The criteria are divided within each class into groups of requirements. These groupings were developed to assure that three basic control objectives for computer security are satisfied and not overlooked. These control objectives deal with:
This section provides a discussion of these general control objectives and their implication in terms of designing trusted systems.
A major goal of the DoD Computer Security Center is to encourage the Computer Industry to develop trusted computer systems and products, making them widely available in the commercial market place. Achievement of this goal will require recognition and articulation by both the public and private sectors of a need and demand for such products.
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.
The term control objective refers to a statement of intent with respect to control over some aspect of an organization's resources, or processes, or both. In terms of a computer system, control objectives provide a framework for developing a strategy for fulfilling a set of security requirements for any given system. Developed in response to generic vulnerabilities, such as the need to manage and handle sensitive data in order to prevent compromise, or the need to provide accountability in order to detect fraud, control objectives have been identified as a useful method of expressing security goals. [ 3]
Examples of control objectives include the three basic design requirements for implementing the reference monitor concept discussed in Section 6. They are:
The three basic control objectives of the criteria are concerned with security policy, accountability, and assurance. The remainder of this section provides a discussion of these basic requirements.
In the most general sense, computer security is concerned with controlling the way in which a computer can be used, i.e., controlling how information processed by it can be accessed and manipulated. However, at closer examination, computer security can refer to a number of areas. Symptomatic of this, FIPS PUB 39, Glossary For Computer Systems Security, does not have a unique definition for computer security. [ 20] Instead there are eleven separate definitions for security which include: ADP systems security, administrative security, data security, etc. A common thread running through these definitions is the word "protection." Further declarations of protection requirements can be found in DoD Directive 5200.28 which describes an acceptable level of protection for classified data to be one that will "assure that systems which process, store, or use classified data and produce classified information will, with reasonable dependability, prevent: a. Deliberate or inadvertent access to classified material by unauthorized persons, and b. Unauthorized manipulation of the computer and its associated peripheral devices." [ 11]
In summary, protection requirements must be defined in terms of the perceived threats, risks, and goals of an organization. This is often stated in terms of a security policy. It has been pointed out in the literature that it is external laws, rules, regulations, etc. that establish what access to information is to be permitted, independent of the use of a computer. In particular, a given system can only be said to be secure with respect to its enforcement of some specific policy. [ 30] Thus, the control objective for security policy is:
SECURITY POLICY CONTROL OBJECTIVE
A statement of intent with regard to control over access to and dissemination of information, to be known as the security policy must be precisely defined and implemented for each system that is used to process sensitive information. The security policy must accurately reflect the laws, regulations, and general policies from which it is derived.
Where a security policy is developed that is to be applied to control of classified or other specifically designated sensitive information, the policy must include detailed rules on how to handle that information throughout its life-cycle. These rules are a function of the various sensitivity designations that the information can assume and the various forms of access supported by the system. Mandatory security refers to the enforcement of a set of access control rules that constrains a subject's access to information on the basis of a comparison of that individual's clearance/authorization to the information, the classification/sensitivity designation of the information, and the form of access being mediated. Mandatory policies either require or can be satisfied by systems that can enforce a partial ordering of designations, namely, the designations must form what is mathematically known as a lattice [ 7]
A clear implication of the above is that the system must assure that the designations associated with sensitive data cannot be arbitrarily changed, since this could permit individuals who lack the appropriate authorization to access sensitive information. Also implied is the requirement that the system control the flow of information so that data cannot be stored with lower sensitivity designations unless its "downgrading" has been authorized. The control objective is:
MANDATORY SECURITY CONTROL OBJECTIVE
Security policies defined for systems that are used to process classified or other specifically categorized sensitive information must include provisions for the enforcement of mandatory access control rules. That is, they must include a set of rules for controlling access based directly on a comparison of the individual's clearance or authorization for the information and the classification or sensitivity designation of the information being sought, and indirectly on considerations of physical and other environmental factors of control. The mandatory access control rules must accurately reflect the laws, regulations, and general policies from which they are derived.
Discretionary security is the principal type of access control available in computer systems today. The basis of this kind of security is that an individual user, or program operating on his behalf, is allowed to specify explicitly the types of access other users may have to information under his control. Discretionary security differs from mandatory security in that it implements an access control policy on the basis of an individual's need-to-know as opposed to mandatory controls which are driven by the classification or sensitivity designation of the information.
Discretionary controls are not a replacement for mandatory controls. In an environment in which information is classified (as in the DoD) discretionary security provides for a finer granularity of control within the overall constraints of the mandatory policy. Access to classified information requires effective implementation of both types of controls as precondition to granting that access. In general, no person may have access to classified information unless: (a) that person has been determined to be trustworthy, i.e., granted a personnel security clearance -- MANDATORY, and (b) access is necessary for the performance of official duties, i.e., determined to have a need-to-know -- DISCRETIONARY. In other words, discretionary controls give individuals discretion to decide on which of the permissible accesses will actually be allowed to which users, consistent with overriding mandatory policy restrictions. The control objective is:
DISCRETIONARY SECURITY CONTROL OBJECTIVE
Security policies defined for systems that are used to process classified or other sensitive information must include provisions for the enforcement of discretionary access control rules. That is, they must include a consistent set of rules for controlling and limiting access based on identified individuals who have been determined to have a need-to-know for the information.
To implement a set of mechanisms that will put into effect a mandatory security policy, it is necessary that the system mark information with appropriate classification or sensitivity labels and maintain these markings as the information moves through the system. Once information is unalterably and accurately marked, comparisons required by the mandatory access control rules can be accurately and consistently made. An additional benefit of having the system maintain the classification or sensitivity label internally is the ability to automatically generate properly "labeled" output. The labels, if accurately and integrally maintained by the system, remain accurate when output from the system. The control objective is:
MARKING CONTROL OBJECTIVE
Systems that are designed to enforce a mandatory security policy must store and preserve the integrity of classification or other sensitivity labels for all information. Labels exported from the system must be accurate representations of the corresponding internal sensitivity labels being exported.
The second basic control objective addresses one of the fundamental principles of security, i.e., individual accountability. Individual accountability is the key to securing and controlling any system that processes information on behalf of individuals or groups of individuals. A number of requirements must be met in order to satisfy this objective.
The first requirement is for individual user identification. Second, there is a need for authentication of the identification. Identification is functionally dependent on authentication. Without authentication, user identification has no credibility. Without a credible identity, neither mandatory nor discretionary security policies can be properly invoked because there is no assurance that proper authorizations can be made.
The third requirement is for dependable audit capabilities. That is, a trusted computer system must provide authorized personnel with the ability to audit any action that can potentially cause access to, generation of, or effect the release of classified or sensitive information. The audit data will be selectively acquired based on the auditing needs of a particular installation and/or application. However, there must be sufficient granularity in the audit data to support tracing the auditable events to a specific individual who has taken the actions or on whose behalf the actions were taken. The control objective is:
ACCOUNTABILITY CONTROL OBJECTIVE
Systems that are used to process or handle classified or other sensitive information must assure individual accountability whenever either a mandatory or discretionary security policy is invoked. Furthermore, to assure accountability, the capability must exist for an authorized and competent agent to access and evaluate accountability information by a secure means, within a reasonable amount of time, and without undue difficulty.
The third basic control objective is concerned with guaranteeing or providing confidence that the security policy has been implemented correctly and that the protection-relevant elements of the system do, indeed, accurately mediate and enforce the intent of that policy. By extension, assurance must include a guarantee that the trusted portion of the system works only as intended. To accomplish these objectives, two types of assurance are needed. They are life-cycle assurance and operational assurance.
Life-cycle assurance refers to steps taken by an organization to ensure that the system is designed, developed, and maintained using formalized and rigorous controls and standards. [ 21]
Computer systems that process and store sensitive or classified information depend on the hardware and software to protect that information. It follows that the hardware and software themselves must be protected against unauthorized changes that could cause protection mechanisms to malfunction or be bypassed completely. For this reason trusted computer systems must be carefully evaluated and tested during the design and development phases and reevaluated whenever changes are made that could affect the integrity of the protection mechanisms. Only in this way can confidence be provided that the hardware and software interpretation of the security policy is maintained accurately and without distortion.
While life-cycle assurance is concerned with procedures for managing system design, development, and maintenance; operational assurance focuses on features and system architecture used to ensure that the security policy is uncircumventably enforced during system operation. That is, the security policy must be integrated into the hardware and software protection features of the system. Examples of steps taken to provide this kind of confidence include: methods for testing the operational hardware and software for correct operation, isolation of protection- critical code, and the use of hardware and software to provide distinct domains. The control objective is:
ASSURANCE CONTROL OBJECTIVE
Systems that are used to process or handle classified or other sensitive information must be designed to guarantee correct and accurate interpretation of the security policy and must not distort the intent of that policy. Assurance must be provided that correct implementation and operation of the policy exists throughout the system's life-cycle.