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Orange Book Parts I and II: THE CRITERIA and RATIONALE AND GUIDELINES: RATIONALE BEHIND THE EVALUATION CLASSES Previous Next Table of Contents

6. RATIONALE BEHIND THE EVALUATION CLASSES

6.1 THE REFERENCE MONITOR CONCEPT

In October of 1972, the Computer Security Technology Planning Study, conducted by James P. Anderson & Co., produced a report for the Electronic Systems Division (ESD) of the United States Air Force. [ 1]

In that report, the concept of "a reference monitor which enforces the authorized access relationships between subjects and objects of a system" was introduced. The reference monitor concept was found to be an essential element of any system that would provide multilevel secure computing facilities and controls.

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: [ 1]

Extensive peer review and continuing research and development activities have sustained the validity of the Anderson Committee's findings. Early examples of the reference validation mechanism were known as security kernels. The Anderson Report described the security kernel as "that combination of hardware and software which implements the reference monitor concept." [ 1] In this vein, it will be noted that the security kernel must support the three reference monitor requirements listed above.

6.2 A FORMAL SECURITY POLICY MODEL

Following the publication of the Anderson report, considerable research was initiated into formal models of security policy requirements and of the mechanisms that would implement and enforce those policy models as a security kernel. Prominent among these efforts was the ESD-sponsored development of the Bell and LaPadula model, an abstract formal treatment of DoD security policy. [ 2] Using mathematics and set theory, the model precisely defines the notion of secure state, fundamental modes of access, and the rules for granting subjects specific modes of access to objects. Finally, a theorem is proven to demonstrate that the rules are security-preserving operations, so that the application of any sequence of the rules to a system that is in a secure state will result in the system entering a new state that is also secure. This theorem is known as the Basic Security Theorem.

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.

6.3 THE TRUSTED COMPUTING BASE

In order to encourage the widespread commercial availability of trusted computer systems, these evaluation criteria have been designed to address those systems in which a security kernel is specifically implemented as well as those in which a security kernel has not been implemented. The latter case includes those systems in which objective (c) is not fully supported because of the size or complexity of the reference validation mechanism. For convenience, these evaluation criteria use the term Trusted Computing Base to refer to the reference validation mechanism, be it a security kernel, front-end security filter, or the entire trusted computer system.

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.

6.4 ASSURANCE

The third reference monitor design objective is currently interpreted as meaning that the TCB must be of sufficiently simple organization and complexity to be subjected to analysis and tests, the completeness of which can be assured.

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.

6.5 THE CLASSES

It is highly desirable that there be only a small number of overall evaluation classes. Three major divisions have been identified in the evaluation criteria with a fourth division reserved for those systems that have been evaluated and found to offer unacceptable security protection. Within each major evaluation division, it was found that "intermediate" classes of trusted system design and development could meaningfully be defined. These intermediate classes have been designated in the criteria because they identify systems that:

Except within division A it is not anticipated that additional "intermediate" evaluation classes satisfying the two characteristics described above will be identified.

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.

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