CSC-STD-00l-83, dtd l5 Aug 83
Library No. S225,7ll
December 26, 1985
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)
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.
2.0 DIVISION C: DISCRETIONARY PROTECTION
5.0 CONTROL OBJECTIVES FOR TRUSTED COMPUTER SYSTEMS
9.0 A GUIDELINE ON CONFIGURING MANDATORY ACCESS CONTROL FEATURES
10.0 A GUIDELINE ON SECURITY TESTING
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. 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. 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. 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. The criteria presented in this document have evolved from the earlier NBS and MITRE evaluation material.
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.
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.
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.
The following are minimal requirements for systems assigned a class (A1) rating:
As described in the introduction to this document, efforts to define the problems and develop solutions associated with processing nationally sensitive information, as well as other sensitive data such as financial, medical, and personnel information used by the National Security Establishment, have been underway for a number of years. The criteria, as described in Part I, represent the culmination of these efforts and describe basic requirements for building trusted computer systems. To date, however, these systems have been viewed by many as only satisfying National Security needs. As long as this perception continues the consensus needed to motivate manufacture of trusted systems will be lacking.
The purpose of this section is to describe in detail the fundamental control objectives. These objectives lay the foundation for the requirements outlined in the criteria. The goal is to explain the foundations so that those outside the National Security Establishment can assess their universality and, by extension, the universal applicability of the criteria requirements to processing all types of sensitive applications whether they be for National Security or the private sector.
Examples of control objectives include the three basic design requirements for implementing the reference monitor concept discussed in Section 6. They are:
The Anderson report went on to define the reference validation mechanism as "an implementation of the reference monitor concept . . . that validates each reference to data or programs by any user (program) against a list of authorized types of reference for that user." It then listed the three design requirements that must be met by a reference validation mechanism:
A subject can act on behalf of a user or another subject. The subject is created as a surrogate for the cleared user and is assigned a formal security level based on their classification. The state transitions and invariants of the formal policy model define the invariant relationships that must hold between the clearance of the user, the formal security level of any process that can act on the user's behalf, and the formal security level of the devices and other objects to which any process can obtain specific modes of access. The Bell and LaPadula model, for example, defines a relationship between formal security levels of subjects and objects, now referenced as the "dominance relation." From this definition, accesses permitted between subjects and objects are explicitly defined for the fundamental modes of access, including read-only access, read/write access, and write-only access. The model defines the Simple Security Condition to control granting a subject read access to a specific object, and the *-Property (read "Star Property") to control granting a subject write access to a specific object. Both the Simple Security Condition and the *-Property include mandatory security provisions based on the dominance relation between formal security levels of subjects and objects the clearance of the subject and the classification of the object. The Discretionary Security Property is also defined, and requires that a specific subject be authorized for the particular mode of access required for the state transition. In its treatment of subjects (processes acting on behalf of a user), the model distinguishes between trusted subjects (i.e., not constrained within the model by the *-Property) and untrusted subjects (those that are constrained by the *-Property).
From the Bell and LaPadula model there evolved a model of the method of proof required to formally demonstrate that all arbitrary sequences of state transitions are security-preserving. It was also shown that the *- Property is sufficient to prevent the compromise of information by Trojan Horse attacks.
The heart of a trusted computer system is the Trusted Computing Base (TCB) which contains all of the elements of the system responsible for supporting the security policy and supporting the isolation of objects (code and data) on which the protection is based. The bounds of the TCB equate to the "security perimeter" referenced in some computer security literature. In the interest of understandable and maintainable protection, a TCB should be as simple as possible consistent with the functions it has to perform. Thus, the TCB includes hardware, firmware, and software critical to protection and must be designed and implemented such that system elements excluded from it need not be trusted to maintain protection. Identification of the interface and elements of the TCB along with their correct functionality therefore forms the basis for evaluation.
For general-purpose systems, the TCB will include key elements of the operating system and may include all of the operating system. For embedded systems, the security policy may deal with objects in a way that is meaningful at the application level rather than at the operating system level. Thus, the protection policy may be enforced in the application software rather than in the underlying operating system. The TCB will necessarily include all those portions of the operating system and application software essential to the support of the policy. Note that, as the amount of code in the TCB increases, it becomes harder to be confident that the TCB enforces the reference monitor requirements under all circumstances.
Clearly, as the perceived degree of risk increases (e.g., the range of sensitivity of the system's protected data, along with the range of clearances held by the system's user population) for a particular system's operational application and environment, so also must the assurances be increased to substantiate the degree of trust that will be placed in the system. The hierarchy of requirements that are presented for the evaluation classes in the trusted computer system evaluation criteria reflect the need for these assurances.
As discussed in Section 5.3, the evaluation criteria uniformly require a statement of the security policy that is enforced by each trusted computer system. In addition, it is required that a convincing argument be presented that explains why the TCB satisfies the first two design requirements for a reference monitor. It is not expected that this argument will be entirely formal. This argument is required for each candidate system in order to satisfy the assurance control objective.
The systems to which security enforcement mechanisms have been added, rather than built-in as fundamental design objectives, are not readily amenable to extensive analysis since they lack the requisite conceptual simplicity of a security kernel. This is because their TCB extends to cover much of the entire system. Hence, their degree of trustworthiness can best be ascertained only by obtaining test results. Since no test procedure for something as complex as a computer system can be truly exhaustive, there is always the possibility that a subsequent penetration attempt could succeed. It is for this reason that such systems must fall into the lower evaluation classes.
On the other hand, those systems that are designed and engineered to support the TCB concepts are more amenable to analysis and structured testing. Formal methods can be used to analyze the correctness of their reference validation mechanisms in enforcing the system's security policy. Other methods, including less-formal arguments, can be used in order to substantiate claims for the completeness of their access mediation and their degree of tamper-resistance. More confidence can be placed in the results of this analysis and in the thoroughness of the structured testing than can be placed in the results for less methodically structured systems. For these reasons, it appears reasonable to conclude that these systems could be used in higher-risk environments. Successful implementations of such systems would be placed in the higher evaluation classes.
Distinctions in terms of system architecture, security policy enforcement, and evidence of credibility between evaluation classes have been defined such that the "jump" between evaluation classes would require a considerable investment of effort on the part of implementors. Correspondingly, there are expected to be significant differentials of risk to which systems from the higher evaluation classes will be exposed.
Security guidance for Federal automated information systems is provided by the Office of Management and Budget. Two specifically applicable Circulars have been issued. OMB Circular No. A-71, Transmittal Memorandum No. 1, "Security of Federal Automated Information Systems," directs each executive agency to establish and maintain a computer security program. It makes the head of each executive branch, department and agency responsible "for assuring an adequate level of security for all agency data whether processed in-house or commercially. This includes responsibility for the establishment of physical, administrative and technical safeguards required to adequately protect personal, proprietary or other sensitive data not subject to national security regulations, as well as national security data."[26, para. 4 p. 2]
OMB Circular No. A-123, "Internal Control Systems," issued to help eliminate fraud, waste, and abuse in government programs requires: (a) agency heads to issue internal control directives and assign responsibility, (b) managers to review programs for vulnerability, and (c) managers to perform periodic reviews to evaluate strengths and update controls. Soon after promulgation of OMB Circular A-123, the relationship of its internal control requirements to building secure computer systems was recognized. While not stipulating computer controls specifically, the definition of Internal Controls in A-123 makes it clear that computer systems are to be included:
* i.e., NASA, Commerce Department, GSA, State Department, Small Business Administration, National Science Foundation, Treasury Department, Transportation Department, Interior Department, Agriculture Department, U.S. Information Agency, Labor Department, Environmental Protection Agency, Justice Department, U.S. Arms Control and Disarmament Agency, Federal Emergency Management Agency, Federal Reserve System, and U.S. General Accounting Office.
For ADP systems, these information security requirements are further amplified and specified in: 1) DoD Directive 5200.28  and DoD Manual 5200.28-M , for DoD components; and 2) Section XIII of DoD 5220.22-M  for contractors. DoD Directive 5200.28, "Security Requirements for Automatic Data Processing (ADP) Systems," stipulates: "Classified material contained in an ADP system shall be safeguarded by the continuous employment of protective features in the system's hardware and software design and configuration . . . ."[8, sec. IV] Furthermore, it is required that ADP systems that "process, store, or use classified data and produce classified information will, with reasonable dependability, prevent:
DoD Directive 5200.28 provides the security requirements for ADP systems. For some types of information, such as Sensitive Compartmented Information (SCI), DoD Directive 5200.28 states that other minimum security requirements also apply. These minima are found in DCID l/l6 (new reference number 5) which is implemented in DIAM 50-4 (new reference number 6) for DoD and DoD contractor ADP systems.
From requirements imposed by these regulations, directives and circulars, the three components of the Security Policy Control Objective, i.e., Mandatory and Discretionary Security and Marking, as well as the Accountability and Assurance Control Objectives, can be functionally defined for DoD applications. The following discussion provides further specificity in Policy for these Control Objectives.
This control objective is supported by the following citations:
DoD Directive 5200.28 (VI.A.1) states: "Each user's identity shall be positively established, and his access to the system, and his activity in the system (including material accessed and actions taken) controlled and open to scrutiny."
DoD Manual 5200.28-M (Section V 5-100) states: "An audit log or file (manual, machine, or a combination of both) shall be maintained as a history of the use of the ADP System to permit a regular security review of system activity. (e.g., The log should record security related transactions, including each access to a classified file and the nature of the access, e.g., logins, production of accountable classified outputs, and creation of new classified files. Each classified file successfully accessed [regardless of the number of individual references] during each 'job' or 'interactive session' should also be recorded in the audit log. Much of the material in this log may also be required to assure that the system preserves information entrusted to it.)"
DoD Manual 5200.28-M (Section IV 4-305f) states: "Where needed to assure control of access and individual accountability, each user or specific group of users shall be identified to the ADP System by appropriate administrative or hardware/software measures. Such identification measures must be in sufficient detail to enable the ADP System to provide the user only that material which he is authorized."
DoD Manual 5200.28-M
A basis for this objective can be found in the following sections of DoD Directive 5200.28:
DoD Directive 5200.28 (IV.B.1) stipulates: "Generally, security of an ADP system is most effective and economical if the system is designed originally to provide it. Each Department of Defense Component undertaking design of an ADP system which is expected to process, store, use, or produce classified material shall: From the beginning of the design process, consider the security policies, concepts, and measures prescribed in this Directive."
DoD Directive 5200.28 (IV.C.5.a) states: "Provision may be made to permit adjustment of ADP system area controls to the level of protection required for the classification category and type(s) of material actually being handled by the system, provided change procedures are developed and implemented which will prevent both the unauthorized access to classified material handled by the system and the unauthorized manipulation of the system and its components. Particular attention shall be given to the continuous protection of automated system security measures, techniques and procedures when the personnel security clearance level of users having access to the system changes."
DoD Directive 5200.28 (VI.A.2) states: "Environmental Control. The ADP System shall be externally protected to minimize the likelihood of unauthorized access to system entry points, access to classified information in the system, or damage to the system."
DoD Manual 5200.28-M (Section I 1-102b) states:
"Component's Designated Approving Authorities, or their designees for this purpose . . will assure:
. . . . . . . . . . . . . . . . .
A major component of assurance, life-cycle assurance, as described in DoD Directive 7920.1, is concerned with testing ADP systems both in the development phase as well as during operation (17). DoD Directive 5215.1 (Section F.2.C.(2)) requires "evaluations of selected industry and government-developed trusted computer systems against these criteria."
From a security perspective, covert channels with low bandwidths represent a lower threat than those with high bandwidths. However, for many types of covert channels, techniques used to reduce the bandwidth below a certain rate (which depends on the specific channel mechanism and the system architecture) also have the effect of degrading the performance provided to legitimate system users. Hence, a trade-off between system performance and covert channel bandwidth must be made. Because of the threat of compromise that would be present in any multilevel computer system containing classified or sensitive information, such systems should not contain covert channels with high bandwidths. This guideline is intended to provide system developers with an idea of just how high a "high" covert channel bandwidth is.
A covert channel bandwidth that exceeds a rate of one hundred (100) bits per second is considered "high" because 100 bits per second is the approximate rate at which many computer terminals are run. It does not seem appropriate to call a computer system "secure" if information can be compromised at a rate equal to the normal output rate of some commonly used device.
In any multilevel computer system there are a number of relatively low-bandwidth covert channels whose existence is deeply ingrained in the system design. Faced with the large potential cost of reducing the bandwidths of such covert channels, it is felt that those with maximum bandwidths of less than one (1) bit per second are acceptable in most application environments. Though maintaining acceptable performance in some systems may make it impractical to eliminate all covert channels with bandwidths of 1 or more bits per second, it is possible to audit their use without adversely affecting system performance. This audit capability provides the system administration with a means of detecting -- and procedurally correcting -- significant compromise. Therefore, a Trusted Computing Base should provide, wherever possible, the capability to audit the use of covert channel mechanisms with bandwidths that may exceed a rate of one (1) bit in ten (10) seconds.
The covert channel problem has been addressed by a number of authors. The interested reader is referred to references , , , , , , and .
As in Part I, highlighting is used to indicate changes in the guidelines from the next lower division.
"Department of Defense Trusted Computer System Evaluation Criteria" forms the basis upon which the Computer Security Center will carry out the commercial computer security evaluation process. This process is focused on commercially produced and supported general-purpose operating system products that meet the needs of government departments and agencies. The formal evaluation is aimed at "off-the-shelf" commercially supported products and is completely divorced from any consideration of overall system performance, potential applications, or particular processing environments. The evaluation provides a key input to a computer system security approval/accreditation. However, it does not constitute a complete computer system security evaluation. A complete study (e.g., as in reference ) must consider additional factors dealing with the system in its unique environment, such as it's proposed security mode of operation, specific users, applications, data sensitivity, physical and personnel security, administrative and procedural security, TEMPEST, and communications security.
The product evaluation process carried out by the Computer Security Center has three distinct elements:
Since it is generally very difficult to add effective security measures late in a product's life cycle, the Center is interested in working with system vendors in the early stages of product design. A preliminary product evaluation allows the Center to consult with computer vendors on computer security issues found in products that have not yet been formally announced.
A preliminary evaluation is typically initiated by computer system vendors who are planning new computer products that feature security or major security-related upgrades to existing products. After an initial meeting between the vendor and the Center, appropriate non-disclosure agreements are executed that require the Center to maintain the confidentiality of any proprietary information disclosed to it. Technical exchange meetings follow in which the vendor provides details about the proposed product (particularly its internal designs and goals) and the Center provides expert feedback to the vendor on potential computer security strengths and weaknesses of the vendor's design choices, as well as relevant interpretation of the criteria. The preliminary evaluation is typically terminated when the product is completed and ready for field release by the vendor. Upon termination, the Center prepares a wrap-up report for the vendor and for internal distribution within the Center. Those reports containing proprietary information are not available to the public.
During preliminary evaluation, the vendor is under no obligation to actually complete or market the potential product. The Center is, likewise, not committed to conduct a formal product evaluation. A preliminary evaluation may be terminated by either the Center or the vendor when one notifies the other, in writing, that it is no longer advantageous to continue the evaluation.
Formal Product Evaluation
The formal product evaluation provides a key input to certification of a computer system for use in National Security Establishment applications and is the sole basis for a product being placed on the Evaluated Products List.
A formal product evaluation begins with a request by a vendor for the Center to evaluate a product for which the product itself and accompanying documentation needed to meet the requirements defined by this publication are complete. Non-disclosure agreements are executed and a formal product evaluation team is formed by the Center. An initial meeting is then held with the vendor to work out the schedule for the formal evaluation. Since testing of the implemented product forms an important part of the evaluation process, access by the evaluation team to a working version of the system is negotiated with the vendor. Additional support required from the vendor includes complete design documentation, source code, and access to vendor personnel who can answer detailed questions about specific portions of the product. The evaluation team tests the product against each requirement, making any necessary interpretations of the criteria with respect to the product being evaluated.
The evaluation team writes a final report on their findings about the system. The report is publicly available (containing no proprietary or sensitive information) and contains the overall class rating assigned to the system and the details of the evalution team's findings when comparing the product against the evaluation criteria. Detailed information concerning vulnerabilities found by the evaluation team is furnished to the system developers and designers as each is found so that the vendor has a chance to eliminate as many of them as possible prior to the completion of the Formal Product Evaluation. Vulnerability analyses and other proprietary or sensitive information are controlled within the Center through the Vulnerability Reporting Program and are distributed only within the U.S. Government on a strict need-to-know and non-disclosure basis, and to the vendor.
The divisions of systems recognized under the trusted computer system evaluation criteria are as follows. Each division represents a major improvement in the overall confidence one can place in the system to protect classified and other sensitive information.
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.
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.
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.
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.
The classes of systems recognized under the trusted computer system evaluation criteria are as follows. They are presented in the order of increasing desirablity from a computer security point of view.
Class (D): Minimal Protection
This class is reserved for those systems that have been evaluated but that fail to meet the requirements for a higher evaluation class.
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.
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.
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 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.
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.
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. 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.
This appendix lists requirements defined in "Department of Defense Trusted Computer System Evaluation Criteria" alphabetically rather than by class. It is provided to assist in following the evolution of a requirement through the classes. For each requirement, three types of criteria may be present. Each will be preceded by the word: NEW, CHANGE, or ADD to indicate the following:
NEW: Any criteria appearing in a lower class are superseded by the criteria that follow.
CHANGE: The criteria that follow have appeared in a lower class but are changed for this class. Highlighting is used to indicate the specific changes to previously stated criteria.
ADD: The criteria that follow have not been required for any lower class, and are added in this class to the previously stated criteria for this requirement.
Abbreviations are used as follows:
NR: (No Requirement) This requirement is not included in this class.
NAR: (No Additional Requirements) This requirement does not change from the previous class.
The reader is referred to Part I of this document when placing new criteria for a requirement into the complete context for that class.
Figure 1 provides a pictorial summary of the evolution of requirements through the classes.
C2: NEW: 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. 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. The ADP system administrator shall be able to selectively audit the actions of any one or more users based on individual identity.
B1: CHANGE: 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.
ADD: The TCB shall also be able to audit any override of human-readable output markings.
B2: ADD: The TCB shall be able to audit the identified events that may be used in the exploitation of covert storage channels.
B3: ADD: 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 lease disruptive action to terminate the event.
B2: NEW: 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.
A1: CHANGE: 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. 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.
ADD: 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.
Covert Channel Analysis
B2: NEW: 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.)
B3: CHANGE: 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.
A1: ADD: Formal methods shall be used in the analysis.
C1: NEW: Documentation shall be available that provides a description of the manufacturer's philosophy of protection and an explanation of how this philosophy is translated into the TCB. If the TCB is composed of distinct modules, the interfaces between these modules shall be described.
B1: ADD: 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.
B2: CHANGE: 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.
ADD: 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.)
B3: ADD: 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.
A1: CHANGE: 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.
ADD: 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.
Design Specification and Verification
B1: NEW: An informal or formal model of the security policy supported by the TCB shall be maintained over the life cycle of the ADP system that is shown to be consistent with its axioms.
B2: CHANGE: 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.
ADD: 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.
B3: ADD: A convincing argument shall be given that the DTLS is consistent with the model.
A1: CHANGE: 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.
ADD: 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. 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.
B2: NEW: 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.
Discretionary Access Control
C1: NEW: The TCB shall define and control access between named users and named objects (e.g., files and programs) in the ADP system. The enforcement mechanism (e.g., self/group/public controls, access control lists) shall allow users to specify and control sharing of those objects by named individuals or defined groups or both.
C2: CHANGE: The enforcement mechanism (e.g., self/group/public controls, access control lists) shall allow users to specify and control sharing of those objects by named individuals, or defined groups of individuals, or by both, and shall provide controls to limit propagation of access rights.
ADD: 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.
B3: CHANGE: The enforcement mechanism (e.g., access control lists) shall allow users to specify and control sharing of those objects, and shall provide controls to limit propagation of access rights. These access controls shall be capable of specifying, for each named object, a list of named individuals and a list of groups of named individuals with their respective modes of access to that object.
ADD: 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.
Exportation of Labeled Information
B1: NEW: 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.
Exportation to Multilevel Devices
B1: NEW: 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.
Exportation to Single-Level Devices
B1: NEW: 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.
Identification and Authentication
C1: NEW: The TCB shall require users to identify themselves to it before beginning to perform any other actions that the TCB is expected to mediate. Furthermore, the TCB shall use a protected mechanisms (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.
C2: ADD: 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.
B1: CHANGE: 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.
B1: NEW: 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.
Labeling Human-Readable Output
B1: NEW: 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.
B1: NEW: 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.
B2: CHANGE: 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.
Mandatory Access Control
B1: NEW: 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.
B2: CHANGE: 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. 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:
C2: NEW: All authorizations to the information contained within a storage object shall be revoked prior to initial assignment, allocation or reallocation to a subject from the TCB's pool of unused storage objects. No information, including encrypted representations of information, produced by a prior subject's actions is to be available to any subject that obtains access to an object that has been released back to the system.
Security Features User's Guide
C1: NEW: 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.
C1: NEW: 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. (See the Security Testing guidelines.)
C2: ADD: 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.
B1: NEW: 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.)
B2: CHANGE: 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.
ADD: The TCB shall be found relatively resistant to penetration. Testing shall demonstrate that the TCB implementation is consistent with the descriptive top-level specification.
B3: CHANGE: The TCB shall be found resistant to penetration.
ADD: 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.
A1: CHANGE: Testing shall demonstrate that the TCB implementation is consistent with the formal top-level specification.
ADD: Manual or other mapping of the FTLS to the source code may form a basis for penetration testing.
Subject Sensitivity Labels
B2: NEW: 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.
C1: NEW: The TCB shall maintain a domain for its own execution that protects it from external interference or tampering (e.g., by modification of its code or data structures). Resources controlled by the TCB may be a defined subset of the subjects and objects in the ADP system.
C2: ADD: The TCB shall isolate the resources to be protected so that they are subject to the access control and auditing requirements.
B1: ADD: The TCB shall maintain process isolation through the provision of distinct address spaces under its control.
B2: NEW: The TCB shall maintain a domain for its own execution that protects it from external interference or tampering (e.g., by modification of its code or data structures). 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.
B3: ADD: 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.
C1: NEW: 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.
C1: NEW: 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.
B2: ADD: It shall include results of testing the effectiveness of the methods used to reduce covert channel bandwidths.
A1: ADD: The results of the mapping between the formal top-level specification and the TCB source code shall be given.
A1: NEW: 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.
Trusted Facility Management
B2: NEW: The TCB shall support separate operator and administrator functions.
B3: ADD: 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.
Trusted Facility Manual
C1: NEW: A manual addressed to the ADP system administrator shall present cautions about functions and privileges that should be controlled when running a secure facility.
C2: ADD: 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.
B1: ADD: The manual shall describe the operator and administrator functions related to security, to include changing the 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.
B2: ADD: 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.
B3: ADD: It shall include the procedures to ensure that the system is initially started in a secure manner. Procedures shall also be included to resume secure system operation after any lapse in system operation.
B2: NEW: 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.
B3: CHANGE: 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.
B3: NEW: 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.
Approval/Accreditation - The official authorization that is granted to an ADP system to process sensitive information in its operational environment, based upon comprehensive security evaluation of the system's hardware, firmware, and software security design, configuration, and implementation and of the other system procedural, administrative, physical, TEMPEST, personnel, and communications security controls.
Audit Trail - A set of records that collectively provide documentary evidence of processing used to aid in tracing from original transactions forward to related records and reports, and/or backwards from records and reports to their component source transactions.
Authenticate - To establish the validity of a claimed identity.
Automatic Data Processing (ADP) System - An assembly of computer hardware, firmware, and software configured for the purpose of classifying, sorting, calculating, computing, summarizing, transmitting and receiving, storing, and retrieving data with a minimum of human intervention.
Bandwidth - A characteristic of a communication channel that is the amount of information that can be passed through it in a given amount of time, usually expressed in bits per second.
Bell-LaPadula Model - A formal state transition model of computer security policy that describes a set of access control rules. In this formal model, the entities in a computer system are divided into abstract sets of subjects and objects. The notion of a secure state is defined and it is proven that each state transition preserves security by moving from secure state to secure state; thus, inductively proving that the system is secure. A system state is defined to be "secure" if the only permitted access modes of subjects to objects are in accordance with a specific security policy. In order to determine whether or not a specific access mode is allowed, the clearance of a subject is compared to the classification of the object and a determination is made as to whether the subject is authorized for the specific access mode. The clearance/classification scheme is expressed in terms of a lattice. See also: Lattice, Simple Security Property, *-Property.
Certification - The technical evaluation of a system's security features, made as part of and in support of the approval/accreditation process, that establishes the extent to which a particular computer system's design and implementation meet a set of specified security requirements.
Channel - An information transfer path within a system. May also refer to the mechanism by which the path is effected.
Covert Channel - A communication channel that allows a process to transfer information in a manner that violates the system's security policy. See also: Covert Storage Channel, Covert Timing Channel.
Covert Storage Channel - A covert channel that involves the direct or indirect writing of a storage location by one process and the direct or indirect reading of the storage location by another process. Covert storage channels typically involve a finite resource (e.g., sectors on a disk) that is shared by two subjects at different security levels.
Covert Timing Channel - A covert channel in which one process signals information to another by modulating its own use of system resources (e.g., CPU time) in such a way that this manipulation affects the real response time observed by the second process.
Data - Information with a specific physical representation.
Data Integrity - The state that exists when computerized data is the same as that in the source documents and has not been exposed to accidental or malicious alteration or destruction.
Descriptive Top-Level Specification (DTLS) - A top-level specification that is written in a natural language (e.g., English), an informal program design notation, or a combination of the two.
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 (unless restrained by mandatory access control).
Domain - The set of objects that a subject has the ability to access.
Dominate - Security level S1 is said to dominate security level S2 if the hierarchical classification of S1 is greater than or equal to that of S2 and the non-hierarchical categories of S1 include all those of S2 as a subset.
Exploitable Channel - Any channel that is useable or detectable by subjects external to the Trusted Computing Base.
Flaw Hypothesis Methodology - A system analysis and penetration technique where specifications and documentation for the system are analyzed and then flaws in the system are hypothesized. The list of hypothesized flaws is then prioritized on the basis of the estimated probability that a flaw actually exists and, assuming a flaw does exist, on the ease of exploiting it and on the extent of control or compromise it would provide. The prioritized list is used to direct the actual testing of the system.
Flaw - An error of commission, omission, or oversight in a system that allows protection mechanisms to be bypassed.
Formal Proof - A complete and convincing mathematical argument, presenting the full logical justification for each proof step, for the truth of a theorem or set of theorems. The formal verification process uses formal proofs to show the truth of certain properties of formal specification and for showing that computer programs satisfy their specifications.
Formal Security Policy Model - A mathematically precise statement of a security policy. To be adequately precise, such a model must represent the initial state of a system, the way in which the system progresses from one state to another, and a definition of a "secure" state of the system. To be acceptable as a basis for a TCB, the model must be supported by a formal proof that if the initial state of the system satisfies the definition of a "secure" state and if all assumptions required by the model hold, then all future states of the system will be secure. Some formal modeling techniques include: state transition models, temporal logic models, denotational semantics models, algebraic specification models. An example is the model described by Bell and LaPadula in reference . See also: Bell-LaPadula Model, Security Policy Model.
Formal Top-Level Specification (FTLS) - A Top-Level Specification that is written in a formal mathematical language to allow theorems showing the correspondence of the system specification to its formal requirements to be hypothesized and formally proven.
Formal Verification - The process of using formal proofs to demonstrate the consistency (design verification) between a formal specification of a system and a formal security policy model or (implementation verification) between the formal specification and its program implementation.
Front-End Security Filter - A process that is invoked to process data accordint to a specified security policy prior to releasing the data outside the processing environment or upon receiving data from an external source.
Functional Testing - The portion of security testing in which the advertised features of a system are tested for correct operation.
General-Purpose System - A computer system that is designed to aid in solving a wide variety of problems.
Granularity - The relative fineness or coarseness by which a mechanism can be adjusted. The phrase "the granularity of a single user" means the access control mechanism can be adjusted to include or exclude any single user.
Lattice - A partially ordered set for which every pair of elements has a greatest lower bound and a least upper bound.
Least Privilege - This principle requires that each subject in a system be granted the most restrictive set of privileges (or lowest clearance) needed for the performance of authorized tasks. The application of this principle limits the damage that can result from accident, error, or unauthorized use.
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.
Multilevel Device - A device that is used in a manner that permits it to simultaneously process data of two or more security levels without risk of compromise. To accomplish this, sensitivity labels are normally stored on the same physical medium and in the same form (i.e., machine-readable or human-readable) as the data being processed.
Multilevel Secure - A class of system containing information with different sensitivities that simultaneously permits access by users with different security clearances and needs-to-know, but prevents users from obtaining access to information for which they lack authorization.
Object - A passive entity that contains or receives information. Access to an object potentially implies access to the information it contains. Examples of objects are: records, blocks, pages, segments, files, directories, directory trees, and programs, as well as bits, bytes, words, fields, processors, video displays, keyboards, clocks, printers, network nodes, etc.
Object Reuse - The reassignment to some subject of a medium (e.g., page frame, disk sector, magnetic tape) that contained one or more objects. To be securely reassigned, such media must contain no residual data from the previously contained object(s).
Output - Information that has been exported by a TCB.
Password - A private character string that is used to authenticate an identity.
Penetration Testing - The portion of security testing in which the penetrators attempt to circumvent the security features of a system. The penetrators may be assumed to use all system design and implementation documentation, which may include listings of system source code, manuals, and circuit diagrams. The penetrators work under no constraints other than those that would be applied to ordinary users.
Process - A program in execution. It is completely characterized by a single current execution point (represented by the machine state) and address space.
Protection-Critical Portions of the TCB - Those portions of the TCB whose normal function is to deal with the control of access between subjects and objects.
Protection Philosophy - An informal description of the overall design of a system that delineates each of the protection mechanisms employed. A combination (appropriate to the evaluation class) of formal and informal techniques is used to show that the mechanisms are adequate to enforce the security policy.
Read - A fundamental operation that results only in the flow of information from an object to a subject.
Read Access - Permission to read information.
Read-Only Memory (ROM) - A storage area in which the contents can be read but not altered during normal computer processing.
Reference Monitor Concept - An access control concept that refers to an abstract machine that mediates all accesses to objects by subjects.
Resource - Anything used or consumed while performing a function. The categories of resources are: time, information, objects (information containers), or processors (the ability to use information). Specific examples are: CPU time; terminal connect time; amount of directly-addressable memory; disk space; number of I/O requests per minute, etc.
Security Kernel - The hardware, firmware, and software elements of a Trusted Computing Base that implement the reference monitor concept. It must mediate all accesses, be protected from modification, and be verifiable as correct.
Security Level - The combination of a hierarchical classification and a set of non-hierarchical categories that represents the sensitivity of information.
Security Policy - The set of laws, rules, and practices that regulate how an organization manages, protects, and distributes sensitive information.
Security Policy Model - An informal presentation of a formal security policy model.
Security Relevant Event - Any event that attempts to change the security state of the system, (e.g., change discretionary access controls, change the security level of the subject, change user password, etc.). Also, any event that attempts to violate the security policy of the system, (e.g., too many attempts to login, attempts to violate the mandatory access control limits of a defice, attempts to downgrade a file, etc.).
Security Testing - A process used to determine that the security features of a system are implemented as designed and that they are adequate for a proposed application environment. This process includes hands-on functional testing, penetration testing, and verification. See also: Functional Testing, Penetration Testing, Verification.
Sensitive Information - Information that, as determined by a competent authority, must be protected because its unauthorized disclosure, alteration, loss, or destruction will at least cause perceivable damage to someone or something.
Sensitivity Label - A piece of information that represents the security level of an object and that describes the sensitivity (e.g., classification) of the data in the object. Sensitivity labels are used by the TCB as the basis for mandatory access control decisions.
Simple Security Condition - A Bell-LaPadula security model rule allowing a subject read access to an object only if the security level of the subject dominates the security level of the object.
Single-Level Device - A device that is used to process data of a single security level at any one time. Since the device need not be trusted to separate data of different security levels, sensitivity labels do not have to be stored with the data being processed.
*-Property (Star Property) - A Bell-LaPadula security model rule allowing a subject write access to an object only if the security level of the subject is dominated by the security level of the object. Also known as the Confinement Property.
Storage Object - An object that supports both read and write accesses.
Subject - An active entity, generally in the form of a person, process, or device that causes information to flow among objects or changes the system state. Technically, a process/domain pair.
Subject Security Level - A subject's security level is equal to the security level of the objects to which it has both read and write access. A subject's security level must always be dominated by the clearance of the user the subject is associated with.
TEMPEST - The study and control of spurious electronic signals emitted from ADP equipment.
Top-Level Specification (TLS) - A non-procedural description of system behavior at the most abstract level. Typically a functional specification that omits all implementation details.
Trap Door - A hidden software or hardware mechanism that permits system protection mechanisms to be circumvented. It is activated in some non-apparent manner (e.g., special "random" key sequence at a terminal).
Trojan Horse - A computer program with an apparently or actually useful function that contains additional (hidden) functions that surreptitiously exploit the legitimate authorizations of the invoking process to the detriment of security. For example, making a "blind copy" of a sensitive file for the creator of the Trojan Horse.
Trusted Computer System - A system that employs sufficient hardware and software integrity measures to allow its use for processing simultaneously a range of sensitive or classified information.
Trusted Computing Base (TCB) - The totality of protection mechanisms within a computer system -- including hardware, firmware, and software -- the combination of which is responsible for enforcing a security policy. A TCB consists of one or more components that together enforce a unified security policy over a product or system. The ability of a trusted computing base to correctly enforce a security policy depends solely on the mechanisms within the TCB and on the correct input by system administrative personnel of parameters (e.g., a user's clearance) related to the security policy.
Trusted Path - A mechanism by which a person at a terminal can communicate directly with the Trusted Computing Base. This mechanism can only be activated by the person or the Trusted Computing Base and cannot be imitated by untrusted software.
Trusted Software - The software portion of a Trusted Computing Base.
User - Any person who interacts directly with a computer system.
Verification - The process of comparing two levels of system specification for proper correspondence (e.g., security policy model with top-level specification, TLS with source code, or source code with object code). This process may or may not be automated.
Write - A fundamental operation that results only in the flow of information from a subject to an object.
Write Access - Permission to write an object.
2. Bell, D. E. and LaPadula, L. J. Secure Computer Systems: Unified Exposition and Multics Interpretation, MTR-2997 Rev. 1, MITRE Corp., Bedford, Mass., March 1976.
3. Brand, S. L. "An Approach to Identification and Audit of Vulnerabilities and Control in Application Systems," in Audit and Evaluation of Computer Security II: System Vulnerabilities and Controls, Z. Ruthberg, ed., NBS Special Publication #500-57, MD78733, April 1980.
4. Brand, S. L. "Data Processing and A-123," in Proceedings of the Computer Performance Evaluation User's Group 18th Meeting, C. B. Wilson, ed., NBS Special Publication #500-95, October 1982.
5. DCID l/l6, Security of Foreign Intelligence in Automated Data Processing Systems and Networks (U), 4 January l983.
6. DIAM 50-4, Security of Compartmented Computer Operations (U), 24 June l980.
7. Denning, D. E. "A Lattice Model of Secure Information Flow," in Communications of the ACM, vol. 19, no. 5 (May 1976), pp. 236-243.
8. Denning, D. E. Secure Information Flow in Computer Systems, Ph.D. dissertation, Purdue Univ., West Lafayette, Ind., May 1975.
9. DoD Directive 5000.29, Management of Computer Resources in Major Defense Systems, 26 April l976.
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