From: brian@ucsd.Edu (Brian Kantor) Newsgroups: comp.doc Subject: Computer Security - 1983 Orange Book [part 5 of 5] Date: 23 Jun 90 15:12:30 GMT Distribution: usa Organization: The Avant-Garde of the Now, Ltd. Design Specification and Verification C1: NR. C2: NR. B1: NEW: An informal or formal model of the security policy supported by the TCB shall be maintained 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 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. Device Labels C1: NR. C2: NR. B1: NR. 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. B3: NAR. A1: NAR. 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. 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. B1: NAR. B2: NAR. B3: CHANGE: The enforcement mechanism (e.g., access control lists) shall allow users to specify and control sharing of those objects. 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. A1: NAR. Exportation of Labeled Information C1: NR. C2: NR. 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 current security level associated with a single-level communication channel or I/O device. B2: NAR. B3: NAR. A1: NAR. Exportation to Multilevel Devices C1: NR. C2: NR. 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. B2: NAR. B3: NAR. A1: NAR. Exportation to Single-Level Devices C1: NR. C2: NR. 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. B2: NAR. B3: NAR. A1: NAR. 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 mechanism (e.g., passwords) to authenticate the user's identity. The TCB shall protect authentication data so that it cannot be accessed by any unauthorized user. 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 determine the security level and authorizations of subjects that may be created to act on behalf of the individual user. B2: NAR. B3: NAR. A1: NAR. Label Integrity C1: NR. C2: NR. 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. B2: NAR. B3: NAR. A1: NAR. Labeling Human-Readable Output C1: NR. C2: NR. 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. B2: NAR. B3: NAR. A1: NAR. ____________________________________________________________ * The hierarchical classification component in human-readable sensitivity labels shall be equal to the greatest hierarchical classification of any of the information in the output that the labels refer to; the non-hierarchical category component shall include all of the non-hierarchical categories of the information in the output the labels refer to, but no other non-hierarchical categories. ____________________________________________________________ Labels C1: NR. C2: NR. 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) that is directly or indirectly accessible by subjects external to the TCB shall be maintained by the TCB. B3: NAR. A1: NAR. Mandatory Access Control C1: NR. C2: NR. 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. 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: B3: NAR. A1: NAR. Object Reuse C1: NR. C2: NEW: When a storage object is initially assigned, allocated, or reallocated to a subject from the TCB's pool of unused storage objects, the TCB shall assure that the object contains no data for which the subject is not authorized. B1: NAR. B2: NAR. B3: NAR. A1: NAR. 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. C2: NAR. B1: NAR. B2: NAR. B3: NAR. A1: NAR. Security Testing 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 C1: NR. C2: NR. B1: NR. 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. B3: NAR. A1: NAR. System Architecture 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. A1: NAR. System Integrity 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. C2: NAR. B1: NAR. B2: NAR. B3: NAR. A1: NAR. Test Documentation C1: NEW: The system developer shall provide to the evaluators a document that describes the test plan and results of the security mechanisms' functional testing. C2: NAR. B1: NAR. B2: ADD: It shall include results of testing the effectiveness of the methods used to reduce covert channel bandwidths. B3: NAR. A1: ADD: The results of the mapping between the formal top-level specification and the TCB source code shall be given. Trusted Distribution C1: NR. C2: NR. B1: NR. B2: NR. B3: NR. 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 C1: NR. C2: NR. B1: NR. 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. A1: NAR. 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. A1: NAR. Trusted Path C1: NR. C2: NR. B1: NR. 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. A1: NAR. Trusted Recovery C1: NR. C2: NR. B1: NR. B2: NR. 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. A1: NAR. (this page is reserved for Figure 1) GLOSSARY Access - A specific type of interaction between a subject and an object that results in the flow of information from one to the other. 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. 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 [2]. 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. 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. 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. 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 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 Property - 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. It creates a basic protection environment and provides additional user services required for a trusted computer 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. REFERENCES 1. Anderson, J. P. Computer Security Technology Planning Study, ESD-TR-73-51, vol. I, ESD/AFSC, Hanscom AFB, Bedford, Mass., October 1972 (NTIS AD-758 206). 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. Denning, D. E. "A Lattice Model of Secure Information Flow," in Communications of the ACM, vol. 19, no. 5 (May 1976), pp. 236-243. 6. Denning, D. E. Secure Information Flow in Computer Systems, Ph.D. dissertation, Purdue Univ., West Lafayette, Ind., May 1975. 7. DoD 5200.1-R, Information Security Program Regulation, August 1982. 8. 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