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. The TCB shall be able to enforce individual accountability by providing the capability to uniquely identify each individual ADP system user. The TCB shall also provide the capability of associating this identity with all auditable actions taken by that individual.

2.2.2.2 Audit

2.2.3 Assurance

2.2.3.1 Operational Assurance

2.2.3.1.1 System Architecture

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 controlled by the TCB may be a defined subset of the subjects and objects in the ADP system. The TCB shall isolate the resources to be protected so that they are subject to the access control and auditing requirements.

2.2.3.1.2 System Integrity

Hardware and/or software features shall be provided that can be used to periodically validate the correct operation of the on-site hardware and firmware elements of the TCB.

2.2.3.2 Life-Cycle Assurance

2.2.3.2.1 Security Testing

The security mechanisms of the ADP system shall be tested and found to work as claimed in the system documentation. Testing shall be done to assure that there are no obvious ways for an unauthorized user to bypass or otherwise defeat the security protection mechanisms of the TCB. Testing shall also include a search for obvious flaws that would allow violation of resource isolation, or that would permit unauthorized access to the audit or authentication data. (See the Security Testing guidelines.)

2.2.4 Documentation

2.2.4.1 Security Features User's Guide

A single summary, 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.

2.2.4.2 Trusted Facility Manual

2.2.4.3 Test Documentation

2.2.4.4 Design Documentation

3.0 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.

3.1 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. The following are minimal requirements for systems assigned a class (B1) rating:

3.1.1 Security Policy

3.1.1.1 Discretionary Access Control

3.1.1.2 Object Reuse

3.1.1.3 Labels

3.1.1.3.1 Label Integrity

3.1.1.3.2 Exportation of Labeled Information

3.1.1.3.2.1 Exportation to Multilevel Devices

3.1.1.3.2.2 Exportation to Single-Level Devices

3.1.1.3.2.3 Labeling Human-Readable Output

3.1.1.4 Mandatory Access Control

3.1.2 Accountability

3.1.2.1 Identification and Authentication

3.1.2.2 Audit

3.1.3 Assurance

3.1.3.1 Operational Assurance

3.1.3.1.1 System Architecture

3.1.3.1.2 System Integrity

3.1.3.2 Life-Cycle Assurance

3.1.3.2.1 Security Testing

3.1.3.2.2 Design Specification and Verification

3.1.4 Documentation

3.1.4.1 Security Features User's Guide
3.1.4.2 Trusted Facility Manual
3.1.4.3 Test Documentation
3.1.4.4 Design Documentation

3.1.1 Security Policy

3.1.1.1 Discretionary Access Control

3.1.1.2 Object Reuse

3.1.1.3 Labels

Sensitivity labels associated with each subject and storage object under its control (e.g., process, file, segment, device) shall be maintained by the TCB. These labels shall be used as the basis for mandatory access control decisions. In order to import non-labeled data, the TCB shall request and receive from an authorized user the security level of the data, and all such actions shall be auditable by the TCB.

3.1.1.3.1 Label Integrity

Sensitivity labels shall accurately represent security levels of the specific subjects or objects with which they are associated. When exported by the TCB, sensitivity labels shall accurately and unambiguously represent the internal labels and shall be associated with the information being exported.

3.1.1.3.2 Exportation of Labeled Information

The TCB shall designate each communication channel and I/O device as either single-level or multilevel. Any change in this designation shall be done manually and shall be auditable by the TCB. The TCB shall maintain and be able to audit any change in the security level or levels associated with a communication channel or I/O device.

3.1.1.3.2.1 Exportation to Multilevel Devices

When the TCB exports an object to a multilevel I/O device, the sensitivity label associated with that object shall also be exported and shall reside on the same physical medium as the exported information and shall be in the same form (i.e., machine-readable or human-readable form). When the TCB exports or imports an object over a multilevel communication channel, the protocol used on that channel shall provide for the unambiguous pairing between the sensitivity labels and the associated information that is sent or received.

3.1.1.3.2.2 Exportation to Single-Level Devices

Single-level I/O devices and single-level communication channels are not required to maintain the sensitivity labels of the information they process. However, the TCB shall include a mechanism by which the TCb and an authorized user reliably communicate to designate the single security level of information imported or exported via single-level communication channels or I/O devices.

3.1.1.3.2.3 Labeling Human-Readable Output

The ADP system administrator shall be able to specify the printable label names associated with exported sensitivity labels. The TCB shall mark the beginning and end of all human-readable, paged, hardcopy output (e.g., line printer output) with human-readable sensitivity labels that properly represent the sensitivity of the output. The TCB shall, be default, mark the top and bottom of each page of human-readable, paged, hardcopy output (e.g., line printer output) with human-readable sensitivity labels that properly represent the overall sensitivity of the output or that properly represent the sensitivity of the information on the page. The TCB shall, by default and in an appropriate manner, mark other forms of human-readable output (e.g., maps, graphics) with human-readable sensitivity labels that properly represent the sensitivity of the touput. Any override of these marking defaults shall be auditable by the TCB.

3.1.1.4 Mandatory Access Control

The TCB shall enforce a mandatory access control policy over all subjects and storage objects under its control (e.g., processes, files, segments, devices). These subjects and objects shall be assigned sensitivity labels that are a combination of hierarchical classification levels and non-hierarchical categories, and the labels shall be used as the basis for mandatory access control decisions. The TCB shall be able to support two or more such security levels. (See the Mandatory Access Control Guidelines.) The following requirements shall hold for all accesses between subjects and objects controlled by the TCB: a subject can read an object only if the hierarchical classification in the subject's security level is greater than or equal to the hierarchical classification in the object's security level and the non-hierarchical categories in the subject's security level include all the non-hierarchical categories in the object's security level. A subject can write an object only if the hierarchical classification in the subject's security level is less than or equal to the hierarchical classification in the object's security level and all the non-hierarchical categories in the subject's security level are included in the non-hierarchical categories in the object's security level. Identification and authentication data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authorization of subjects external to the TCB that may be created to act on behalf of the individual user are dominated by the clearance and authorization of that user.

3.1.2 Accountability

3.1.2.1 Identification and Authentication

The TCB shall require users to identify themselves to it before beginning to perform any other actions that the TCB expected to mediate. Furthermore, the TCB shall maintain authentication data that includes information for verifying the identity of individual users (e.g., passwords) as well as information for determining the clearance and authorizations or individual users. This data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authorizations of subjects external to the TCB that may be created to act on behalf of the individual user are dominated by the clearance and authorization of that user. The TCB shall protect authentication data so that it cannot be accessed by any unauthorized user. The TCB shall be able to enforce individual accountability by providing the capability to uniquely identify each individual ADP system user. The TCB shall also provide the capability of associating this identity with all auditable actions taken by that individual.

3.1.2.2 Audit

The TCB shall 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 shall 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, and actions taken by computer operators and system administrators and/or system security officers and other security relevant events. The TCB shall also be able to audit any override of human-readable output markings. 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 identification/authentication events the origin of request (e.g., terminal ID) shall 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 and the object's security level. The ADP system administrator shall be able to selectively audit the actions of any one or more users based on individual identity and/or object security level.

3.1.3 Assurance

3.1.3.1 Operational Assurance

3.1.3.1.1 System Architecture

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 controlled by the TCB may be a defined subset of the subjects and objects in the ADP system. The TCB shall maintain process isolation through the provision of distinct address spaces under its control. The TCB shall isolate the resources to be protected so that they are subject to the access control and auditing requirements.

3.1.3.1.2 System Integrity

Hardware and/or software features shall be provided that can be used to periodically validate the correct operation of the on-site hardware and firmware elements of the TCB.

3.1.3.2 Life-Cycle Assurance

3.1.3.2.1 Security Testing

The security mechanisms of the ADP system shall be tested and found to work as claimed in the system documentation. A team of individuals who thoroughly understand the specific implementation of the TCB shall subject its design documentation, source code, and object code to thorough analysis and testing. Their objectives shall be: to uncover all design and implementation flaws that would permit a subject external to the TCB to read, change, or delete data normally denied under the mandatory or discretionary security policy enforced by the TCB; as well as to assure that no subject (without authorization to do so) is able to cause the TCB to enter a state such that it is unable to respond to communications initiated by other users. All discovered flaws shall be removed or neutralized and the TCB retested to demonstrate that they have been eliminated and that new flaws have not been introduced. (See the Security Testing Guidelines.)

3.1.3.2.2 Design Specification and Verification

An informal or formal model of the security policy supported by the TCB shall be maintained over the life cycle of the ADP system and demonstrated to be consistent with its axioms.

3.1.4 Documentation

3.1.4.1 Security Features User's Guide

A single summary, 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.

3.1.4.2 Trusted Facility Manual

3.1.4.3 Test Documentation

3.1.4.4 Design Documentation

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. An informal or formal description of the security policy model enforced by the TCB shall be available and an explanation provided to show that it is sufficient to enforce the security policy. The specific TCB protection mechanisms shall be identified and an explanation given to show that they satisfy the model.

3.2 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 class (B1) systems be extended to all subjects and objects in the ADP system. In addition, covert channels are addressed. The TCB must be carefully structured into protection-critical and non-protection-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 system administrator and operator functions, and stringent configuration management controls are imposed. The system is relatively resistant to penetration. The following are minimal requirements for systems assigned a class (B2) rating:

3.2.1 Security Policy

3.2.1.1 Discretionary Access Control

3.2.1.2 Object Reuse

3.2.1.3 Labels

3.2.1.3.1 Label Integrity

3.2.1.3.2 Exportation of Labeled Information

3.2.1.3.2.1 Exportation to Multilevel Devices

3.2.1.3.2.2 Exportation to Single-Level Devices

3.2.1.3.2.3 Labeling Human-Readable Output

3.2.1.3.3 Subject Sensitivity Labels

3.2.1.3.4 Device Labels

3.2.1.4 Mandatory Access Control

3.2.2 Accountability

3.2.2.1 Identification and Authentication

3.2.2.1.1 Trusted Path

3.2.2.2 Audit

3.2.3 Assurance

3.2.3.1 Operational Assurance

3.2.3.1.1 System Architecture

3.2.3.1.2 System Integrity

3.2.3.1.3 Covert Channel Analysis

3.2.3.1.4 Trusted Facility Management

3.2.3.2 Life-Cycle Assurance

3.2.3.2.1 Security Testing

3.2.3.2.2 Design Specification and Verification

3.2.3.2.3 Configuration Management

3.2.4 Documentation

3.2.4.1 Security Features User's Guide

3.2.4.2 Trusted Facility Manual

3.2.4.3 Test Documentation

3.2.4.4 Design Documentation

3.2.1 Security Policy

3.2.1.1 Discretionary Access Control

3.2.1.2 Object Reuse

3.2.1.3 Labels

Sensitivity labels associated with each ADP system resource (e.g., subject, storage object, ROM) that is directly or indirectly accessible by subjects external to the TCB shall be maintained by the TCB. These labels shall be used as the basis for mandatory access control decisions. In order to import non-labeled data, the TCB shall request and receive from an authorized user the security level of the data, and all such actions shall be auditable by the TCB.

3.2.1.3.1 Label Integrity

3.2.1.3.2 Exportation of Labeled Information

3.2.1.3.2.1 Exportation to Multilevel Devices

3.2.1.3.2.2 Exportation to Single-Level Devices

Single-level I/O devices and single-level communication channels are not required to maintain the sensitivity labels of the information they process. However, the TCB shall include a mechanism by which the TCB and an authorized user reliably communicate to designate the single security level of information imported or exported via single-level communication channels or I/O devices.

3.2.1.3.2.3 Labeling Human-Readable Output

The ADP system administrator shall be able to specify the printable label names associated with exported sensitivity labels. The TCB shall mark the beginning and end of all human-readable, paged, hardcopy output (e.g., line printer output) with human-readable sensitivity labels that properly represent the sensitivity of the output. The TCB shall, by default, mark the top and bottom of each page of human-readable, paged, hardcopy output (e.g., line printer output) with human-readable sensitivity labels that properly represent the overall sensitivity of the output or that properly represent the sensitivity of the information on the page. The TCB shall, by default and in an appropriate manner, mark other forms of human-readable output (e.g., maps, graphics) with human-readable sensitivity labels that properly represent the sensitivity of the output. Any override of these marking defaults shall be auditable by the TCB.

3.2.1.3.3 Subject Sensitivity Labels

The TCB shall immediately notify a terminal user of each change in the security level associated with that user during an interactive session. A terminal user shall be able to query the TCB as desired for a display of the subject's complete sensitivity label.

3.2.1.3.4 Device Labels

The TCB shall support the assignment of minimum and maximum security levels to all attached physical devices. These security levels shall be used by the TCB to enforce constraints imposed by the physical environments in which the devices are located.

3.2.1.4 Mandatory Access Control

The TCB shall enforce a mandatory access control policy over all resources (i.e., subjects, storage objects, and I/ O devices that are directly or indirectly accessible by subjects external to the TCB. These subjects and objects shall be assigned sensitivity labels that are a combination of hierarchical classification levels and non-hierarchical categories, and the labels shall be used as the basis for mandatory access control decisions. The TCB shall be able to support two or more such security levels. (See the Mandatory Access Control guidelines.) The following requirements shall hold for all accesses between all subjects external to the TCB and all objects directly or indirectly accessible by these subjects: A subject can read an object only if the hierarchical classification in the subject's security level is greater than or equal to the hierarchical classification in the object's security level and the non-hierarchical categories in the subject's security level include all the non-hierarchical categories in the object's security level. A subject can write an object only if the hierarchical classification in the subject's security level is less than or equal to the hierarchical classification in the object's security level and all the non-hierarchical categories in the subject's security level are included in the non-hierarchical categories in the object's security level. Identification and authentication data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authorization of subjects external to the TCB that may be created to act on behalf of the individual user are dominated by the clearance and authorization of that user.

3.2.2 Accountability

3.2.2.1 Identification and Authentication

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 maintain authentication data that includes information for verifying the identity of individual users (e.g., passwords) as well as information for determining the clearance and authorizations of individual users. This data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authorizations of subjects external to the TCB that may be created to act on behalf of the individual user are dominated by the clearance and authorization of that user. The TCB shall protect authentication data so that it cannot be accessed by any unauthorized user. The TCB shall be able to enforce individual accountability by providing the capability to uniquely identify each individual ADP system user. The TCB shall also provide the capability of associating this identity with all auditable actions taken by that individual.

3.2.2.1.1 Trusted Path

The TCB shall support a trusted communication path between itself and user for initial login and authentication. Communications via this path shall be initiated exclusively by a user.

3.2.2.2 Audit

The TCB shall 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 shall 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, and actions taken by computer operators and system administrators and/or system security officers, and other security relevant events. The TCB shall also be able to audit any override of human-readable output markings. 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 identification/authentication events the origin of request (e.g., terminal ID) shall 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 and the object's security level. The ADP system administrator shall be able to selectively audit the actions of any one or more users based on individual identity and/or object security level. The TCB shall be able to audit the identified events that may be used in the exploitation of covert storage channels.

3.2.3 Assurance

3.2.3.1 Operational Assurance

3.2.3.1.1 System Architecture

3.2.3.1.2 System Integrity

Hardware and/or software features shall be provided that can be used to periodically validate the correct operation of the on-site hardware and firmware elements of the TCB.

3.2.3.1.3 Covert Channel Analysis

The system developer shall conduct a thorough search for covert storage channels and make a determination (either by actual measurement or by engineering estimation) of the maximum bandwidth of each identified channel. (See the covert channels guideline section.)

3.2.3.1.4 Trusted Facility Management

The TCB shall support separate operator and administrator functions.

3.2.3.2 Life-Cycle Assurance

3.2.3.2.1 Security Testing

The security mechanisms of the ADP system shall be tested and found to work as claimed in the system documentation. A team of individuals who thoroughly understand the specific implementation of the TCB shall subject its design documentation, source code, and object code to thorough analysis and testing. Their objectives shall be: to uncover all design and implementation flaws that would permit a subject external to the TCB to read, change, or delete data normally denied under the mandatory or discretionary security policy enforced by the TCB; as well as to assure that no subject (without authorization to do so) is able to cause the TCB to enter a state such that it is unable to respond to communications initiated by other users. The TCB shall be found relatively resistant to penetration. All discovered flaws shall be corrected and the TCB retested to demonstrate that they have been eliminated and that new flaws have not been introduced. Testing shall demonstrate that the TCB implementation is consistent with the descriptive top-level specification. (See the Security Testing Guidelines.)

3.2.3.2.2 Design Specification and Verification

A formal model of the security policy supported by the TCB shall be maintained over the life cycle of the ADP system that is proven consistent with its axioms. A descriptive top-level specification (DTLS) of the TCB shall be maintained that completely and accurately describes the TCB in terms of exceptions, error messages, and effects. It shall be shown to be an accurate description of the TCB interface.

3.2.3.2.3 Configuration Management

During development and maintenance of the TCB, a configuration management system shall be in place that maintains control of changes to the descriptive top-level specification, other design data, implementation documentation, source code, the running versionof the object code, and test fixtures and documentation. The configuration management system shall assure a consistent mapping among all documentation and code associated with the current version of the TCB. Tools shall be provided for generation of a new version of the TCB from source code. Also available shall be tools for comparing a newly generated version with the previous TCB version in order to ascertain that only the intended changes have been made in the code that will actually be used as the new version of the TCB.

3.2.4 Documentation

3.2.4.1 Security Features User's Guide

A single summary, 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.

3.2.4.2 Trusted Facility Manual

A manual addressed to the ADP system administrator shall present cautions about functions and privileges that should be controlled when running a secure facility. The procedures for examining and maintaining the audit files as well as the detailed audit record structure for each type of audit event shall be given. The manual shall describe the operator and administrator functions related to security, to include changing the security characteristics of a user. It shall provide guidelines on the consistent and effective use of the protection features of the system, how they interact, how to securely generate a new TCB, and facility procedures, warnings, and privileges that need to be controlled in order to operate the facility in a secure manner. The TCB modules that contain the reference validation mechanism shall be identified. The procedures for secure generation of a new TCB from source after modification of any modules in the TCB shall be described.

3.2.4.3 Test Documentation

The system developer shall provide to the evaluators a document that describes the test plan, test procedures that show how the security mechanisms were tested, and results of the security mechanisms' functional testing. It shall include results of testing the effectiveness of the methods used to reduce covert channel bandwidths.

3.2.4.4 Design Documentation

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. The interfaces between the TCB modules shall be described. A formal description of the security policy model enforced by the TCB shall be available and proven that it is sufficient to enforce the security policy. The specific TCB protection mechanisms shall be identified and an explanation given to show that they satisfy the model. The descriptive top-level specification (DTLS) shall be shown to be an accurate description of the TCB interface. Documentation shall describe how the TCB implements the reference monitor concept and give an explanation why it is tamper resistant, cannot be bypassed, and is correctly implemented. Documentation shall describe how the TCB is structured to facilitate testing and to enforce least privilege. This documentation shall also present the results of the covert channel analysis and the tradeoffs involved in restricting the channels. All auditable events that may be used in the exploitation of known covert storage channels shall be identified. The bandwidths of known covert storage channels the use of which is not detectable by the auditing mechanisms, shall be provided. (See the Covert Channel Guideline section.)

3.3 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 system 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. The following are minimal requirements for systems assigned a class (B3) rating:

3.3.1 Security Policy

3.3.1.1 Discretionary Access Control

3.3.1.2 Object Reuse

3.3.1.3 Labels

3.3.1.3.1 Label Integrity

3.3.1.3.2 Exportation of Labeled Information

3.3.1.3.2.1 Exportation to Multilevel Devices

3.3.1.3.2.2 Exportation to Single-Level Devices

3.3.1.3.2.3 Labeling Human-Readable Output

3.3.1.3.3 Subject Sensitivity Labels

3.3.1.3.4 Device Labels

3.3.1.4 Mandatory Access Control

3.3.2 Accountability

3.3.2.1 Identification and Authentication

3.3.2.1.1 Trusted Path

3.3.2.2 Audit

3.3.3 Assurance

3.3.3.1 Operational Assurance

3.3.3.1.1 System Architecture

3.3.3.1.2 System Integrity

3.3.3.1.3 Covert Channel Analysis

3.3.3.1.4 Trusted Facility Management

3.3.3.1.5 Trusted Recovery

3.3.3.2 Life-Cycle Assurance

3.3.3.2.1 Security Testing

3.3.3.2.2 Design Specification and Verification

3.3.3.2.3 Configuration Management

3.3.4 Documentation

3.3.4.1 Security Features User's Guide

3.3.4.2 Trusted Facility Manual

3.3.4.3 Test Documentation

3.3.4.4 Design Documentation

3.3.1 Security Policy

3.3.1.1 Discretionary Access Control

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., access control lists) shall allow users to specify and control sharing of those objects, and shall provide controls to limit propagation of access rights. The discretionary access control mechanism shall, either by explicit user action or by default, provide that objects are protected from unauthorized access. 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. Furthermore, 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. Access permission to an object by users not already possessing access permission shall only be assigned by authorized users.

3.3.1.2 Object Reuse

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 subjects actions is to be available to any subject that obtains access to an object that has been released back to the system.

3.3.1.3 Labels

3.3.1.3.1 Label Integrity

3.3.1.3.2 Exportation of Labeled Information

3.3.1.3.2.1 Exportation to Multilevel Devices

3.3.1.3.2.2 Exportation to Single-Level Devices

Single-level I/O devices and single-level communication channels are not required to maintain the sensitivity labels of the information they process. However, the TCB shall include a mechanism by which the TCB and an authorized user reliably communicate to designate the single security level of information imported or exported via single-level communication channels or I/O devices.

3.3.1.3.2.3 Labeling Human-Readable Output

The ADP system administrator shall be able to specify the printable label names associated with exported sensitivity labels. The TCB shall mark the beginning and end of all human-readable, paged, hardcopy output (e.g., line printer output) with human-readable sensitivity labels that properly represent the sensitivity of the output. The TCB shall, by default, mark the top and bottom of each page of human-readable, paged, hardcopy output (e.g., line printer output) with human-readable sensitivity labels that properly represent the overall sensitivity of the output or that properly represent the sensitivity of the information on the page. The TCB shall, by default and in an appropriate manner, mark other forms of human-readable output (e.g., maps, graphics) with human-readable sensitivity labels that properly represent the sensitivity of the output. Any override of these marking defaults shall be auditable by the TCB.

3.3.1.3.3 Subject Sensitivity Labels

3.3.1.3.4 Device Labels

3.3.1.4 Mandatory Access Control

The TCB shall enforce a mandatory access control policy over all resources (i.e., subjects, storage objects, and I/O devices) that are directly or indirectly accessible by subjects external to the TCB. These subjects and objects shall be assigned sensitivity labels that are a combination of hierarchical classification levels and non-hierarchical categories, and the labels shall be used as the basis for mandatory access control decisions. The TCB shall be able to support two or more such security levels. (See the Mandatory Access Control guidelines.) The following requirements shall hold for all accesses between all subjects external to the TCB and all objects directly or indirectly accessible by these subjects: A subject can read an object only if the hierarchical classification in the subject's security level is greater than or equal to the hierarchical classification in the object's security level and the non-hierarchical categories in the subject's security level include all the non-hierarchical categories in the object's security level. A subject can write an object only if the hierarchical classification in the subject's security level is less than or equal to the hierarchical classification in the object's security level and all the non-hierarchical categories in the subject's security level are included in the non- hierarchical categories in the object's security level. Identification and authentication data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authorization of subjects external to the TCB that may be created to act on behalf of the individual user are dominated by the clearance and authorization of that user.

3.3.2 Accountability

3.3.2.1 Identification and Authentication

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 maintain authentication data that includes information for verifying the identity of individual users (e.g., passwords) as well as information for determining the clearance and authorizations of individual users. This data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authorizations of subjectsexternal to the TCB that may be created to act on behalf of the individual user are dominated by the clearance and authorization of that user. The TCB shall protect authentication data so that it cannot be accessed by any unauthorized user. The TCB shall be able to enforce individual accountability by providing the capability to uniquely identify each individual ADP system user. The TCB shall also provide the capability of associating this identity with all auditable actions taken by that individual.

3.3.2.1.1 Trusted Path

The TCB shall support a trusted communication path between itself and users for use when a positive TCB-to-user connection is required (e.g., login, change subject security level). Communications via this trusted path shall be activated exclusively by a user of the TCB and shall be logically isolated and unmistakably distinguishable from other paths.

3.3.2.2 Audit

3.3.3 Assurance

3.3.3.1 Operational Assurance

3.3.3.1.1 System Architecture

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). The TCB shall maintain process isolation through the provision of distinct address spaces under its control. The TCB shall be internally structured into well-defined largely independent modules. It shall make effective use of available hardware to separate those elements that are protection-critical from those that are not. The TCB modules shall be designed such that the principle of least privilege is enforced. Features in hardware, such as segmentation, shall be used to support logically distinct storage objects with separate attributes (namely: readable, writeable). The user interface to the TCB shall be identified. The TCB shall be designed and structured to use a complete, conceptually simple protection mechanism with precisely defined semantics. This mechanism shall play a central role in enforcing the internal structuring of the TCB and the system. The TCB shall incorporate significant use of layering, abstraction and data hiding. Significant system engineering shall be directed toward minimizing the complexity of the TCB and excluding from the TCB modules that are not protection-critical.

3.3.3.1.2 System Integrity

Hardware and/or software features shall be provided that can be used to periodically validate the correct operation of the on-site hardware and firmware elements of the TCB.

3.3.3.1.3 Covert Channel Analysis

The system developer shall conduct a thorough search for covert channels and make a determination (either by actual measurement or by engineering estimation) of the maximum bandwidth of each identified channel. (See the Covert Channels Guideline section.)

3.3.3.1.4 Trusted Facility Management

The TCB shall support separate operator and administrator functions. The functions performed in the role of a security administrator shall be identified. The ADP system administrative personnel shall only be able to perform security administrator functions after taking a distinct auditable action to assume the security administrator role on the ADP system. Non-security functions that can be performed in the security administration role shall be limited strictly to those essential to performing the security role effectively.

3.3.3.1.5 Trusted Recovery

Procedures and/or mechanisms shall be provided to assure that, after an ADP system failure or other discontinuity, recovery without a protection compromise is obtained.

3.3.3.2 Life-Cycle Assurance

3.3.3.2.1 Security Testing

The security mechanisms of the ADP system shall be tested and found to work as claimed in the system documentation. A team of individuals who thoroughly understand the specific implementation of the TCB shall subject its design documentation, source code, and object code to thorough analysis and testing. Their objectives shall be: to uncover all design and implementation flaws that would permit a subject external to the TCB to read, change, or delete data normally denied under the mandatory or discretionary security policy enforced by the TCB; as well as to assure that no subject (without authorization to do so) is able to cause the TCB to enter a state such that it is unable to respond to communications initiated by other users. The TCB shall be found resistant to penetration. All discovered flaws shall be corrected and the TCB retested to demonstrate that they have been eliminated and that new flaws have not been introduced. Testing shall demonstrate that the TCB implementation is consistent with the descriptive top-level specification. (See the Security Testing Guidelines.) No design flaws and no more than a few correctable implementation flaws may be found during testing and there shall be reasonable confidence that few remain.

3.3.3.2.2 Design Specification and Verification

3.3.3.2.3 Configuration Management

During development and maintenance of the TCB, a configuration management system shall be in place that maintains control of changes to the descriptive top-level specification, other design data, implementation documentation, source code, the running version of the object code, and test fixtures and documentation. The configuration management system shall assure a consistent mapping among all documentation and code associated with the current version of the TCB. Tools shall be provided for generation of a new version of the TCB from source code. Also available shall be tools for comparing a newly generated version with the previous TCB version in order to ascertain that only the intended changes have been made in the code that will actually be used as the new version of the TCB.

3.3.4 Documentation

3.3.4.1 Security Features User's Guide

A single summary, 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.

3.3.4.2 Trusted Facility Manual

3.3.4.3 Test Documentation

3.3.4.4 Design Documentation

Documentation shall be available that provides a description of the manufacturer's philosophy of protection and explanation of how this philosophy is translated into the TCB. The interfaces between the TCB modules shall be described. A formal description of the security policy model enforced by the TCB shall be available and proven that it is sufficient to enforce the security policy. The specific TCB protection mechanisms shall be identified and an explanation given to show that they satisfy the model. The descriptive top-level specification (DTLS) shall be shown to be an accurate description of the TCB interface. Documentation shall describe how the TCB implements the reference monitor concept and give an explanation why it is tamper resistant, cannot be bypassed, and is correctly implemented. The TCB implementation (i.e., in hardware, firmware, and software) shall be informally shown to be consistent with the DTLS. The elements of the DTLS shall be shown, using informal techniques, to correspond to the elements of the TCB. Documentation shall describe how the TCB is structured to facilitate testing and to enforce least privilege. This documentation shall also present the results of the covert channel analysis and the tradeoffs involved in restricting the channels. All auditable events that may be used in the exploitation of known covert storage channels shall be identified. The bandwidths of known covert storage channels, the use of which is not detectable by the auditing mechanisms, shall be provided. (See the Covert Channel Guideline section.)

4.0 DIVISION A: VERIFIED PROTECTION

This division is characterized by the use of formal security verification methods to assure that the mandatory and discretionary security controls employed in the system can effectively protect classified or 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.

4.1 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 are 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 the security policy and a formal top-level specification (FTLS) of the design. Independent of the particular specification language or verification system used, there are five important criteria for class (A1) design verification:

A formal model of the security policy must be clearly identified and documented, including a mathematical proof that the model is consistent with its axioms and is sufficient to support the security policy.
An FTLS must be produced that includes abstract definitions of the functions the TCB performs and of the hardware and/or firmware mechanisms that are used to support separate execution domains.
The FTLS of the TCB must be shown to be consistent with the model by formal techniques where possible (i.e., where verification tools exist) and informal ones otherwise.
The TCB implementation (i.e., in hardware, firmware, and software) must be informally shown to be consistent with the FTLS. The elements of the FTLS must be shown, using informal techniques, to correspond to the elements of the TCB. The FTLS must express the unified protection mechanism required to satisfy the security policy, and it is the elements of this protection mechanism that are mapped to the elements of the TCB.
Formal analysis techniques must be used to identify and analyze covert channels. Informal techniques may be used to identify covert timing channels. The continued existence of identified covert channels in the system must be justified.

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.

The following are minimal requirements for systems assigned a class (A1) rating:

4.1.1 Security Policy

4.1.1.1 Discretionary Access Control

4.1.1.2 Object Reuse

4.1.1.3 Labels

4.1.1.3.1 Label Integrity

4.1.1.3.2 Exportation of Labeled Information

4.1.1.3.3 Exportation to Multilevel Devices

4.1.1.3.3.1 Exportation to Single-Level Devices

4.1.1.3.3.2 Labeling Human-Readable Output

4.1.1.3.4 Subject Sensitivity Labels

4.1.1.3.5 Device Labels

4.1.1.4 Mandatory Access Control

4.1.2 Accountability

4.1.2.1 Identification and Authentication

4.1.2.1.1 Trusted Path

4.1.2.2 Audit

4.1.3 Assurance

4.1.3.1 Operational Assurance

4.1.3.1.1 System Architecture

4.1.3.1.2 System Integrity

4.1.3.1.3 Covert Channel Analysis

4.1.3.1.4 Trusted Facility Management

4.1.3.1.5 Trusted Recovery

4.1.3.2 Life-Cycle Assurance

4.1.3.2.1 Security Testing

4.1.3.2.2 Design Specification and Verification

4.1.3.2.3 Configuration Management

4.1.3.2.4 Trusted Distribution

4.1.4 Documentation

4.1.4.1 Security Features User's Guide

4.1.4.2 Trusted Facility Manual

4.1.4.3 Test Documentation

4.1.4.4 Design Documentation

4.1.1 Security Policy

4.1.1.1 Discretionary Access Control

4.1.1.2 Object Reuse

4.1.1.3 Labels

4.1.1.3.1 Label Integrity

4.1.1.3.2 Exportation of Labeled Information

4.1.1.3.3 Exportation to Multilevel Devices

4.1.1.3.3.1 Exportation to Single-Level Devices

Single-level I/O devices and single-level communication channels are not required to maintain the sensitivity labels of the information they process. However, the TCB shall include a mechanism by which the TCB and an authorized user reliably communicate to designate the single security level of information imported or exported via single-level communication channels or I/O devices.

4.1.1.3.3.2 Labeling Human-Readable Output

The ADP system administrator shall be able to specify the printable label names associated with exported sensitivity labels. The TCB shall mark the beginning and end of all human-readable, paged, hardcopy output (e.g., line printer output) with human-readable sensitivity labels that properly represent the sensitivity of the output. The TCB shall, by default, mark the top and bottom of each page of human-readable, paged, hardcopy output (e.g., line printer output) with human-readable sensitivity labels that properly represent the overall sensitivity of the output or that properly represent the sensitivity of the information on the page. The TCB shall, by default and in an appropriate manner, mark other forms of human-readable output (e.g., maps, graphics) with human-readable sensitivity labels that properly represent the sensitivity of the output. Any override of these marking defaults shall be auditable by the TCB.

4.1.1.3.4 Subject Sensitivity Labels

4.1.1.3.5 Device Labels

4.1.1.4 Mandatory Access Control

4.1.2 Accountability

4.1.2.1 Identification and Authentication

4.1.2.1.1 Trusted Path

The TCB shall support a trusted communication path between itself and users for use when a positive TCB-to-user connection is required (e.g., login, change subject security level). Communications via this trusted path shall be activated exclusively by a user or the TCB and shall be logically isolated and unmistakably distinguishable from other paths.

4.1.2.2 Audit

The TCB shall 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 shall 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, and actions taken by computer operators and system administrators and/or system security officers, and other security relevant events. The TCB shall also be able to audit any override of human-readable output markings. 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 identification/authentication events the origin of request (e.g., terminal ID) shall 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 and the object's security level. The ADP system administrator shall be able to selectively audit the actions of any one or more users based on individual identity and/or object security level. The TCB shall be able to audit the identified events that may be used in the exploitation of covert storage channels. The TCB shall 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 security relevant events continues, the system shall take the least disruptive action to terminate the event.

4.1.3 Assurance

4.1.3.1 Operational Assurance

4.1.3.1.1 System Architecture

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). The TCB shall maintain process isolation through the provision of distinct address spaces under its control. The TCB shall be internally structured into well-defined largely independent modules. It shall make effective use of available hardware to separate those elements that are protection-critical from those that are not. The TCB modules shall be designed such that the principle of least privilege is enforced. Features in hardware, such as segmentation, shall be used to support logically distinct storage objects with separate attributes (namely: readable, writeable). The user interface to the TCB shall be completely defined and all elements of the TCB identified. The TCB shall be designed and structured to use a complete, conceptually simple protection mechanism with precisely defined semantics. This mechanism shall play a central role in enforcing the internal structuring of the TCB and the system. The TCB shall incorporate significant use of layering, abstraction and data hiding. Significant system engineering shall be directed toward minimizing the complexity of the TCB and excluding from the TCB modules that are not protection-critical.

4.1.3.1.2 System Integrity

Hardware and/or software features shall be provided that can be used to periodically validate the correct operation of the on-site hardware and firmware elements of the TCB.

4.1.3.1.3 Covert Channel Analysis

4.1.3.1.4 Trusted Facility Management

4.1.3.1.5 Trusted Recovery

Procedures and/or mechanisms shall be provided to assure that, after an ADP system failure or other discontinuity, recovery without a protection compromise is obtained.

4.1.3.2 Life-Cycle Assurance

4.1.3.2.1 Security Testing

The security mechanisms of the ADP system shall be tested and found to work as claimed in the system documentation. A team of individuals who thoroughly understand the specific implementation of the TCB shall subject its design documentation, source code, and object code to thorough analysis and testing. Their objectives shall be: to uncover all design and implementation flaws that would permit a subject external to the TCB to read, change, or delete data normally denied under the mandatory or discretionary security policy enforced by the TCB; as well as to assure that no subject (without authorization to do so) is able to cause the TCB to enter a state such that it is unable to respond to communications initiated by other users. The TCB shall be found resistant to penetration. All discovered flaws shall be corrected and the TCB retested to demonstrate that they have been eliminated and that new flaws have not been introduced. Testing shall demonstrate that the TCB implementation is consistent with the formal top- level specification. (See the Security Testing Guidelines.) No design flaws and no more than a few correctable implementation flaws may be found during testing and there shall be reasonable confidence that few remain. Manual or other mapping of the FTLS to the source code may form a basis for penetration testing.

4.1.3.2.2 Design Specification and Verification

A formal model of the security policy supported by the TCB shall be maintained over the life-cycle of the ADP system that is proven consistent with its axioms. A descriptive top-level specification (DTLS) of the TCB shall be maintained that completely and accurately describes the TCB in terms of exceptions, error messages, and effects. A formal top-level specification (FTLS) of the TCB shall be maintained that accurately describes the TCB in terms of exceptions, error messages, and effects. The DTLS and FTLS shall include those components of the TCB that are implemented as hardware and/or firmware if their properties are visible at the TCB interface. The FTLS shall be shown to be an accurate description of the TCB interface. A convincing argument shall be given that the DTLS is consistent with the model and a combination of formal and informal techniques shall be used to show that the FTLS is consistent with the model. This verification evidence shall be consistent with that provided within the state-of-the-art of the particular computer security center-endorsed formal specification and verification system used. Manual or other mapping of the FTLS to the TCB source code shall be performed to provide evidence of correct implementation.

4.1.3.2.3 Configuration Management

During the entire life-cycle, i.e., during the design,development, and maintenance of the TCB, a configuration management system shall be in place for all security-relevant hardware, firmware, and software that maintains control of changes to the formal model, the descriptive and formal top-level specifications, other design data, implementation documentation, source code, the running version of the object code, and test fixtures and documentation. The configuration management system shall assure a consistent mapping among all documentation and code associated with the current version of the TCB. Tools shall be provided for generation of a new version of the TCB from source code. Also available shall be tools, maintained under strict configuration control, for comparing a newly generated version with the previous TCB version in order to ascertain that only the intended changes have been made in the code that will actually be used as the new version of the TCB. A combination of technical, physical, and procedural safeguards shall be used to protect from unauthorized modification or destruction the master copy or copies of all material used to generate the TCB.

4.1.3.2.4 Trusted Distribution

A trusted ADP system control and distribution facility shall be provided for maintaining the integrity of the mapping between the master data describing the current version of the TCB and the on-site master copy of the code for the current version. Procedures (e.g., site security acceptance testing) shall exist for assuring that the TCB software, firmware, and hardware updates distributed to a customer are exactly as specified by the master copies.

4.1.4 Documentation

4.1.4.1 Security Features User's Guide

A single summary, 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.

4.1.4.2 Trusted Facility Manual

4.1.4.3 Test Documentation

4.1.4.4 Design Documentation

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. The interfaces between the TCB modules shall be described. A formal description of the security policy model enforced by the TCB shall be available and proven that it is sufficient to enforce the security policy. The specific TCB protection mechanisms shall be identified and an explanation given to show that they satisfy the model. The descriptive top-level specification (DTLS) shall be shown to be an accurate description of the TCB interface. Documentation shall describe how the TCB implements the reference monitor concept and give an explanation why it is tamper resistant, cannot be bypassed, and is correctly implemented. The TCB implementation (i.e., in hardware, firmware, and software) shall be informally shown to be consistent with the formal top-level specification (FTLS). The elements of the FTLS shall be shown, using informal techniques, to correspond to the elements of the TCB. Documentation shall describe how the TCB is structured to facilitate testing and to enforce least privilege. This documentation shall also present the results of the covert channel analysis and the tradeoffs involved in restricting the channels. All auditable events that may be used in the exploitation of known covert storage channels shall be identified. The bandwidths of known covert storage channels, the use of which is not detectable by the auditing mechanisms, shall be provided. (See the Covert Channel Guideline section.) Hardware, firmware, and software mechanisms not dealt with in the FTLS but strictly internal to the TCB (e.g., mapping registers, direct memory access I/O) shall be clearly described.

4.2 BEYOND CLASS (A1)

Most of the security enhancements envisioned for systems that will provide features and assurance in addition to that already provided by class (Al) systems are beyond current technology. The discussion below is intended to guide future work and is derived from research and development activities already underway in both the public and private sectors. As more and better analysis techniques are developed, the requirements for these systems will become more explicit. In the future, use of formal verification will be extended to the source level and covert timing channels will be more fully addressed. At this level the design environment will become important and testing will be aided by analysis of the formal top-level specification. Consideration will be given to the correctness of the tools used in TCB development (e.g., compilers, assemblers, loaders) and to the correct functioning of the hardware/firmware on which the TCB will run. Areas to be addressed by systems beyond class (A1) include:

System Architecture

A demonstration (formal or otherwise) must be given showing that requirements of self-protection and completeness for reference monitors have been implemented in the TCB.

Security Testing

Although beyond the current state-of-the-art, it is envisioned that some test-case generation will be done automatically from the formal top-level specification or formal lower-level specifications.

Formal Specification and Verification

The TCB must be verified down to

DEPARTMENT OF DEFENSE STANDARD

DEPARTMENT OFDEFENSETRUSTED COMPUTERSYSTEM EVALUATIONCRITERIA

DECEMBER 1985

CONTENTS

PART I: CRITERIA

PART II: RATIONAL AND GUIDELINES

Historical Perspective

Scope

Purpose

Fundamental Computer Security Requirements

Policy

Accountability

Assurance

Structure of the Document

Structure of the Criteria

PART I: THE CRITERIA

DEPARTMENT OF
DEFENSE
TRUSTED COMPUTER
SYSTEM EVALUATION
CRITERIA