DoD 5200 28-STD Supersedes CSC-STD-00l-83 dtd l5 Aug 83 Library No S225 7ll DEPARTMENT OF DEFENSE STANDARD DEPARTMENT OF DEFENSE TRUSTED COMPUTER SYSTEM EVALUATION CRITERIA DECEMBER l985 December 26 l985 Page 1 FOREWORD This publication DoD 5200 28-STD Department of Defense Trusted Computer System Evaluation Criteria is issued under the authority of an in accordance with DoD Directive 5200 28 Security Requirements for Automatic Data Processing ADP Systems and in furtherance of responsibilities assigned by DoD Directive 52l5 l Computer Security Evaluation Center Its purpose is to provide technical hardware firmware software security criteria and associated technical evaluation methodologies in support of the overall ADP system security policy evaluation and approval accreditation responsibilities promulgated by DoD Directive 5200 28 The provisions of this document apply to the Office of the Secretary of Defense ASD the Military Departments the Organization of the Joint Chiefs of Staff the Unified and Specified Commands the Defense Agencies and activities administratively supported by OSD hereafter called DoD Components This publication is effective immediately and is mandatory for use by all DoD Components in carrying out ADP system technical security evaluation activities applicable to the processing and storage of classified and other sensitive DoD information and applications as set forth herein Recommendations for revisions to this publication are encouraged and will be reviewed biannually by the National Computer Security Center through a formal review process Address all proposals for revision through appropriate channels to National Computer Security Center Attention Chief Computer Security Standards DoD Components may obtain copies of this publication through their own publications channels Other federal agencies and the public may obtain copies from Office of Standards and Products National Computer Security Center Fort Meade MD 20755-6000 Attention Chief Computer Security Standards _________________________________ Donald C Latham Assistant Secretary of Defense Command Control Communications and Intelligence Page 2 ACKNOWLEDGEMENTS Special recognition is extended to Sheila L Brand National Computer Security Center NCSC who integrated theory policy and practice into and directed the production of this document Acknowledgment is also given for the contributions of Grace Hammonds and Peter S Tasker the MITRE Corp Daniel J Edwards NCSC Roger R Schell former Deputy Director of NCSC Marvin Schaefer NCSC and Theodore M P Lee Sperry Corp who as original architects formulated and articulated the technical issues and solutions presented in this document Jeff Makey formerly NCSC Warren F Shadle NCSC and Carole S Jordan NCSC who assisted in the preparation of this document James P Anderson James P Anderson Co Steven B Lipner Digital Equipment Corp Clark Weissman System Development Corp LTC Lawrence A Noble formerly U S Air Force Stephen T Walker formerly DoD Eugene V Epperly DoD and James E Studer formerly Dept of the Army who gave generously of their time and expertise in the review and critique of this document and finally thanks are given to the computer industry and others interested in trusted computing for their enthusiastic advice and assistance throughout this effort Page 3 CONTENTS FOREWORD i ACKNOWLEDGMENTS ii PREFACE v INTRODUCTION 1 PART I THE CRITERIA 1 0 DIVISION D MINIMAL PROTECTION 9 2 0 DIVISION C DISCRETIONARY PROTECTION 11 2 1 Class C1 Discretionary Security Protection 12 2 2 Class C2 Controlled Access Protection 15 3 0 DIVISION B MANDATORY PROTECTION 3 1 Class B1 Labeled Security Protection 3 2 Class B2 Structured Protection 3 3 Class B3 Security Domains 4 0 DIVISION A VERIFIED PROTECTION 41 4 1 Class A1 Verified Design 42 4 2 Beyond Class A1 51 PART II 19 20 26 33 RATIONALE AND GUIDELINES 5 0 CONTROL OBJECTIVES FOR TRUSTED COMPUTER SYSTEMS 5 1 A Need for Consensus 5 2 Definition and Usefulness 5 3 Criteria Control Objective 55 56 56 56 6 0 RATIONALE BEHIND THE EVALUATION CLASSES 6 1 The Reference Monitor Concept 6 2 A Formal Security Policy Model 6 3 The Trusted Computing Base 6 4 Assurance 6 5 The Classes 63 64 64 65 65 66 7 0 THE RELATIONSHIP BETWEEN POLICY AND THE CRITERIA 7 1 Established Federal Policies 7 2 DoD Policies 7 3 Criteria Control Objective For Security Policy 7 4 Criteria Control Objective for Accountability 7 5 Criteria Control Objective for Assurance 69 70 70 71 74 76 8 0 A GUIDELINE ON COVERT CHANNELS 79 Page 4 9 0 A GUIDELINE ON CONFIGURING MANDATORY ACCESS CONTROL FEATURES 81 10 0 A GUIDELINE ON SECURITY TESTING 10 1 Testing for Division C 10 2 Testing for Division B 10 3 Testing for Division A 83 84 84 85 APPENDIX A Commercial Product Evaluation Process 87 APPENDIX B Summary of Evaluation Criteria Divisions 89 APPENDIX C Sumary of Evaluation Criteria Classes 91 APPENDIX D Requirement Directory 93 GLOSSARY 109 REFERENCES 115 Page 5 PREFACE The trusted computer system evaluation criteria defined in this document classify systems into four broad hierarchical divisions of enhanced security protection They provide a basis for the evaluation of effectiveness of security controls built into automatic data processing system products The criteria were developed with three objectives in mind a to provide users with a yardstick with which to assess the degree of trust that can be placed in computer systems for the secure processing of classified or other sensitive information b to provide guidance to manufacturers as to what to build into their new widely-available trusted commercial products in order to satisfy trust requirements for sensitive applications and c to provide a basis for specifying security requirements in acquisition specifications Two types of requirements are delineated for secure processing a specific security feature requirements and b assurance requirements Some of the latter requirements enable evaluation personnel to determine if the required features are present and functioning as intended The scope of these criteria is to be applied to the set of components comprising a trusted system and is not necessarily to be applied to each system component individually Hence some components of a system may be completely untrusted while others may be individually evaluated to a lower or higher evaluation class than the trusted product considered as a whole system In trusted products at the high end of the range the strength of the reference monitor is such that most of the components can be completely untrusted Though the criteria are intended to be application-independent the specific security feature requirements may have to be interpreted when applying the criteria to specific systems with their own functional requirements applications or special environments e g communications processors process control computers and embedded systems in general The underlying assurance requirements can be applied across the entire spectrum of ADP system or application processing environments without special interpretation Page 6 INTRODUCTION Historical Perspective In October 1967 a task force was assembled under the auspices of the Defense Science Board to address computer security safeguards that would protect classified information in remote-access resource-sharing computer systems The Task Force report Security Controls for Computer Systems published in February 1970 made a number of policy and technical recommendations on actions to be taken to reduce the threat of compromise of classified information processed on remote-access computer systems 34 Department of Defense Directive 5200 28 and its accompanying manual DoD 5200 28-M published in 1972 and 1973 respectively responded to one of these recommendations by establishing uniform DoD policy security requirements administrative controls and technical measures to protect classified information processed by DoD computer systems 8 9 Research and development work undertaken by the Air Force Advanced Research Projects Agency and other defense agencies in the early and mid 70's developed and demonstrated solution approaches for the technical problems associated with controlling the flow of information in resource and information sharing computer systems 1 The DoD Computer Security Initiative was started in 1977 under the auspices of the Under Secretary of Defense for Research and Engineering to focus DoD efforts addressing computer security issues 33 Concurrent with DoD efforts to address computer security issues work was begun under the leadership of the National Bureau of Standards NBS to define problems and solutions for building evaluating and auditing secure computer systems 17 As part of this work NBS held two invitational workshops on the subject of audit and evaluation of computer security 20 28 The first was held in March 1977 and the second in November of 1978 One of the products of the second workshop was a definitive paper on the problems related to providing criteria for the evaluation of technical computer security effectiveness 20 As an outgrowth of recommendations from this report and in support of the DoD Computer Security Initiative the MITRE Corporation began work on a set of computer security evaluation criteria that could be used to assess the degree of trust one could place in a computer system to protect classified data 24 25 31 The preliminary concepts for computer security evaluation were defined and expanded upon at invitational workshops and symposia whose participants represented computer security expertise drawn from industry and academia in addition to the government Their work has since been subjected to much peer review and constructive technical criticism from the DoD industrial research and development organizations universities and computer manufacturers The DoD Computer Security Center the Center was formed in January 1981 to staff and expand on the work started by the DoD Computer Security Initiative 15 A major goal of the Center as given in its DoD Charter is to encourage the widespread availability of trusted computer systems for use by those who process classified or other sensitive information 10 The criteria presented in this document have evolved from the earlier NBS and MITRE evaluation material Scope Page 7 The trusted computer system evaluation criteria defined in this document apply primarily to trusted commercially available automatic data processing ADP systems They are also applicable as amplified below the the evaluation of existing systems and to the specification of security requirements for ADP systems acquisition Included are two distinct sets of requirements 1 specific security feature requirements and 2 assurance requirements The specific feature requirements encompass the capabilities typically found in information processing systems employing general-purpose operating systems that are distinct from the applications programs being supported However specific security feature requirements may also apply to specific systems with their own functional requirements applications or special environments e g communications processors process control computers and embedded systems in general The assurance requirements on the other hand apply to systems that cover the full range of computing environments from dedicated controllers to full range multilevel secure resource sharing systems Purpose As outlined in the Preface the criteria have been developedto serve a number of intended purposes To provide a standard to manufacturers as to what security features to build into their new and planned commercial products in order to provide widely available systems that satisfy trust requirements with particular emphasis on preventing the disclosure of data for sensitive applications To provide DoD components with a metric with which to evaluate the degree of trust that can be placed in computer systems for the secure processing of classified and other sensitive information To provide a basis for specifying security requirements in acquisition specifications With respect to the second purpose for development of the criteria i e providing DoD components with a security evaluation metric evaluations can be delineated into two types a an evaluation can be performed on a computer product from a perspective that excludes the application environment or b it can be done to assess whether appropriate security measures have been taken to permit the system to be used operationally in a specific environment The former type of evaluation is done by the Computer Security Center through the Commercial Product Evaluation Process That process is described in Appendix A The latter type of evaluation i e those done for the purpose of assessing a system's security attributes with respect to a specific operational mission is known as a certification evaluation It must be understood that the completion of a formal product evaluation does not constitute certification or accreditation for the system to be used in any specific application environment On the contrary the evaluation report only provides a trusted computer system's evaluation rating along with supporting data describing the product system's strengths and weaknesses from a computer security point of Page 8 view The system security certification and the formal approval accreditation procedure done in accordance with the applicable policies of the issuing agencies must still be followed-before a system can be approved for use in processing or handling classified information 8 9 Designated Approving Authorities DAAs remain ultimately responsible for specifying security of systems they accredit The trusted computer system evaluation criteria will be used directly and indirectly in the certification process Along with applicable policy it will be used directly as technical guidance for evaluation of the total system and for specifying system security and certification requirements for new acquisitions Where a system being evaluated for certification employs a product that has undergone a Commercial Product Evaluation reports from that process will be used as input to the certification evaluation Technical data will be furnished to designers evaluators and the Designated Approving Authorities to support their needs for making decisions Fundamental Computer Security Requirements Any discussion of computer security necessarily starts from a statement of requirements i e what it really means to call a computer system secure In general secure systems will control through use of specific security features access to information such that only properly authorized individuals or processes operating on their behalf will have access to read write create or delete information Six fundamental requirements are derived from this basic statement of objective four deal with what needs to be provided to control access to information and two deal with how one can obtain credible assurances that this is accomplished in a trusted computer system Policy Requirement 1 - SECURITY POLICY - There must be an explicit and well-defined security policy enforced by the system Given identified subjects and objects there must be a set of rules that are used by the system to determine whether a given subject can be permitted to gain access to a specific object Computer systems of interest must enforce a mandatory security policy that can effectively implement access rules for handling sensitive e g classified information 7 These rules include requirements such as No person lacking proper personnel security clearance shall obtain access to classified information In addition discretionary security controls are required to ensure that only selected users or groups of users may obtain access to data e g based on a need-to-know Requirement 2 - MARKING - Access control labels must be associated with objects In order to control access to information stored in a computer according to the rules of a mandatory security policy it must be possible to mark every object with a label that reliably identifies the object's sensitivity level e g classification and or the modes of access accorded those subjects who may potentially access the object Page 9 Accountability Requirement 3 - IDENTIFICATION - Individual subjects must be identified Each access to information must be mediated based on who is accessing the information and what classes of information they are authorized to deal with This identification and authorization information must be securely maintained by the computer system and be associated with every active element that performs some security-relevant action in the system Requirement 4 - ACCOUNTABILITY - Audit information must be selectively kept and protected so that actions affecting security can be traced to the responsible party A trusted system must be able to record the occurrences of security-relevant events in an audit log The capability to select the audit events to be recorded is necessary to minimize the expense of auditing and to allow efficient analysis Audit data must be protected from modification and unauthorized destruction to permit detection and after-the-fact investigations of security violations Assurance Requirement 5 - ASSURANCE - The computer system must contain hardware software mechanisms that can be independently evaluated to provide sufficient assurance that the system enforces requirements 1 through 4 above In order to assure that the four requirements of Security Policy Marking Identification and Accountability are enforced by a computer system there must be some identified and unified collection of hardware and software controls that perform those functions These mechanisms are typically embedded in the operating system and are designed to carry out the assigned tasks in a secure manner The basis for trusting such system mechanisms in their operational setting must be clearly documented such that it is possible to independently examine the evidence to evaluate their sufficiency Requirement 6 - CONTINUOUS PROTECTION - The trusted mechanisms that enforce these basic requirements must be continuously protected against tampering and or unauthorized changes No computer system can be considered truly secure if the basic hardware and software mechanisms that enforce the security policy are themselves subject to unauthorized modification or subversion The continuous protection requirement has direct implications throughout the computer system's life-cycle These fundamental requirements form the basis for the individual evaluation criteria applicable for each evaluation division and class The interested reader is referred to Section 5 of this document Control Objectives for Trusted Computer Systems for a more complete discussion and further amplification of these fundamental requirements as they apply to general-purpose information processing systems and to Section 7 for amplification of the relationship between Policy and these requirements Structure of the Document The remainder of this document is divided into two parts four appendices and a glossary Part I Sections 1 through 4 presents the detailed criteria derived from the fundamental requirements described above and relevant to the Page 10 rationale and policy excerpts contained in Part II Part II Sections 5 through 10 provides a discussion of basic objectives rationale and national policy behind the development of the criteria and guidelines for developers pertaining to mandatory access control rules implementation the covert channel problem and security testing It is divided into six sections Section 5 discusses the use of control objectives in general and presents the three basic control objectives of the criteria Section 6 provides the theoretical basis behind the criteria Section 7 gives excerpts from pertinent regulations directives OMB Circulars and Executive Orders which provide the basis for many trust requirements for processing nationally sensitive and classified information with computer systems Section 8 provides guidance to system developers on expectations in dealing with the covert channel problem Section 9 provides guidelines dealing with mandatory security Section 10 provides guidelines for security testing There are four appendices including a description of the Trusted Computer System Commercial Products Evaluation Process Appendix A summaries of the evaluation divisions Appendix B and classes Appendix C and finally a directory of requirements ordered alphabetically In addition there is a glossary Structure of the Criteria The criteria are divided into four divisions D C B and A ordered in a hierarchical manner with the highest division A being reserved for systems providing the most comprehensive security Each division represents a major improvement in the overall confidence one can place in the system for the protection of sensitive information Within divisions C and B there are a number of subdivisions known as classes The classes are also ordered in a hierarchical manner with systems representative of division C and lower classes of division B being characterized by the set of computer security mechanisms that they possess Assurance of correct and complete design and implementation for these systems is gained mostly through testing of the security- relevant portions of the system The security-relevant portions of a system are referred to throughout this document as the Trusted Computing Base TCB Systems representative of higher classes in division B and division A derive their security attributes more from their design and implementation structure Increased assurance that the required features are operative correct and tamperproof under all circumstances is gained through progressively more rigorous analysis during the design process Within each class four major sets of criteria are addressed The first three represent features necessary to satisfy the broad control objectives of Security Policy Accountability and Assurance that are discussed in Part II Section 5 The fourth set Documentation describes the type of written evidence in the form of user guides manuals and the test and design documentation required for each class A reader using this publication for the first time may find it helpful to first read Part II before continuing on with Part I Page 11 PART I THE CRITERIA Highlighting UPPERCASE is used in Part I to indicate criteria not contained in a lower class or changes and additions to already defined criteria Where there is no highlighting requirements have been carried over from lower classes without addition or modification Page 12 1 0 DIVISION D MINIMAL PROTECTION This division contains only one class It is reserved for those systems that have been evaluated but that fail to meet the requirements for a higher evaluation class Page 13 2 0 DIVISION C DISCRETIONARY PROTECTION Classes in this division provide for discretionary need-to-know protection and through the inclusion of audit capabilities for accountability of subjects and the actions they initiate Page 14 2 1 CLASS C1 DISCRETIONARY SECURITY PROTECTION The Trusted Computing Base TCB of a class C1 system nominally satisfies the discretionary security requirements by providing separation of users and data It incorporates some form of credible controls capable of enforcing access limitations on an individual basis i e ostensibly suitable for allowing users to be able to protect project or private information and to keep other users from accidentally reading or destroying their data The class C1 environment is expected to be one of cooperating users processing data at the same level s of sensitivity The following are minimal requirements for systems assigned a class C1 rating 2 1 1 Security Policy 2 1 1 1 Discretionary Access Control 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 2 1 2 Accountability 2 1 2 1 Identification and Authentication The TCB shall require users to identify themselves to it before beginning to perform any other actions that the TCB 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 2 1 3 Assurance 2 1 3 1 Operational Assurance 2 1 3 1 1 System Architecture The TCB shall maintain a domain for its own execution protects it from external interference or tampering e g by modification of its code or data strucutres Resources controlled by the TCB may be a defined subset of the subjects and objects in the ADP system 2 1 3 1 2 System Integrity Hardware and or software features shall be provided that can be used to periodically validate the correct operation of the on-site hardware and firmware elements of the TCB 2 1 3 2 Life-Cycle Assurance Page 15 2 1 3 2 1 Security Testing The security mechanisms of the ADP system shall be tested and found to work as claimed in the system documentation 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 2 1 4 Documentation 2 1 4 1 Security Features User's Guide A single summary chapter or manual in user documentation shall describe the protection mechanisms provided by the TCB guidelines on their use and how they interact with one another 2 1 4 2 Trusted Facility Manual A manual addressed to the ADP System Administrator shall present cautions about functions and privileges that should be controlled when running a secure facility 2 1 4 3 Test Documentation The system developer shall provide to the evaluators a document that describes the test plan test procedures that show how the the security mechanisms were tested and results of the security mechanisms' functional testing 2 1 4 4 Design Documentation 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 Page 16 2 2 CLASS C2 CONTROLLED ACCESS PROTECTION Systems in this class enforce a more finely grained discretionary access control than C1 systems making users individually accountable for their actions through login procedures auditing of security-relevant events and resource isolation The following are minimal requirements for systems assigned a class C2 rating 2 2 1 Security Policy 2 2 1 1 Discretionary Access Control The TCB shall define and control access between named users and named objects e g files and programs in the ADP system The enforcement mechanism e g self group public controls access control lists shall allow users to specify and control sharing of those objects by named individuals or defined groups of individuals or by both and shall provide controls to limit propagation of access rights The discretionary access control mechanism shall either by explicit user action or by default provide that objects are protected from unauthorized access These access controls shall be capable of including or excluding access to the granularity of a single user Access permission to an object by users not already possessing access permission shall only be assigned by authorized users 2 2 1 2 Object Reuse All authorizations to the information contained within a storage object shall be revoked prior to initial assignment allocation or reallocation to a subject from the TCB's pool of unused storage objects No information including encrypted representations of information produced by a prior subject's actions is to be available to any subject that obtains access to an object that has been released back to the system 2 2 2 Accountability 2 2 2 1 Identification and Authentication The TCB shall require users to identify themselves to it before beginning to perform any other actions that the TCB is expected to mediate Furthermore the TCB shall use a protected mechanism e g passwords to authenticate the user's identity The TCB shall protect authentication data so that it cannot be accessed by any unauthorized user The TCB shall be able to enforce individual accountability by providing the capability to uniquely identify each individual ADP system user The TCB shall also provide the capability of associating this identity with all auditable actions taken by that individual 2 2 2 2 Audit Page 17 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 or 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 2 2 3 Assurance 2 2 3 1 Operational Assurance 2 2 3 1 1 System Architecture The TCB shall maintain a domain for its own execution that protects it from external interference or tampering e g by modification of its code or data structures Resources controlled by the TCB may be a defined subset of the subjects and objects in the ADP system The TCB shall isolate the resources to be protected so that they are subject to the access control and auditing requirements 2 2 3 1 2 System Integrity Hardware and or software features shall be provided that can be used to periodically validate the correct operation of the on-site hardware and firmware elements of the TCB 2 2 3 2 Life-Cycle Assurance 2 2 3 2 1 Security Testing The security mechanisms of the ADP system shall be tested and found to work as claimed in the system documentation Testing shall be done to assure that there are no obvious ways for an unauthorized user to bypass or otherwise defeat the security protection mechanisms of the TCB Testing shall also include a search for obvious flaws Page 18 that would allow violation of resource isolation or that would permit unauthorized access to the audit or authentication data See the Security Testing guidelines 2 2 4 Documentation 2 2 4 1 Security Features User's Guide A single summary chapter or manual in user documentation shall describe the protection mechanisms provided by the TCB guidelines on their use and how they interact with one another 2 2 4 2 Trusted Facility Manual A manual addressed to the ADP system administrator shall present cautions about functions and privileges that should be controlled when running a secure facility The procedures for examining and maintaining the audit files as well as the detailed audit record structure for each type of audit event shall be given 2 2 4 3 Test Documentation The system developer shall provide to the evaluators a document that describes the test plan test procedures that show how the security mechanisms were tested and results of the security mechanisms' functional testing 2 2 4 4 Design Documentation Documentation shall be available that provides a description of the manufacturer's philosophy of protection and an explanation of how this philosophy is translated into the TCB If the TCB is composed of distinct modules the interfaces between these modules shall be described Page 19 3 0 DIVISION B MANDATORY PROTECTION The notion of a TCB that preserves the integrity of sensitivity labels and uses them to enforce a set of mandatory access control rules is a major requirement in this division Systems in this division must carry the sensitivity labels with major data structures in the system The system developer also provides the security policy model on which the TCB is based and furnishes a specification of the TCB Evidence must be provided to demonstrate that the reference monitor concept has been implemented Page 20 3 1 CLASS B1 LABELED SECURITY PROTECTION Class B1 systems require all the features required for class C2 In addition an informal statement of the security policy model data labeling and mandatory access control over named subjects and objects must be present The capability must exist for accurately labeling exported information Any flaws identified by testing must be removed The following are minimal requirements for systems assigned a class B1 rating 3 1 1 Security Policy 3 1 1 1 Discretionary Access Control The TCB shall define and control access between named users and named objects e g files and programs in the ADP system The enforcement mechanism e g self group public controls access control lists shall allow users to specify and control sharing of those objects by named individuals or defined groups of individuals or by both and shall provide controls to limit propagation of access rights The discretionary access control mechanism shall either by explicit user action or by default provide that objects are protected from unauthorized access These access controls shall be capable of including or excluding access to the granularity of a single user Access permission to an object by users not already possessing access permission shall only be assigned by authorized users 3 1 1 2 Object Reuse All authorizations to the information contained within a storage object shall be revoked prior to initial assignment allocation or reallocation to a subject from the TCB's pool of unused storage objects No information including encrypted representations of information produced by a prior subject's actions is to be available to any subject that obtains access to an object that has been released back to the system 3 1 1 3 Labels Sensitivity labels associated with each subject and storage object under its control e g process file segment device shall be maintained by the TCB These labels shall be used as the basis for mandatory access control decisions In order to import non-labeled data the TCB shall request and receive from an authorized user the security level of the data and all such actions shall be auditable by the TCB 3 1 1 3 1 Label Integrity Sensitivity labels shall accurately represent security levels of the specific subjects or objects with which they are associated When exported by the TCB Page 21 sensitivity labels shall accurately and unambiguously represent the internal labels and shall be associated with the information being exported 3 1 1 3 2 Exportation of Labeled Information The TCB shall designate each communication channel and I O device as either single-level or miltilevel Any change in this designation shall be done manually and shall be auditable by the TCB The TCB shall maintain and be able to audit any change in the security level or levels associated with a communication channel or I O device 3 1 1 3 2 1 Exportation to Multilevel Devices When the TCB exports an object to a multilevel I O device the sensitivity label associated with that object shall also be exported and shall reside on the same physical medium as the exported information and shall be in the same form i e machine-readable or human-readable form When the TCB exports or imports an object over a multilevel communication channel the protocol used on that channel shall provide for the unambiguous pairing between the sensitivity labels and the associated information that is sent or received 3 1 1 3 2 2 Exportation to Single-Level Devices Single-level I O devices and single-level communication channels are not required to maintain the sensitivity labels of the information they process However the TCB shall include a mechanism by which the TCb and an authorized user reliably communicate to designate the single security level of information imported or exported via single-level communication channels or I O devices 3 1 1 3 2 3 Labeling Human-Readable Output The ADP system administrator shall be able to specify the printable label names associated with exported sensitivity labels The TCB shall mark _____________________________ The hierarchical classification component in human-readable sensitivity labels shall be equal to the greatest hierarchical classification or 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 Page 22 the beginning and end of all human-readable paged hardcopy output e g line printer output with human-readable sensitivity labels that properly represent the sensitivity of the output The TCB shall be default mark the top and bottom of each page of human-readable paged hardcopy output e g line printer output with human-readable sensitivity labels that properly represent the overall sensitivity of the output or that properly represent the sensitivity of the information on the page The TCB shall by default and in an appropriate manner mark other forms of human­ readable output e g maps graphics with human­ readable sensitivity labels that properly represent the sensitivity of the output Any override of these marking defaults shall be auditable by the TCB 3 1 1 4 Mandatory Access Control The TCB shall enforce a mandatory access control policy over all subjects and storage objects under its control e g processes files segments devices These subjects and objects shall be assigned sensitivity labels that are a combination of hierarchical classification levels and non-hierarchical categories and the labels shall be used as the basis for mandatory access control decisions The TCB shall be able to support two or more such security levels See the Mandatory Access Control Guidelines The following requirements shall hold for all accesses between subjects and objects controlled by the TCB a subject can read an object only if the hierarchical classification in the subject's security level is greater than or equal to the hierarchical classification in the object's security level and the non­ hierarchical categories in the subject's security level include all the non-hierarchical categories in the object's security level A subject can write an object only if the hierarchical classification in the subject's security level is less than or equal to the hierarchical classification in the object's security level and all the non-hierarchical categories in the subject's security level are included in the non-hierarchical categories in the object's security level Identification and authentication data shall be used by the TCB to authenti­ cate the user's identity and to ensure that the security level and authorization of subjects external to the TCB that may be created to act on behalf of the individual user are dominated by the clearance and authorization of that user 3 1 2 Accountability 3 1 2 1 Identification and Authentication The TCB shall require users to identify themselves to it before beginning to perform any other actions that the TCB is expected Page 23 to mediate Furthermore the TCB shall maintain authentication data that includes information for verifying the identity of individual users e g passwords as well as information for determining the clearance and authorizations or individual users This data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authorizations of subjects external to the TCB that may be created to act on behalf of the individual user are dominated by the clearance and authorization of that user The TCB shall protect authentication data so that it cannot be accessed by any unauthorized user The TCB shall be able to enforce individual accountability by providing the capability to uniquely identify each individual ADP system user The TCB shall also provide the capability of associating this identity with all auditable actions taken by that individual 3 1 2 2 Audit The TCB shall be able to create maintain and protect from modification or unauthorized access or destruction an audit trail of accesses to the objects it protects The audit data shall be protected by the TCB so that read access to it is limited to those who are authorized for audit data The TCB shall be able to record the following types of events use of identification and authentication mechanisms introduction of objects into a user's address space e g file open program initiation deletion of objects and actions taken by computer operators and system administrators and or system security officers and other security relevant events The TCB shall also be able to audit any override of human-readable output markings For each recorded event the audit record shall identify date and time of the event user type of event and success or failure of the event For identification authentication events the origin of request e g terminal ID shall be included in the audit record For events that introduce an object into a user's address space and for object deletion events the audit record shall include the name of the object and the object's security level The ADP system administrator shall be able to selectively audit the actions of any one or more users based on individual identity and or object security level 3 1 3 Assurance 3 1 3 1 Operational Assurance 3 1 3 1 1 System Architecture The TCB shall maintain a domain for its own execution that protects it from external interference or tampering e g by modification of its code or data structures Resources controlled by the TCB may be a defined subset of the subjects and objects in the ADP system The TCB Page 24 shall maintain process isolation through the provision of distinct address spaces under its control The TCB shall isolate the resources to be protected so that they are subject to the access control and auditing requirements 3 1 3 1 2 System Integrity Hardware and or software features shall be provided that can be used to periodically validate the correct operation of the on-site hardware and firmware elements of the TCB 3 1 3 2 Life-Cycle Assurance 3 1 3 2 1 Security Testing The security mechanisms of the ADP system shall be tested and found to work as claimed in the system documentation A team of individuals who thoroughly understand the specific implementation of the TCB shall subject its design documentation source code and object code to thorough analysis and testing Their objectives shall be to uncover all design and implementation flaws that would permit a subject external to the TCB to read change or delete data normally denied under the mandatory or discretionary security policy enforced by the TCB as well as to assure that no subject without authorization to do so is able to cause the TCB to enter a state such that it is unable to respond to communications initiated by other users All discovered flaws shall be removed or neutralized and the TCB retested to demonstrate that they have been eliminated and that new flaws have not been introduced See the Security Testing Guidelines 3 1 3 2 2 Design Specification and Verification An informal or formal model of the security policy supported by the TCB shall be maintained over the life cycle of the ADP system and demonstrated to be consistent with its axioms 3 1 4 Documentation 3 1 4 1 Security Features User's Guide A single summary chapter or manual in user documentation shall describe the protection mechanisms provided by the TCB guidelines on their use and how they interact with one another 3 1 4 2 Trusted Facility Manual A manual addressed to the ADP system administrator shall Page 25 present cautions about functions and privileges that should be controlled when running a secure facility The procedures for examining and maintaining the audit files as well as the detailed audit record structure for each type of audit event shall be given The manual shall describe the operator and administrator functions related to security to include changing the security characteristics of a user It shall provide guidelines on the consistent and effective use of the protection features of the system how they interact how to securely generate a new TCB and facility procedures warnings and privileges that need to be controlled in order to operate the facility in a secure manner 3 1 4 3 Test Documentation The system developer shall provide to the evaluators a document that describes the test plan test procedures that show how the security mechanisms were tested and results of the security mechanisms' functional testing 3 1 4 4 Design Documentation Documentation shall be available that provides a description of the manufacturer's philosophy of protection and an explanation of how this philosophy is translated into the TCB If the TCB is composed of distinct modules the interfaces between these modules shall be described An informal or formal description of the security policy model enforced by the TCB shall be available and an explanation provided to show that it is sufficient to enforce the security policy The specific TCB protection mechanisms shall be identified and an explanation given to show that they satisfy the model Page 26 3 2 CLASS B2 STRUCTURED PROTECTION In class B2 systems the TCB is based on a clearly defined and documented formal security policy model that requires the discretionary and mandatory access control enforcement found in class B1 systems be extended to all subjects and objects in the ADP system In addition covert channels are addressed The TCB must be carefully structured into protection-critical and non- protection-critical elements The TCB interface is well-defined and the TCB design and implementation enable it to be subjected to more thorough testing and more complete review Authentication mechanisms are strengthened trusted facility management is provided in the form of support for system administrator and operator functions and stringent configuration management controls are imposed The system is relatively resistant to penetration The following are minimal requirements for systems assigned a class B2 rating 3 2 1 Security Policy 3 2 1 1 Discretionary Access Control The TCB shall define and control access between named users and named objects e g files and programs in the ADP system The enforcement mechanism e g self group public controls access control lists shall allow users to specify and control sharing of those objects by named individuals or defined groups of individuals or by both and shall provide controls to limit propagation of access rights The discretionary access control mechanism shall either by explicit user action or by default provide that objects are protected from unauthorized access These access controls shall be capable of including or excluding access to the granularity of a single user Access permission to an object by users not already possessing access permission shall only be assigned by authorized users 3 2 1 2 Object Reuse All authorizations to the information contained within a storage object shall be revoked prior to initial assignment allocation or reallocation to a subject from the TCB's pool of unused storage objects No information including encrypted representations of information produced by a prior subject's actions is to be available to any subject that obtains access to an object that has been released back to the system 3 2 1 3 Labels Sensitivity labels associated with each ADP system resource e g subject storage object ROM that is directly or indirectly accessible by subjects external to the TCB shall be maintained by the TCB These labels shall be used as the basis for mandatory access control decisions In order to import non-labeled data the TCB shall request and receive from an authorized user the security level of the data and all such actions shall be auditable by the TCB Page 27 3 2 1 3 1 Label Integrity Sensitivity labels shall accurately represent security levels of the specific subjects or objects with which they are associated When exported by the TCB sensitivity labels shall accurately and unambiguously represent the internal labels and shall be associated with the information being exported 3 2 1 3 2 Exportation of Labeled Information The TCB shall designate each communication channel and I O device as either single-level or multilevel Any change in this designation shall be done manually and shall be auditable by the TCB The TCB shall maintain and be able to audit any change in the security level or levels associated with a communication channel or I O device 3 2 1 3 2 1 Exportation to Multilevel Devices When the TCB exports an object to a multilevel I O device the sensitivity label associated with that object shall also be exported and shall reside on the same physical medium as the exported information and shall be in the same form i e machine-readable or human-readable form When the TCB exports or imports an object over a multilevel communication channel the protocol used on that channel shall provide for the unambiguous pairing between the sensitivity labels and the associated information that is sent or received 3 2 1 3 2 2 Exportation to Single-Level Devices Single-level I O devices and single-level communication channels are not required to maintain the sensitivity labels of the information they process However the TCB shall include a mechanism by which the TCB and an authorized user reliably communicate to designate the single security level of information imported or exported via single-level communication channels or I O devices 3 2 1 3 2 3 Labeling Human-Readable Output The ADP system administrator shall be able to specify the printable label names associated with exported sensitivity labels The TCB shall mark the beginning and end of all human-readable paged hardcopy output e g line printer output with Page 28 human-readable sensitivity labels that properly represent the sensitivity of the output The TCB shall by default mark the top and bottom of each page of human-readable paged hardcopy output e g line printer output with human-readable sensitivity labels that properly represent the overall sensitivity of the output or that properly represent the sensitivity of the information on the page The TCB shall by default and in an appropriate manner mark other forms of human-readable output e g maps graphics with human-readable sensitivity labels that properly represent the sensitivity of the output Any override of these marking defaults shall be auditable by the TCB 3 2 1 3 3 Subject Sensitivity Labels The TCB shall immediately notify a terminal user of each change in the security level associated with that user during an interactive session A terminal user shall be able to query the TCB as desired for a display of the subject's complete sensitivity label 3 2 1 3 4 Device Labels The TCB shall support the assignment of minimum and maximum security levels to all attached physical devices These security levels shall be used by the TCB to enforce constraints imposed by the physical environments in which the devices are located 3 2 1 4 Mandatory Access Control The TCB shall enforce a mandatory access control policy over all resources i e subjects storage objects and I O devices that are directly or indirectly accessible by subjects external to the TCB These subjects and objects shall be assigned sensitivity labels that are a combination of hierarchical classification levels and non-hierarchical categories and the labels shall be used as the basis for mandatory access control decisions The TCB shall be able to support two or more such security levels See the Mandatory Access Control guidelines The following requirements shall hold for all accesses between all subjects external to the TCB and all objects directly or indirectly accessible by these subjects A subject can read an object only if the hierarchical classification in the subject's security level is greater than or equal to the hierarchical classification in the object's security level and the non-hierarchical categories in the subject's security level include all the non- hierarchical categories in the object's security level A subject can write an object only if the hierarchical classification in the subject's security level is less than or Page 29 equal to the hierarchical classification in the object's security level and all the non-hierarchical categories in the subject's security level are included in the non-hierarchical categories in the object's security level Identification and authentication data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authorization of subjects external to the TCB that may be created to act on behalf of the individual user are dominated by the clearance and authorization of that user 3 2 2 Accountability 3 2 2 1 Identification and Authentication The TCB shall require users to identify themselves to it before beginning to perform any other actions that the TCB is expected to mediate Furthermore the TCB shall maintain authentication data that includes information for verifying the identity of individual users e g passwords as well as information for determining the clearance and authorizations of individual users This data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authorizations of subjects external to the TCB that may be created to act on behalf of the individual user are dominated by the clearance and authorization of that user The TCB shall protect authentication data so that it cannot be accessed by any unauthorized user The TCB shall be able to enforce individual accountability by providing the capability to uniquely identify each individual ADP system user The TCB shall also provide the capability of associating this identity with all auditable actions taken by that individual 3 2 2 1 1 Trusted Path The TCB shall support a trusted communication path between itself and user for initial login and authentication Communications via this path shall be initiated exclusively by a user 3 2 2 2 Audit The TCB shall be able to create maintain and protect from modification or unauthorized access or destruction an audit trail of accesses to the objects it protects The audit data shall be protected by the TCB so that read access to it is limited to those who are authorized for audit data The TCB shall be able to record the following types of events use of identification and authentication mechanisms introduction of objects into a user's address space e g file open program initiation deletion of objects and actions taken by computer operators and system administrators and or system security officers and other security relevant events The TCB shall also be able to audit any override of human-readable output markings For each recorded event the audit record shall Page 30 identify date and time of the event user type of event and success or failure of the event For identification authentication events the origin of request e g terminal ID shall be included in the audit record For events that introduce an object into a user's address space and for object deletion events the audit record shall include the name of the object and the object's security level The ADP system administrator shall be able to selectively audit the actions of any one or more users based on individual identity and or object security level The TCB shall be able to audit the identified events that may be used in the exploitation of covert storage channels 3 2 3 Assurance 3 2 3 1 Operational Assurance 3 2 3 1 1 System Architecture 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 3 2 3 1 2 System Integrity Hardware and or software features shall be provided that can be used to periodically validate the correct operation of the on-site hardware and firmware elements of the TCB 3 2 3 1 3 Covert Channel Analysis The system developer shall conduct a thorough search for covert storage channels and make a determination either by actual measurement or by engineering estimation of the maximum bandwidth of each identified channel See the covert channels guideline section 3 2 3 1 4 Trusted Facility Management The TCB shall support separate operator and administrator Page 31 functions 3 2 3 2 Life-Cycle Assurance 3 2 3 2 1 Security Testing The security mechanisms of the ADP system shall be tested and found to work as claimed in the system documentation A team of individuals who thoroughly understand the specific implementation of the TCB shall subject its design documentation source code and object code to thorough analysis and testing Their objectives shall be to uncover all design and implementation flaws that would permit a subject external to the TCB to read change or delete data normally denied under the mandatory or discretionary security policy enforced by the TCB as well as to assure that no subject without authorization to do so is able to cause the TCB to enter a state such that it is unable to respond to communications initiated by other users The TCB shall be found relatively resistant to penetration All discovered flaws shall be corrected and the TCB retested to demonstrate that they have been eliminated and that new flaws have not been introduced Testing shall demonstrate that the TCB implementation is consistent with the descriptive top-level specification See the Security Testing Guidelines 3 2 3 2 2 Design Specification and Verification A formal model of the security policy supported by the TCB shall be maintained over the life cycle of the ADP system that is proven consistent with its axioms A descriptive top-level specification DTLS of the TCB shall be maintained that completely and accurately describes the TCB in terms of exceptions error messages and effects It shall be shown to be an accurate description of the TCB interface 3 2 3 2 3 Configuration Management During development and maintenance of the TCB a configuration management system shall be in place that maintains control of changes to the descriptive top-level specification other design data implementation documentation source code the running versionof the object code and test fixtures and documentation The configuration management system shall assure a consistent mapping among all documentation and code associated with the current version of the TCB Tools shall be provided for generation of a new version of the TCB from source code Also available shall be tools for comparing a newly generated version with the previous TCB version in order to ascertain that only the intended changes have Page 32 been made in the code that will actually be used as the new version of the TCB 3 2 4 Documentation 3 2 4 1 Security Features User's Guide A single summary chapter or manual in user documentation shall describe the protection mechanisms provided by the TCB guidelines on their use and how they interact with one another 3 2 4 2 Trusted Facility Manual A manual addressed to the ADP system administrator shall present cautions about functions and privileges that should be controlled when running a secure facility The procedures for examining and maintaining the audit files as well as the detailed audit record structure for each type of audit event shall be given The manual shall describe the operator and administrator functions related to security to include changing the security characteristics of a user It shall provide guidelines on the consistent and effective use of the protection features of the system how they interact how to securely generate a new TCB and facility procedures warnings and privileges that need to be controlled in order to operate the facility in a secure manner The TCB modules that contain the reference validation mechanism shall be identified The procedures for secure generation of a new TCB from source after modification of any modules in the TCB shall be described 3 2 4 3 Test Documentation The system developer shall provide to the evaluators a document that describes the test plan test procedures that show how the security mechanisms were tested and results of the security mechanisms' functional testing It shall include results of testing the effectiveness of the methods used to reduce covert channel bandwidths 3 2 4 4 Design Documentation Documentation shall be available that provides a description of the manufacturer's philosophy of protection and an explanation of how this philosophy is translated into the TCB The interfaces between the TCB modules shall be described A formal description of the security policy model enforced by the TCB shall be available and proven that it is sufficient to enforce the security policy The specific TCB protection mechanisms shall be identified and an explanation given to show that they satisfy the model The descriptive top-level specification DTLS shall be shown to be an accurate description of the TCB interface Documentation shall describe how the TCB implements the reference monitor concept and give Page 33 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 Page 34 3 3 CLASS B3 SECURITY DOMAINS The class B3 TCB must satisfy the reference monitor requirements that it mediate all accesses of subjects to objects be tamperproof and be small enough to be subjected to analysis and tests To this end the TCB is structured to exclude code not essential to security policy enforcement with significant system engineering during TCB design and implementation directed toward minimizing its complexity A security administrator is supported audit mechanisms are expanded to signal security- relevant events and system recovery procedures are required The system is highly resistant to penetration The following are minimal requirements for systems assigned a class B3 rating 3 1 1 Security Policy 3 3 1 1 Discretionary Access Control The TCB shall define and control access between named users and named objects e g files and programs in the ADP system The enforcement mechanism e g access control lists shall allow users to specify and control sharing of those objects and shall provide controls to limit propagation of access rights The discretionary access control mechanism shall either by explicit user action or by default provide that objects are protected from unauthorized access These access controls shall be capable of specifying for each named object a list of named individuals and a list of groups of named individuals with their respective modes of access to that object Furthermore for each such named object it shall be possible to specify a list of named individuals and a list of groups of named individuals for which no access to the object is to be given Access permission to an object by users not already possessing access permission shall only be assigned by authorized users 3 3 1 2 Object Reuse All authorizations to the information contained within a storage object shall be revoked prior to initial assignment allocation or reallocation to a subject from the TCB's pool of unused storage objects No information including encrypted representations of information produced by a prior subjects actions is to be available to any subject that obtains access to an object that has been released back to the system 3 3 1 3 Labels Sensitivity labels associated with each ADP system resource e g subject storage object ROM that is directly or indirectly accessible by subjects external to the TCB shall be maintained by the TCB These labels shall be used as the basis for mandatory access control decisions In order to import non-labeled data the TCB shall request and receive from an authorized user the security level of the data and all such Page 35 hold for all accesses between all subjects external to the TCB and all objects directly or indirectly accessible by these actions shall be auditable by the TCB 3 3 1 3 1 Label Integrity Sensitivity labels shall accurately represent security levels of the specific subjects or objects with which they are associated When exported by the TCB sensitivity labels shall accurately and unambiguously represent the internal labels and shall be associated with the information being exported 3 3 1 3 2 Exportation of Labeled Information The TCB shall designate each communication channel and I O device as either single-level or multilevel Any change in this designation shall be done manually and shall be auditable by the TCB The TCB shall maintain and be able to audit any change in the security level or levels associated with a communication channel or I O device 3 3 1 3 2 1 Exportation to Multilevel Devices When the TCB exports an object to a multilevel I O device the sensitivity label associated with that object shall also be exported and shall reside on the same physical medium as the exported information and shall be in the same form i e machine-readable or human-readable form When the TCB exports or imports an object over a multilevel communication channel the protocol used on that channel shall provide for the unambiguous pairing between the sensitivity labels and the associated information that is sent or received 3 3 1 3 2 2 Exportation to Single-Level Devices Single-level I O devices and single-level communication channels are not required to maintain the sensitivity labels of the information they process However the TCB shall include a mechanism by which the TCB and an authorized user reliably communicate to designate the single security level of information imported or exported via single-level communication channels or I O devices 3 3 1 3 2 3 Labeling Human-Readable Output The ADP system administrator shall be able to specify the printable label names associated with Page 36 exported sensitivity labels The TCB shall mark the beginning and end of all human-readable paged hardcopy output e g line printer output with human-readable sensitivity labels that properly represent the sensitivity of the output The TCB shall by default mark the top and bottom of each page of human-readable paged hardcopy output e g line printer output with human- readable sensitivity labels that properly represent the overall sensitivity of the output or that properly represent the sensitivity of the information on the page The TCB shall by default and in an appropriate manner mark other forms of human-readable output e g maps graphics with human-readable sensitivity labels that properly represent the sensitivity of the output Any override of these marking defaults shall be auditable by the TCB 3 3 1 3 3 Subject Sensitivity Labels The TCB shall immediately notify a terminal user of each change in the security level associated with that user during an interactive session A terminal user shall be able to query the TCB as desired for a display of the subject's complete sensitivity label 3 3 1 3 4 Device Labels The TCB shall support the assignment of minimum and maximum security levels to all attached physical devices These security levels shall be used by the TCB to enforce constraints imposed by the physical environments in which the devices are located 3 3 1 4 Mandatory Access Control The TCB shall enforce a mandatory access control policy over all resources i e subjects storage objects and I O devices that are directly or indirectly accessible by subjects external to the TCB These subjects and objects shall be assigned sensitivity labels that are a combination of hierarchical classification levels and non-hierarchical categories and the labels shall be used as the basis for mandatory access control decisions The TCB shall be able to support two or more such security levels See the Mandatory ______________________________ 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 Page 37 Access Control guidelines The following requirements shall subjects A subject can read an object only if the hierarchical classification in the subject's security level is greater than or equal to the hierarchical classification in the object's security level and the non-hierarchical categories in the subject's security level include all the non-hierarchical categories in the object's security level A subject can write an object only if the hierarchical classification in the subject's security level is less than or equal to the hierarchical classification in the object's security level and all the non-hierarchical categories in the subject's security level are included in the non- hierarchical categories in the object's security level Identification and authentication data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authori­ zation of subjects external to the TCB that may be created to act on behalf of the individual user are dominated by the clearance and authorization of that user 3 3 2 Accountability 3 3 2 1 Identification and Authentication The TCB shall require users to identify themselves to it before beginning to perform any other actions that the TCB is expected to mediate Furthermore the TCB shall maintain authentication data that includes information for verifying the identity of individual users e g passwords as well as information for determining the clearance and authorizations of individual users This data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authorizations of subjectsexternal to the TCB that may be created to act on behalf of the individual user are dominated by the clearance and authorization of that user The TCB shall protect authentication data so that it cannot be accessed by any unauthorized user The TCB shall be able to enforce individual accountability by providing the capability to uniquely identify each individual ADP system user The TCB shall also provide the capability of associating this identity with all auditable actions taken by that individual 3 3 2 1 1 Trusted Path The TCB shall support a trusted communication path between itself and users for use when a positive TCB-to­ user connection is required e g login change subject security level Communications via this trusted path shall be activated exclusively by a user of the TCB and shall be logically isolated and unmistakably distinguishable from other paths 3 3 2 2 Audit The TCB shall be able to create maintain and protect from Page 38 modification or unauthorized access or destruction an audit trail of accesses to the objects it protects The audit data shall be protected by the TCB so that read access to it is limited to those who are authorized for audit data The TCB shall be able to record the following types of events use of identification and authentication mechanisms introduction of objects into a user's address space e g file open program initiation deletion of objects and actions taken by computer operators and system administrators and or system security officers and other security relevant events The TCB shall also be able to audit any override of human-readable output markings For each recorded event the audit record shall identify date and time of the event user type of event and success or failure of the event For identification authentication events the origin of request e g terminal ID shall be included in the audit record For events that introduce an object into a user's address space and for object deletion events the audit record shall include the name of the object and the object's security level The ADP system administrator shall be able to selectively audit the actions of any one or more users based on individual identity and or object security level The TCB shall be able to audit the identified events that may be used in the exploitation of covert storage channels The TCB shall contain a mechanism that is able to monitor the occurrence or accumulation of security auditable events that may indicate an imminent violation of security policy This mechanism shall be able to immediately notify the security administrator when thresholds are exceeded and if the occurrence or accumulation of these security relevant events continues the system shall take the least disruptive action to terminate the event 3 3 3 Assurance 3 3 3 1 Operational Assurance 3 3 3 1 1 System Architecture The TCB shall maintain a domain for its own execution that protects it from external interference or tampering e g by modification of its code or data structures The TCB shall maintain process isolation through the provision of distinct address spaces under its control The TCB shall be internally structured into well-defined largely independent modules It shall make effective use of available hardware to separate those elements that are protection-critical from those that are not The TCB modules shall be designed such that the principle of least privilege is enforced Features in hardware such as segmentation shall be used to support logically distinct storage objects with separate attributes namely readable writeable The user interface to the TCB shall be completely defined and all elements of the TCB identified The TCB shall be designed and structured Page 39 to use a complete conceptually simple protection mechanism with precisely defined semantics This mechanism shall play a central role in enforcing the internal structuring of the TCB and the system The TCB shall incorporate significant use of layering abstraction and data hiding Significant system engineering shall be directed toward minimizing the complexity of the TCB and excluding from the TCB modules that are not protection-critical 3 3 3 1 2 System Integrity Hardware and or software features shall be provided that can be used to periodically validate the correct operation of the on-site hardware and firmware elements of the TCB 3 3 3 1 3 Covert Channel Analysis The system developer shall conduct a thorough search for covert channels and make a determination either by actual measurement or by engineering estimation of the maximum bandwidth of each identified channel See the Covert Channels Guideline section 3 3 3 1 4 Trusted Facility Management The TCB shall support separate operator and administrator functions The functions performed in the role of a security administrator shall be identified The ADP system administrative personnel shall only be able to perform security administrator functions after taking a distinct auditable action to assume the security administrator role on the ADP system Non-security functions that can be performed in the security administration role shall be limited strictly to those essential to performing the security role effectively 3 3 3 1 5 Trusted Recovery Procedures and or mechanisms shall be provided to assure that after an ADP system failure or other discontinuity recovery without a protection compromise is obtained 3 3 3 2 Life-Cycle Assurance 3 3 3 2 1 Security Testing The security mechanisms of the ADP system shall be tested and found to work as claimed in the system documentation A team of individuals who thoroughly understand the specific implementation of the TCB shall subject its design documentation source code and object code to thorough analysis and testing Their objectives shall Page 40 be to uncover all design and implementation flaws that would permit a subject external to the TCB to read change or delete data normally denied under the mandatory or discretionary security policy enforced by the TCB as well as to assure that no subject without authorization to do so is able to cause the TCB to enter a state such that it is unable to respond to communications initiated by other users The TCB shall be found resistant to penetration All discovered flaws shall be corrected and the TCB retested to demonstrate that they have been eliminated and that new flaws have not been introduced Testing shall demonstrate that the TCB implementation is consistent with the descriptive top-level specification See the Security Testing Guidelines No design flaws and no more than a few correctable implementation flaws may be found during testing and there shall be reasonable confidence that few remain 3 3 3 2 2 Design Specification and Verification A formal model of the security policy supported by the TCB shall be maintained over the life cycle of the ADP system that is proven consistent with its axioms A descriptive top-level specification DTLS of the TCB shall be maintained that completely and accurately describes the TCB in terms of exceptions error messages and effects It shall be shown to be an accurate description of the TCB interface A convincing argument shall be given that the DTLS is consistent with the model 3 3 3 2 3 Configuration Management During development and maintenance of the TCB a configuration management system shall be in place that maintains control of changes to the descriptive top-level specification other design data implementation documentation source code the running version of the object code and test fixtures and documentation The configuration management system shall assure a consistent mapping among all documentation and code associated with the current version of the TCB Tools shall be provided for generation of a new version of the TCB from source code Also available shall be tools for comparing a newly generated version with the previous TCB version in order to ascertain that only the intended changes have been made in the code that will actually be used as the new version of the TCB 3 3 4 Documentation 3 3 4 1 Security Features User's Guide Page 41 A single summary chapter or manual in user documentation shall describe the protection mechanisms provided by the TCB guidelines on their use and how they interact with one another 3 3 4 2 Trusted Facility Manual A manual addressed to the ADP system administrator shall present cautions about functions and privileges that should be controlled when running a secure facility The procedures for examining and maintaining the audit files as well as the detailed audit record structure for each type of audit event shall be given The manual shall describe the operator and administrator functions related to security to include changing the security characteristics of a user It shall provide guidelines on the consistent and effective use of the protection features of the system how they interact how to securely generate a new TCB and facility procedures warnings and privileges that need to be controlled in order to operate the facility in a secure manner The TCB modules that contain the reference validation mechanism shall be identified The procedures for secure generation of a new TCB from source after modification of any modules in the TCB shall be described 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 3 3 4 3 Test Documentation The system developer shall provide to the evaluators a document that describes the test plan test procedures that show how the security mechanisms were tested and results of the security mechanisms' functional testing It shall include results of testing the effectiveness of the methods used to reduce covert channel bandwidths 3 3 4 4 Design Documentation Documentation shall be available that provides a description of the manufacturer's philosophy of protection and an explanation of how this philosophy is translated into the TCB The interfaces between the TCB modules shall be described A formal description of the security policy model enforced by the TCB shall be available and proven that it is sufficient to enforce the security policy The specific TCB protection mechanisms shall be identified and an explanation given to show that they satisfy the model The descriptive top-level specification DTLS shall be shown to be an accurate description of the TCB interface Documentation shall describe how the TCB implements the reference monitor concept and give an explanation why it is tamper resistant cannot be bypassed and is correctly implemented The TCB implementation i e in hardware firmware and software shall be informally shown to Page 42 be consistent with the DTLS The elements of the DTLS shall be shown using informal techniques to correspond to the elements of the TCB Documentation shall describe how the TCB is structured to facilitate testing and to enforce least privilege This documentation shall also present the results of the covert channel analysis and the tradeoffs involved in restricting the channels All auditable events that may be used in the exploitation of known covert storage channels shall be identified The bandwidths of known covert storage channels the use of which is not detectable by the auditing mechanisms shall be provided See the Covert Channel Guideline section Page 43 4 0 DIVISION A VERIFIED PROTECTION This division is characterized by the use of formal security verification methods to assure that the mandatory and discretionary security controls employed in the system can effectively protect classified or other sensitive information stored or processed by the system Extensive documentation is required to demonstrate that the TCB meets the security requirements in all aspects of design development and implementation Page 44 4 1 CLASS A1 VERIFIED DESIGN Systems in class A1 are functionally equivalent to those in class B3 in that no additional architectural features or policy requirements are added The distinguishing feature of systems in this class is the analysis derived from formal design specification and verification techniques and the resulting high degree of assurance that the TCB is correctly implemented This assurance is developmental in nature starting with a formal model of the security policy and a formal top-level specification FTLS of the design Independent of the particular specification language or verification system used there are five important criteria for class A1 design verification A formal model of the security policy must be clearly identified and documented including a mathematical proof that the model is consistent with its axioms and is sufficient to support the security policy An FTLS must be produced that includes abstract definitions of the functions the TCB performs and of the hardware and or firmware mechanisms that are used to support separate execution domains The FTLS of the TCB must be shown to be consistent with the model by formal techniques where possible i e where verification tools exist and informal ones otherwise The TCB implementation i e in hardware firmware and software must be informally shown to be consistent with the FTLS The elements of the FTLS must be shown using informal techniques to correspond to the elements of the TCB The FTLS must express the unified protection mechanism required to satisfy the security policy and it is the elements of this protection mechanism that are mapped to the elements of the TCB Formal analysis techniques must be used to identify and analyze covert channels Informal techniques may be used to identify covert timing channels The continued existence of identified covert channels in the system must be justified In keeping with the extensive design and development analysis of the TCB required of systems in class A1 more stringent configuration management is required and procedures are established for securely distributing the system to sites A system security administrator is supported The following are minimal requirements for systems assigned a class A1 rating 4 1 1 Security Policy 4 1 1 1 Discretionary Access Control The TCB shall define and control access between named users and named objects e g files and programs in the ADP system Page 45 The enforcement mechanism e g access control lists shall allow users to specify and control sharing of those objects and shall provide controls to limit propagation of access rights The discretionary access control mechanism shall either by explicit user action or by default provide that objects are protected from unauthorized access These access controls shall be capable of specifying for each named object a list of named individuals and a list of groups of named individuals with their respective modes of access to that object Furthermore for each such named object it shall be possible to specify a list of named individuals and a list of groups of named individuals for which no access to the object is to be given Access permission to an object by users not already possessing access permission shall only be assigned by authorized users 4 1 1 2 Object Reuse All authorizations to the information contained within a storage object shall be revoked prior to initial assignment allocation or reallocation to a subject from the TCB's pool of unused storage objects No information including encrypted representations of information produced by a prior subject's actions is to be available to any subject that obtains access to an object that has been released back to the system 4 1 1 3 Labels Sensitivity labels associated with each ADP system resource e g subject storage object ROM that is directly or indirectly accessible by subjects external to the TCB shall be maintained by the TCB These labels shall be used as the basis for mandatory access control decisions In order to import non-labeled data the TCB shall request and receive from an authorized user the security level of the data and all such actions shall be auditable by the TCB 4 1 1 3 1 Label Integrity Sensitivity labels shall accurately represent security levels of the specific subjects or objects with which they are associated When exported by the TCB sensitivity labels shall accurately and unambiguously represent the internal labels and shall be associated with the information being exported 4 1 1 3 2 Exportation of Labeled Information The TCB shall designate each communication channel and I O device as either single-level or multilevel Any change in this designation shall be done manually and shall be auditable by the TCB The TCB shall maintain and be able to audit any change in the security level or levels associated with a communication channel or Page 46 I O device 4 1 1 3 2 1 Exportation to Multilevel Devices When the TCB exports an object to a multilevel I O device the sensitivity label associated with that object shall also be exported and shall reside on the same physical medium as the exported information and shall be in the same form i e machine-readable or human-readable form When the TCB exports or imports an object over a multilevel communication channel the protocol used on that channel shall provide for the unambiguous pairing between the sensitivity labels and the associated information that is sent or received 4 1 1 3 2 2 Exportation to Single-Level Devices Single-level I O devices and single-level communication channels are not required to maintain the sensitivity labels of the information they process However the TCB shall include a mechanism by which the TCB and an authorized user reliably communicate to designate the single security level of information imported or exported via single-level communication channels or I O devices 4 1 1 3 2 3 Labeling Human-Readable Output The ADP system administrator shall be able to specify the printable label names associated with exported sensitivity labels The TCB shall mark the beginning and end of all human-readable paged hardcopy output e g line printer output with human-readable sensitivity labels that properly represent the sensitivity of the output The TCB shall 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 ______________________________ 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 Page 47 manner mark other forms of human-readable output e g maps graphics with human­ readable sensitivity labels that properly represent the sensitivity of the output Any override of these marking defaults shall be auditable by the TCB 4 1 1 3 3 Subject Sensitivity Labels The TCB shall immediately notify a terminal user of each change in the security level associated with that user during an interactive session A terminal user shall be able to query the TCB as desired for a display of the subject's complete sensitivity label 4 1 1 3 4 Device Labels The TCB shall support the assignment of minimum and maximum security levels to all attached physical devices These security levels shall be used by the TCB to enforce constraints imposed by the physical environments in which the devices are located 4 1 1 4 Mandatory Access Control The TCB shall enforce a mandatory access control policy over all resources i e subjects storage objects and I O devices that are directly or indirectly accessible by subjects external to the TCB These subjects and objects shall be assigned sensitivity labels that are a combination of hierarchical classification levels and non-hierarchical categories and the labels shall be used as the basis for mandatory access control decisions The TCB shall be able to support two or more such security levels See the Mandatory Access Control guidelines The following requirements shall hold for all accesses between all subjects external to the TCB and all objects directly or indirectly accessible by these subjects A subject can read an object only if the hierarchical classification in the subject's security level is greater than or equal to the hierarchical classification in the object's security level and the non-hierarchical categories in the subject's security level include all the non-hierarchical categories in the object's security level A subject can write an object only if the hierarchical classification in the subject's security level is less than or equal to the hierarchical classification in the object's security level and all the non-hierarchical categories in the subject's security level are included in the non- hierarchical categories in the object's security level Identification and authentication data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authoriza­ tion 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 Page 48 4 1 2 Accountability 4 1 2 1 Identification and Authentication The TCB shall require users to identify themselves to it before beginning to perform any other actions that the TCB is expected to mediate Furthermore the TCB shall maintain authentication data that includes information for verifying the identity of individual users e g passwords as well as information for determining the clearance and authorizations of individual users This data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authorizations of subjects external to the TCB that may be created to act on behalf of the individual user are dominated by the clearance and authorization of that user The TCB shall protect authentication data so that it cannot be accessed by any unauthorized user The TCB shall be able to enforce individual accountability by providing the capability to uniquely identify each individual ADP system user The TCB shall also provide the capability of associating this identity with all auditable actions taken by that individual 4 1 2 1 1 Trusted Path The TCB shall support a trusted communication path between itself and users for use when a positive TCB-to­ user connection is required e g login change subject security level Communications via this trusted path shall be activated exclusively by a user or the TCB and shall be logically isolated and unmistakably distinguishable from other paths 4 1 2 2 Audit The TCB shall be able to create maintain and protect from modification or unauthorized access or destruction an audit trail of accesses to the objects it protects The audit data shall be protected by the TCB so that read access to it is limited to those who are authorized for audit data The TCB shall be able to record the following types of events use of identification and authentication mechanisms introduction of objects into a user's address space e g file open program initiation deletion of objects and actions taken by computer operators and system administrators and or system security officers and other security relevant events The TCB shall also be able to audit any override of human-readable output markings For each recorded event the audit record shall identify date and time of the event user type of event and success or failure of the event For identification authentication events the origin of request e g terminal ID shall be included in the audit record For events that introduce an object into a user's address space and for object deletion events the audit record shall include the name of the Page 49 object and the object's security level The ADP system administrator shall be able to selectively audit the actions of any one or more users based on individual identity and or object security level The TCB shall be able to audit the identified events that may be used in the exploitation of covert storage channels The TCB shall contain a mechanism that is able to monitor the occurrence or accumulation of security auditable events that may indicate an imminent violation of security policy This mechanism shall be able to immediately notify the security administrator when thresholds are exceeded and if the occurrence or accumulation of these security relevant events continues the system shall take the least disruptive action to terminate the event 4 1 3 Assurance 4 1 3 1 Operational Assurance 4 1 3 1 1 System Architecture The TCB shall maintain a domain for its own execution that protects it from external interference or tampering e g by modification of its code or data structures The TCB shall maintain process isolation through the provision of distinct address spaces under its control The TCB shall be internally structured into well-defined largely independent modules It shall make effective use of available hardware to separate those elements that are protection-critical from those that are not The TCB modules shall be designed such that the principle of least privilege is enforced Features in hardware such as segmentation shall be used to support logically distinct storage objects with separate attributes namely readable writeable The user interface to the TCB shall be completely defined and all elements of the TCB identified The TCB shall be designed and structured to use a complete conceptually simple protection mechanism with precisely defined semantics This mechanism shall play a central role in enforcing the internal structuring of the TCB and the system The TCB shall incorporate significant use of layering abstraction and data hiding Significant system engineering shall be directed toward minimizing the complexity of the TCB and excluding from the TCB modules that are not protection-critical 4 1 3 1 2 System Integrity Hardware and or software features shall be provided that can be used to periodically validate the correct operation of the on-site hardware and firmware elements of the TCB 4 1 3 1 3 Covert Channel Analysis Page 50 The system developer shall conduct a thorough search for covert channels and make a determination either by actual measurement or by engineering estimation of the maximum bandwidth of each identified channel See the Covert Channels Guideline section Formal methods shall be used in the analysis 4 1 3 1 4 Trusted Facility Management The TCB shall support separate operator and administrator functions The functions performed in the role of a security administrator shall be identified The ADP system administrative personnel shall only be able to perform security administrator functions after taking a distinct auditable action to assume the security administrator role on the ADP system Non-security functions that can be performed in the security administration role shall be limited strictly to those essential to performing the security role effectively 4 1 3 1 5 Trusted Recovery Procedures and or mechanisms shall be provided to assure that after an ADP system failure or other discontinuity recovery without a protection compromise is obtained 4 1 3 2 Life-Cycle Assurance 4 1 3 2 1 Security Testing The security mechanisms of the ADP system shall be tested and found to work as claimed in the system documentation A team of individuals who thoroughly understand the specific implementation of the TCB shall subject its design documentation source code and object code to thorough analysis and testing Their objectives shall be to uncover all design and implementation flaws that would permit a subject external to the TCB to read change or delete data normally denied under the mandatory or discretionary security policy enforced by the TCB as well as to assure that no subject without authorization to do so is able to cause the TCB to enter a state such that it is unable to respond to communications initiated by other users The TCB shall be found resistant to penetration All discovered flaws shall be corrected and the TCB retested to demonstrate that they have been eliminated and that new flaws have not been introduced Testing shall demonstrate that the TCB implementation is consistent with the formal top­ level specification See the Security Testing Guidelines No design flaws and no more than a few correctable implementation flaws may be found during testing and there shall be reasonable confidence that few Page 51 remain Manual or other mapping of the FTLS to the source code may form a basis for penetration testing 4 1 3 2 2 Design Specification and Verification A formal model of the security policy supported by the TCB shall be maintained over the life-cycle of the ADP system that is proven consistent with its axioms A descriptive top-level specification DTLS of the TCB shall be maintained that completely and accurately describes the TCB in terms of exceptions error messages and effects A formal top-level specification FTLS of the TCB shall be maintained that accurately describes the TCB in terms of exceptions error messages and effects The DTLS and FTLS shall include those components of the TCB that are implemented as hardware and or firmware if their properties are visible at the TCB interface The FTLS shall be shown to be an accurate description of the TCB interface A convincing argument shall be given that the DTLS is consistent with the model and a combination of formal and informal techniques shall be used to show that the FTLS is consistent with the model This verification evidence shall be consistent with that provided within the state-of-the-art of the particular computer security center-endorsed formal specification and verification system used Manual or other mapping of the FTLS to the TCB source code shall be performed to provide evidence of correct implementation 4 1 3 2 3 Configuration Management During the entire life-cycle i e during the design development and maintenance of the TCB a configuration management system shall be in place for all security­ relevant hardware firmware and software that maintains control of changes to the formal model the descriptive and formal top-level specifications other design data implementation documentation source code the running version of the object code and test fixtures and documentation The configuration management system shall assure a consistent mapping among all documentation and code associated with the current version of the TCB Tools shall be provided for generation of a new version of the TCB from source code Also available shall be tools maintained under strict configuration control for comparing a newly generated version with the previous TCB version in order to ascertain that only the intended changes have been made in the code that will actually be used as the new version of the TCB A combination of technical physical and procedural safeguards shall be used to protect from unauthorized modification or destruction the master copy or copies of all material used to generate the TCB Page 52 4 1 3 2 4 Trusted Distribution A trusted ADP system control and distribution facility shall be provided for maintaining the integrity of the mapping between the master data describing the current version of the TCB and the on-site master copy of the code for the current version Procedures e g site security acceptance testing shall exist for assuring that the TCb software firmware and hardware updates distributed to a customer are exactly as specified by the master copies 4 1 4 Documentation 4 1 4 1 Security Features User's Guide A single summary chapter or manual in user documentation shall describe the protection mechanisms provided by the TCB guidelines on their use and how they interact with one another 4 1 4 2 Trusted Facility Manual A manual addressed to the ADP system administrator shall present cautions about functions and privileges that should be controlled when running a secure facility The procedures for examining and maintaining the audit files as well as the detailed audit record structure for each type of audit event shall be given The manual shall describe the operator and administrator functions related to security to include changing the security characteristics of a user It shall provide guidelines on the consistent and effective use of the protection features of the system how they interact how to securely generate a new TCB and facility procedures warnings and privileges that need to be controlled in order to operate the facility in a secure manner The TCB modules that contain the reference validation mechanism shall be identified The procedures for secure generation of a new TCB from source after modification of any modules in the TCB shall be described 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 4 1 4 3 Test Documentation The system developer shall provide to the evaluators a document that describes the test plan test procedures that show how the security mechanisms were tested and results of the security mechanisms' functional testing It shall include results of testing the effectiveness of the methods used to reduce covert channel bandwidths The results of the mapping between the formal top-level specification and the TCB source code shall be given Page 53 4 1 4 4 Design Documentation Documentation shall be available that provides a description of the manufacturer's philosophy of protection and an explanation of how this philosophy is translated into the TCB The interfaces between the TCB modules shall be described A formal description of the security policy model enforced by the TCB shall be available and proven that it is sufficient to enforce the security policy The specific TCB protection mechanisms shall be identified and an explanation given to show that they satisfy the model The descriptive top-level speci­ fication 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 explana­ tion why it is tamper resistant cannot be bypassed and is correctly implemented The TCB implementation i e in hardware firmware and software shall be informally shown to be consistent with the formal top-level specification FTLS The elements of the FTLS shall be shown using informal techniques to correspond to the elements of the TCB Documentation shall describe how the TCB is structured to facilitate testing and to enforce least privilege This documentation shall also present the results of the covert channel analysis and the tradeoffs involved in restricting the channels All auditable events that may be used in the exploitation of known covert storage channels shall be identified The bandwidths of known covert storage channels the use of which is not detectable by the auditing mechanisms shall be provided See the Covert Channel Guideline section Hardware firmware and software mechanisms not dealt with in the FTLS but strictly internal to the TCB e g mapping registers direct memory access I O shall be clearly described Page 54 4 2 BEYOND CLASS A1 Most of the security enhancements envisioned for systems that will provide features and assurance in addition to that already provided by class Al systems are beyond current technology The discussion below is intended to guide future work and is derived from research and development activities already underway in both the public and private sectors As more and better analysis techniques are developed the requirements for these systems will become more explicit In the future use of formal verification will be extended to the source level and covert timing channels will be more fully addressed At this level the design environment will become important and testing will be aided by analysis of the formal top-level specification Consideration will be given to the correctness of the tools used in TCB development e g compilers assemblers loaders and to the correct functioning of the hardware firmware on which the TCB will run Areas to be addressed by systems beyond class A1 include System Architecture A demonstration formal or otherwise must be given showing that requirements of self-protection and completeness for reference monitors have been implemented in the TCB Security Testing Although beyond the current state-of-the-art it is envisioned that some test-case generation will be done automatically from the formal top-level specification or formal lower-level specifications Formal Specification and Verification The TCB must be verified down to the source code level using formal verification methods where feasible Formal verification of the source code of the security-relevant portions of an operating system has proven to be a difficult task Two important considerations are the choice of a high-level language whose semantics can be fully and formally expressed and a careful mapping through successive stages of the abstract formal design to a formalization of the implementation in low-level specifications Experience has shown that only when the lowest level specifications closely correspond to the actual code can code proofs be successfully accomplished Trusted Design Environment The TCB must be designed in a trusted facility with only trusted cleared personnel Page 55 PART II RATIONALE AND GUIDELINES Page 56 5 0 CONTROL OBJECTIVES FOR TRUSTED COMPUTER SYSTEMS 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 Security Policy Accountability Assurance This section provides a discussion of these general control objectives and their implication in terms of designing trusted systems Page 57 5 1 A NEED FOR CONSENSUS 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 5 2 DEFINITION AND USEFULNESS 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 reference validation mechanism must be tamperproof The reference validation mechanism must always be invoked The reference validation mechanism must be small enough to be subjected to analysis and tests the completeness of which can be assured 1 Page 58 5 3 CRITERIA CONTROL OBJECTIVES 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 5 3 1 Security Policy 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 16 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 8 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 5 3 1 1 Mandatory Security Policy 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 Page 59 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 5 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 5 3 1 2 Discretionary Security Policy 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 Page 60 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 5 3 1 3 Marking 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 5 3 2 Accountability Page 61 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 5 3 3 Assurance 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 17 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 Page 62 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 Page 63 6 0 RATIONALE BEHIND THE EVALUATION CLASSES Page 64 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 a The reference validation mechanism must be tamper proof b The reference validation mechanism must always be invoked c The reference validation mechanism must be small enough to be subject to analysis and tests the completeness of which can be assured 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 Page 65 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 Page 66 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 Page 67 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 are viewed to offer significantly better protection and assurance than would systems that satisfy the basic requirements for their evaluation class and there is reason to believe that systems in the intermediate evaluation classes could eventually be evolved such that they would satisfy the requirements for the next higher evaluation class 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 Page 68 7 0 THE RELATIONSHIP BETWEEN POLICY AND THE CRITERIA Section 1 presents fundamental computer security requirements and Section 5 presents the control objectives for Trusted Computer Systems They are general requirements useful and necessary for the development of all secure systems However when designing systems that will be used to process classified or other sensitive information functional requirements for meeting the Control Objectives become more specific There is a large body of policy laid down in the form of Regulations Directives Presidential Executive Orders and OMB Circulars that form the basis of the procedures for the handling and processing of Federal information in general and classified information specifically This section presents pertinent excerpts from these policy statements and discusses their relationship to the Control Objectives These excerpts are examples to illustrate the relationship of the policies to criteria and may not be complete Page 69 7 1 ESTABLISHED FEDERAL POLICIES A significant number of computer security policies and associated requirements have been promulgated by Federal government elements The interested reader is referred to reference 32 which analyzes the need for trusted systems in the civilian agencies of the Federal government as well as in state and local governments and in the private sector This reference also details a number of relevant Federal statutes policies and requirements not treated further below 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 26 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 27 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 4 While not stipulating computer controls specifically the definition of Internal Controls in A-123 makes it clear that computer systems are to be included Internal Controls - The plan of organization and all of the methods and measures adopted within an agency to safeguard its resources assure the accuracy and reliability of its information assure adherence to applicable laws regulations and policies and promote operational economy and efficiency 27 sec 4 C The matter of classified national security information processed by ADP systems was one of the first areas given serious and extensive concern in computer security The computer security policy documents promulgated as a result contain generally more specific and structured requirements than most keyed in turn to an authoritative basis that itself provides a rather clearly articulated and structured information security policy This basis Executive Order 12356 National Security Information sets forth requirements for the classification declassification and safeguarding of national security information per se 14 7 2 DOD POLICIES Within the Department of Defense these broad requirements are implemented and further specified primarily through two vehicles 1 DoD Regulation 5200 1-R Page 70 7 which applies to all components of the DoD as such and 2 DoD 5220 22-M Industrial Security Manual for Safeguarding Classified Information 11 which applies to contractors included within the Defense Industrial Security Program Note that the latter transcends DoD as such since it applies not only to any contractors handling classified information for any DoD component but also to the contractors of eighteen other Federal organizations for whom the Secretary of Defense is authorized to act in rendering industrial security services For ADP systems these information security requirements are further amplified and specified in 1 DoD Directive 5200 28 8 and DoD Manual 5200 28-M 9 for DoD components and 2 Section XIII of DoD 5220 22-M 11 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 a Deliberate or inadvertent access to classified material by unauthorized persons and b Unauthorized manipulation of the computer and its associated peripheral devices 8 sec I B 3 Requirements equivalent to these appear within DoD 5200 28-M 9 and in DoD 5220 22-M 11 DoD Directove 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 ______________________________ 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 Page 71 7 3 1 7 3 Marking CRITERIA CONTROL OBJECTIVE FOR SECURITY POLICY The control objective for marking is 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 corresonding internal sensitivity labels being exported DoD 5220 22-M Industrial Security Manual for Safeguarding Classified Information explains in paragraph 11 the reasons for marking information a General Classification designation by physical marking notation or other means serves to warn and to inform the holder what degree of protection against unauthorized disclosure is reqired for that information or material 14 Marking requirements are given in a number of policy statements Executive Order 12356 Sections 1 5 a and 1 5 a 1 requires that classification markings shall be shown on the face of all classified documents or clearly associated with other forms of classified information in a manner appropriate to the medium involved 14 DoD Regulation 5200 1-R Section 1-500 requires that information or material that requires protection against unauthorized disclosure in the interest of national security shall be classified in one of three designations namely 'Top Secret ' 'Secret' or 'Confidential ' 7 By extension for use in computer processing the unofficial designation Unclassified is used to indicate information that does not fall under one of the other three designations of classified information DoD Regulation 5200 1-R Section 4-304b requires that ADP systems and word processing systems employing such media shall provide for internal classification marking to assure that classified information contained therein that is reproduced or generated will bear applicable classification and associated markings This regulation provides for the exemption of certain existing systems where internal classification and applicable associated markings cannot be implemented without extensive system modifications 7 However it is clear that future DoD ADP systems must be able to provide applicable and accurate labels for classified and other sensitive information DoD Manual 5200 28-M Section IV 4-305d requires the following Security Labels - All classified material accessible by or within Page 72 the ADP system shall be identified as to its security classification and access or dissemination limitations and all output of the ADP system shall be appropriately marked 9 7 3 2 Mandatory Security The control objective for mandatory security is 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 There are a number of policy statements that are related to mandatory security Executive Order 12356 Section 4 1 a states that a person is eligible for access to classified information provided that a determination of trustworthiness has been made by agency heads or designated officials and provided that such access is essential to the accomplishment of lawful and authorized Government purposes 14 DoD Regulation 5200 1-R Chapter I Section 3 defines a Special Access Program as any program imposing 'need-to-know' or access controls beyond those normally provided for access to Confidential Secret or Top Secret information Such a program includes but is not limited to special clearance adjudication or investigative requirements special designation of officials authorized to determine 'need-to-know' or special lists of persons determined to have a 'need-to- know ' 7 para 1-328 This passage distinguishes between a 'discretionary' determination of need-to-know and formal need-to-know which is implemented through Special Access Programs DoD Regulation 5200 1-R paragraph 7-100 describes general requirements for trustworthiness clearance and need-to-know and states that the individual with possession knowledge or control of classified information has final responsibility for determining if conditions for access have been met This regulation further stipulates that no one has a right to have access to classified information solely by virtue of rank or position 7 para 7-100 DoD Manual 5200 28-M Section II 2-100 states that Personnel who develop test debug maintain or use programs which are classified or which will be used to access or develop classified material shall have a personnel security clearance and an access authorization need-to-know as appropriate for the highest classified and most restrictive category of classified material Page 73 which they will access under system constraints 9 DoD Manual 5220 22-M Paragraph 3 a defines access as the ability and opportunity to obtain knowledge of classified information An individual in fact may have access to classified information by being in a place where such information is kept if the security measures which are in force do not prevent him from gaining knowledge of the classified information 11 The above mentioned Executive Order Manual Directives and Regulations clearly imply that a trusted computer system must assure that the classification labels associated with sensitive data cannot be arbitrarily changed since this could permit individuals who lack the appropriate clearance to access classified information Also implied is the requirement that a trusted computer system must control the flow of information so that data from a higher classification cannot be placed in a storage object of lower classification unless its downgrading has been authorized 7 3 3 Discretionary Security The term discretionary security refers to a computer system's ability to control information on an individual basis It stems from the fact that even though an individual has all the formal clearances for access to specific classified information each individual's access to information must be based on a demonstrated need-to-know Because of this it must be made clear that this requirement is not discretionary in a take it or leave it sense The directives and regulations are explicit in stating that the need-to-know test must be satisfied before access can be granted to the classified information The control objective for discretionary security is 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 DoD Regulation 5200 1-R Paragraph 7-100 In addition to excerpts already provided that touch on need-to- know this section of the regulation stresses the need- to-know principle when it states no person may have access to classified information unless access is necessary for the performance of official duties 7 Also DoD Manual 5220 22-M Section III 20 a states that an individual shall be permitted to have access to classified information only when the contractor determines that access is necessary in the performance of tasks or services essential to the fulfillment of a contract or program i e the individual has a need-to-know 11 Page 74 7 4 CRITERIA CONTROL OBJECTIVE FOR ACCOUNTABILITY The control objective for accountability is 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 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 8 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 9 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 9 DoD Manual 5200 28-M Section I 1-102b states Component's Designated Approving Authorities or their designees for this purpose will assure 4 Maintenance of documentation on operating systems O S and all modifications thereto and its retention for a sufficient period of time to enable tracing of security­ related defects to their point of origin or inclusion in the system 6 Establishment of procedures to discover recover Page 75 handle and dispose of classified material improperly disclosed through system malfunction or personnel action 7 Proper disposition and correction of security deficiencies in all approved ADP Systems and the effective use and disposition of system housekeeping or audit records records of security violations or security-related system malfunctions and records of tests of the security features of an ADP System 9 DoD Manual 5220 22-M Section XIII 111 states Audit Trails a The general security requirement for any ADP system audit trail is that it provide a documented history of the use of the system An approved audit trail will permit review of classified system activity and will provide a detailed activity record to facilitate reconstruction of events to determine the magnitude of compromise if any should a security malfunction occur To fulfill this basic requirement audit trail systems manual automated or a combination of both must document significant events occurring in the following areas of concern i preparation of input data and dissemination of output data i e reportable interactivity between users and system support personnel ii activity involved within an ADP environment e g ADP support personnel modification of security and related controls and iii internal machine activity b The audit trail for an ADP system approved to process classified information must be based on the above three areas and may be stylized to the particular system All systems approved for classified processing should contain most if not all of the audit trail records listed below The contractor's SPP documentation must identify and describe those applicable 1 Personnel access 2 Unauthorized and surreptitious entry into the central computer facility or remote terminal areas 3 Start stop time of classified processing indicating pertinent systems security initiation and termination events e g upgrading downgrading actions pursuant to paragraph 107 4 All functions initiated by ADP system console operators 5 Disconnects of remote terminals and peripheral devices paragraph 107c 6 Log-on and log-off user activity Page 76 7 Unauthorized attempts to access files or programs as well as all open close create and file destroy actions 8 Program aborts and anomalies including identification information i e user program name time and location of incident etc 9 System hardware additions deletions and maintenance actions 10 Generations and modifications affecting the security features of the system software c The ADP system security supervisor or designee shall review the audit trail logs at least weekly to assure that all pertinent activity is properly recorded and that appropriate action has been taken to correct any anomaly The majority of ADP systems in use today can develop audit trail systems in accord with the above however special systems such as weapons communications communications security and tactical data exchange and display systems may not be able to comply with all aspects of the above and may require individualized consideration by the cognizant security office d Audit trail records shall be retained for a period of one inspection cycle 11 7 5 CRITERIA CONTROL OBJECTIVE FOR ASSURANCE The control objective for assurance is 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 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 8 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 Page 77 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 8 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 8 DoD Manual 5200 28-M Section I 1-102b states Component's Designated Approving Authorities or their designees for this purpose will assure 5 Supervision monitoring and testing as appropriate of changes in an approved ADP System which could affect the security features of the system so that a secure system is maintained 7 Proper disposition and correction of security deficiencies in all approved ADP Systems and the effective use and disposition of system housekeeping or audit records records of security violations or security-related system malfunctions and records of tests of the security features of an ADP System 8 Conduct of competent system ST E timely review of system ST E reports and correction of deficiencies needed to support conditional or final approval or disapproval of an ADP System for the processing of classified information 9 Establishment where appropriate of a central ST E coordination point for the maintenance of records of selected techniques procedures standards and tests used in the testing and evaluation of security features of ADP Systems which may be suitable for validation and use by other Department of Defense Components 9 DoD Manual 5220 22-M Section XIII 103a requires the initial approval in writing of the cognizant security office prior to processing any classified information in an ADP system This section requires reapproval by the cognizant security office for major system modifications made subsequent to initial approval Reapprovals will be required because of i major changes in personnel access requirements ii relocation or structural modification of the central computer facility iii additions deletions or changes to main frame storage or Page 78 input output devices iv system software changes impacting security protection features v any change in clearance declassification audit trail or hardware software maintenance procedures and vi other system changes as determined by the cognizant security office 11 A major component of assurance life-cycle assurance as described in DoD Directive 7920 l 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 10 Page 79 8 0 A GUIDELINE ON COVERT CHANNELS A covert channel is any communication channel that can be exploited by a process to transfer information in a manner that violates the system's security policy There are two types of covert channels storage channels and timing channels Covert storage channels include all vehicles that would allow the direct or indirect writing of a storage location by one process and the direct or indirect reading of it by another Covert timing channels include all vehicles that would allow one process to signal information to another process by modulating its own use of system resources in such a way that the change in response time observed by the second process would provide information 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 5 6 19 21 22 23 and 29 Page 80 9 0 A GUIDELINE ON CONFIGURING MANDATORY ACCESS CONTROL FEATURES The Mandatory Access Control requirement includes a capability to support an unspecified number of hierarchical classifications and an unspecified number of non-hierarchical categories at each hierarchical level To encourage consistency and portability in the design and development of the National Security Establishment trusted computer systems it is desirable for all such systems to be able to support a minimum number of levels and categories The following suggestions are provided for this purpose The number of hierarchical classifications should be greater than or equal to sixteen 16 The number of non-hierarchical categories should be greater than or equal to sixty-four 64 Page 81 10 0 A GUIDELINE ON SECURITY TESTING These guidelines are provided to give an indication of the extent and sophistication of testing undertaken by the DoD Computer Security Center during the Formal Product Evaluation process Organizations wishing to use Department of Defense Trusted Computer System Evaluation Criteria for performing their own evaluations may find this section useful for planning purposes As in Part I highlighting is used to indicate changes in the guidelines from the next lower division Page 82 10 1 10 1 1 TESTING FOR DIVISION C Personnel The security testing team shall consist of at least two individuals with bachelor degrees in Computer Science or the equivalent Team members shall be able to follow test plans prepared by the system developer and suggest additions shall be familiar with the flaw hypothesis or equivalent security testing methodology and shall have assembly level programming experience Before testing begins the team members shall have functional knowledge of and shall have completed the system developer's internals course for the system being evaluated 10 1 2 Testing The team shall have hands-on involvement in an independent run of the tests used by the system developer The team shall independently design and implement at least five system-specific tests in an attempt to circumvent the security mechanisms of the system The elapsed time devoted to testing shall be at least one month and need not exceed three months There shall be no fewer than twenty hands-on hours spent carrying out system developer-defined tests and test team-defined tests 10 2 10 2 1 TESTING FOR DIVISION B Personnel The security testing team shall consist of at least two individuals with bachelor degrees in Computer Science or the equivalent and at least one individual with a master's degree in Computer Science or equivalent Team members shall be able to follow test plans prepared by the system developer and suggest additions shall be conversant with the flaw hypothesis or equivalent security testing methodology shall be fluent in the TCB implementation language s and shall have assembly level programming experience Before testing begins the team members shall have functional knowledge of and shall have completed the system developer's internals course for the system being evaluated At least one team member shall have previously completed a security test on another system 10 2 2 Testing The team shall have hands-on involvement in an independent run of the test package used by the system developer to test security-relevant hardware and software The team shall independently design and implement at least fifteen system­ specific tests in an attempt to circumvent the security mechanisms of the system The elapsed time devoted to testing Page 83 shall be at least two months and need not exceed four months There shall be no fewer than thirty hands-on hours per team member spent carrying out system developer-defined tests and test team-defined tests 10 3 10 3 1 TESTING FOR DIVISION A Personnel The security testing team shall consist of at least one individual with a bachelor's degree in Computer Science or the equivalent and at least two individuals with masters' degrees in Computer Science or equivalent Team members shall be able to follow test plans prepared by the system developer and suggest additions shall be conversant with the flaw hypothesis or equivalent security testing methodology shall be fluent in the TCB implementation language s and shall have assembly level programming experience Before testing begins the team members shall have functional knowledge of and shall have completed the system developer's internals course for the system being evaluated At least one team member shall be familiar enough with the system hardware to understand the maintenance diagnostic programs and supporting hardware documentation At least two team members shall have previously completed a security test on another system At least one team member shall have demonstrated system level programming competence on the system under test to a level of complexity equivalent to adding a device driver to the system 10 3 2 Testing The team shall have hands-on involvement in an independent run of the test package used by the system developer to test security-relevant hardware and software The team shall independently design and implement at least twenty-five system­ specific tests in an attempt to circumvent the security mechanisms of the system The elapsed time devoted to testing shall be at least three months and need not exceed six months There shall be no fewer than fifty hands-on hours per team member spent carrying out system developer-defined tests and test team-defined tests Page 84 APPENDIX A COMMERCIAL PRODUCE EVALUATION PROCESS 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 18 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 Preliminary Product Evaluation - An informal dialogue between a vendor and the Center in which technical information is exchanged to create a common understanding of the vendor's product the criteria and the rating that may be expected to result from a formal product evaluation Formal Product Evaluation - A formal evaluation by the Center of a product that is available to the DoD and that results in that product and its assigned rating being placed on the Evaluated Products List Evaluated Products List - A list of products that have been subjected to formal product evaluation and their assigned ratings Preliminary Product Evaluation 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 Page 85 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 Page 86 APPENDIX B SUMMARY OF EVALUATION CRITERIA DIVISIONS 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 Page 87 APPENDIX C SUMMARY OF EVALUATION CRITERIA CLASSES 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 Page 88 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 Page 89 APPENDIX D REQUIREMENT DIRECTORY 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 Audit C1 NR 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 Page 90 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 A1 NAR Configuration Management C1 NR C2 NR B1 NR 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 B3 NAR Page 91 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 C1 NR C2 NR B1 NR B2 NEW The system developer shall conduct a storage channels and make a determination measurement or by engineering estimation each identified channel See the Covert section thorough search for covert either by actual of the maximum bandwidth of Channels Guideline 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 Design Documentation 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 C2 NAR 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 Page 92 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 C1 NR C2 NR 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 Page 93 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 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 Page 94 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 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 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 security level or levels associated with a 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 Page 95 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 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 B2 NAR Page 96 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 ______________________________ 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 Page 97 categories A1 NAR 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 ROM 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 Identification and Page 98 authentication data shall be used by the TCB to authenticate the user's identity and to ensure that the security level and authori­ zation 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 B3 NAR A1 NAR Object Reuse C1 NR 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 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 Page 99 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 Page 100 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 Page 101 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 test procedures that show how the security mechanisms were tested 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 Page 102 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 Page 103 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 Page 104 this page is reserved for Figure l PagelOS 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 Page 106 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 Page 107 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 Front-End Security Filter - A process that is invoked to process Page 108 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 Page 109 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 Page 110 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 Page 111 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 Page 112 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 Page 113 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 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