5
Criteria to Evaluate Computer and Network Security

Characterizing a computer system as being secure presupposes some criteria, explicit or implicit, against which the system in question is measured or evaluated. Documents such as the National Computer Security Center's (NCSC's) Trusted Computer System Evaluation Criteria (TCSEC, or Orange Book; U.S. DOD, 1985d) and its Trusted Network Interpretation (TNI, or Red Book; U.S. DOD, 1987), and the harmonized Information Technology Security Evaluation Criteria (ITSEC; Federal Republic of Germany, 1990) of France, Germany, the Netherlands, and the United Kingdom provide standards against which computer and network systems can be evaluated with respect to security characteristics. As described below in "Comparing National Criteria Sets," these documents embody different approaches to security evaluation, and the differences are a result of other, perhaps less obvious purposes that security evaluation criteria can serve.

This chapter describes the competing goals that influence the development of criteria and how current criteria reflect trade-offs among these goals. It discusses how U.S. criteria should be restructured to reflect the emergence of foreign evaluation criteria and the experience gained from the use of current NCSC criteria. While building on experience gained in the use of Orange Book criteria, the analysis contributes to the arguments for a new construct, Generally Accepted System Security Principles, or GSSP. As recommended by the committee, GSSP would provide a broader set of criteria and drive a more flexible and comprehensive process for evaluating single-vendor (and conglomerate) systems.



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Computers at Risk: Safe Computing in the Information Age 5 Criteria to Evaluate Computer and Network Security Characterizing a computer system as being secure presupposes some criteria, explicit or implicit, against which the system in question is measured or evaluated. Documents such as the National Computer Security Center's (NCSC's) Trusted Computer System Evaluation Criteria (TCSEC, or Orange Book; U.S. DOD, 1985d) and its Trusted Network Interpretation (TNI, or Red Book; U.S. DOD, 1987), and the harmonized Information Technology Security Evaluation Criteria (ITSEC; Federal Republic of Germany, 1990) of France, Germany, the Netherlands, and the United Kingdom provide standards against which computer and network systems can be evaluated with respect to security characteristics. As described below in "Comparing National Criteria Sets," these documents embody different approaches to security evaluation, and the differences are a result of other, perhaps less obvious purposes that security evaluation criteria can serve. This chapter describes the competing goals that influence the development of criteria and how current criteria reflect trade-offs among these goals. It discusses how U.S. criteria should be restructured to reflect the emergence of foreign evaluation criteria and the experience gained from the use of current NCSC criteria. While building on experience gained in the use of Orange Book criteria, the analysis contributes to the arguments for a new construct, Generally Accepted System Security Principles, or GSSP. As recommended by the committee, GSSP would provide a broader set of criteria and drive a more flexible and comprehensive process for evaluating single-vendor (and conglomerate) systems.

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Computers at Risk: Safe Computing in the Information Age SECURITY EVALUATION CRITERIA IN GENERAL At a minimum, security evaluation criteria provide a standard language for expressing security characteristics and establish an objective basis for evaluating a product relative to these characteristics. Thus one can critique such criteria based on how well security characteristics can be expressed and evaluated relative to the criteria. Security evaluation criteria also serve as frameworks for users (purchasers) and for vendors. Users employ criteria in the selection and acquisition of computer and network products, for example, by relying on independent evaluations to validate vendor claims for security and by using ratings as a basis for concisely expressing computer and network security requirements. Vendors rely on criteria for guidance in the development of products and use evaluations as a means of product differentiation. Thus it is also possible to critique security evaluation criteria based on their utility to users and vendors in support of these goals. These goals of security evaluation criteria are not thoroughly complementary. Each of the national criteria sets in use (or proposed) today reflects somewhat different goals and the trade-offs made by the criteria developers relative to these goals. A separate issue with regard to evaluating system security is how applicable criteria of the sort noted above are to complete systems, as opposed to individual computer or network products. This question is addressed below in "System Certification vs. Product Evaluation." Before discussing in more detail the goals for product criteria, it is useful to examine the nature of the security characteristics addressed in evaluation criteria. Security Characteristics Most evaluation criteria reflect two potentially independent aspects of security: functionality and assurance. Security functionality refers to the facilities by which security services are provided to users. These facilities may include, for example, various types of access control mechanisms that allow users to constrain access to data, or authentication mechanisms that verify a user's claimed identity. Usually it is easy to understand differences in security functionality, because they are manifested by mechanisms with which the user interacts (perhaps indirectly). Systems differ in the number, type, and combination of security mechanisms available. In contrast, security assurance often is not represented by any user-visible mechanisms and so can be difficult to evaluate. A product rating intended to describe security assurance expresses an evaluator's

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Computers at Risk: Safe Computing in the Information Age degree of confidence in the effectiveness of the implementation of security functionality. Personal perceptions of "degree of confidence" are relative, and so criteria for objectively assessing security assurance are based primarily on requirements for increasingly rigorous development practices, documentation, analysis, configuration management, and testing. Relative degrees of assurance also may be indicated by rankings based on the relative strength of the underlying mechanisms (e.g., cryptographic algorithms). Thus two products that appear to provide the same security functionality to a user may actually provide different levels of assurance because of the particulars (e.g., relative strength or quality) of the mechanisms used to implement the functionality or because of differences in the development methodology, documentation, or analysis accorded each implementation. Such differences in the underlying mechanisms of implementation should be recognized in an evaluation of security. Their significance can be illustrated by analogy: two painted picnic tables may appear to be identical outwardly, but one is constructed of pressure-treated lumber and the other of untreated lumber. Although the functionality of both with regard to table size and seating capacity is identical, the former table may be more durable than the latter because of the materials used to construct (implement) it. Another example illustrates more subtle determinants of assurance. A product might be evaluated as providing a high level of assurance because it was developed by individuals holding U.S. government top-secret clearances and working in a physically secure facility, and because it came with reams of documentation detailing the system design and attesting to the rigorous development practices used. But an identical product developed by uncleared individuals in a nonsecured environment and not accompanied by equivalent documentation, would probably receive a much lower assurance rating. Although the second product in this example is not necessarily less secure than the first, an evaluator probably would have less confidence in the security of the second product due to the lack of supporting evidence provided by its implementors, and perhaps, less confidence in the trustworthiness of the implementors themselves.1 Somewhat analogous is the contrast between buying a picnic table from a well-known manufacturer with a reputation for quality (a member of the "Picnic Table Manufacturers of America") versus purchasing a table from someone who builds picnic tables as an avocation. One may have confidence that the former manufacturer will use good materials and construction techniques (to protect his corporate image), whereas the latter may represent a greater risk (unless one knows the builder or has references from satisfied customers), irrespective of

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Computers at Risk: Safe Computing in the Information Age the actual quality of materials and workmanship. For computers and networks, the technology is sufficiently complex that users cannot, in general, personally evaluate the security assurance and therefore the quality of the product as they might the quality of a picnic table. Even evaluators cannot thoroughly examine every aspect of a computer system to the depth one would prefer, hence the reliance on evidence of good development practices, extensive documentation, and so on. Security assurance is evaluated in these indirect ways in part because testing, specification, and verification technology is not sufficiently mature to permit more direct rankings of assurance. In principle one could begin by specifying, using a formal specification language, the security policies that a target product should implement. Then one could use verification tools (programs) to establish the correspondence between this specification and a formal top-level specification (FTLS) for the product. This FTLS could, in turn, be shown to match the actual implementation of the product in a (high-level) programming language. The output of the compiler used to translate the high-level language into executable code would also have to be shown to correspond to the high-level language. This process could be continued to include firmware and hardware modules and logic design if one were to impose even more stringent assurance standards. As described in Chapter 4 of this report, state-of-the-art specification and verification technology does not allow for such a thorough, computer-driven process to demonstrate that a computer or network correctly supports a security policy. Experience has shown that there are numerous opportunities for human subversion of such a process unless it is carried through to the step that includes examination of the executable code (Thompson, 1984), and unless extreme measures, currently beyond the state of the art, are taken to ensure the correctness of the verification tools, compilers, and so on. Testing is a useful adjunct to the process, but the interfaces to the products of interest are sufficiently complex so as to preclude exhaustive testing to detect security flaws. Thus testing can contribute to an evaluator's confidence that security functionality is correctly implemented, but it cannot be the sole basis for providing a rating based on assurance as well. This explains, in large part, the reliance on indirect evidence of assurance (e.g., documentation requirements, trusted developers, and use of a secure development environment). Assurance Evaluation There are actually two stages of assurance evaluation: design evaluation and implementation evaluation. Design evaluation attempts to assure

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Computers at Risk: Safe Computing in the Information Age that a particular proposed system design actually provides the functionality it attempts rather than simply appearing to do so. Some early systems were constructed that associated passwords with files, rather than with users, as a form of access control. This approach gave the appearance of providing the required functionality but in fact failed to provide adequate accountability. This is an example of a design flaw that would likely be detected and remedied by a design evaluation process. Design evaluation is insurance against making a fundamental design error and embedding this error so deeply in a system that it cannot later be changed for any reasonable cost. To support the requirement of confidentiality, the possible mechanisms are well enough understood that design evaluation may not be needed to ensure a good design. But for newer areas of functionality, such as supporting the requirement for integrity or secure distributed systems, there is less experience with design options. This committee considers explicit design evaluation to be very important. There are many ways to obtain such review, and vendor prudence may be sufficient in some circumstances to ensure that this step is part of system design. However, in general, the committee endorses design evaluation by an independent team (involving personnel not employed by the vendor) as a standard part of secure system design and encourages that this step be undertaken whenever possible. Implementation evaluation is also important, but generally is more difficult, more time consuming, and more costly. For the level of assurance generally required in the commercial market, it may be sufficient to carry out a minimal implementation evaluation (as part of overall system quality assurance procedures, including initial operational or Beta testing) prior to system release if a good design evaluation is performed. Moreover, if the incident reporting and tracking system proposed in Chapters 1 and 6 is instituted, implementation flaws can be identified and fixed in the normal course of system releases. (Of course, well-known systems with well-known design flaws continue to be used, and continue to be penetrated. But for systems with modest security pretensions, many attacks exploit implementation flaws that could be corrected through diligent incident reporting and fixing of reported flaws.) By contrast the current implementation evaluation process as practiced by NCSC is very time consuming, and because it must occur after implementation, it slows the delivery of evaluated systems to the marketplace.2 For systems attempting to conform to a baseline set of GSSP as recommended by the committee (see Chapter 1, "Overview and Recommendations," and Chapter 2, "Concepts of Information Security"),

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Computers at Risk: Safe Computing in the Information Age the committee recommends that in the short term a process of evaluating installed systems (field evaluation), rather than the a priori implementation evaluation now carried out by NCSC, be used to increase the level of implementation quality. This process of field evaluation, while it shares the basic goal of the current NCSC process, differs from that process in several ways that the committee views as advantageous. First, because such field evaluation is less time consuming, it may be viewed as less onerous than the current method for implementation evaluation. It should also be less costly, which would increase its acceptability. One side effect is that the early customers of a system subject to field evaluation would not have the full benefit of evaluated security mechanisms, a situation that would prompt customers with relatively high concern for security to delay purchase. In exchange for this limitation for early customers, the system would reach the market promptly and then continue to improve as a result of field experience. This process would also accommodate new releases and revisions of a system more easily than the current NCSC procedure, the Rating Maintenance Phase (RAMP). New releases that revise the function of the system should receive an incremental design review. But revisions to fix bugs would naturally be covered by the normal process of field testing. Indeed, it would be hoped that revisions would follow naturally from the implementation evaluation. This field evaluation process, if explicitly organized, can focus market forces in an effective way and lead to the recognition of outside evaluation as a valuable part of system assurance. The committee is concerned that, outside of the DOD, where the NCSC process is mandated, there is little appreciation of the importance of evaluation as an explicit step. Instead, the tendency initially is to accept security claims at face value, which can result in a later loss of credibility for a set of requirements. For example, customers have confused a bad implementation for a bad specification, and rejected a specification when one system implemented it badly. Thus the committee has linked its recommendation for the establishment of a broad set of criteria, GSSP, with a recommendation to establish methods, guidelines, and facilities for evaluating products with respect to GSSP. The committee believes that the way to achieve a system evaluation process supported by vendors and users alike is to begin with a design evaluation, based on GSSP itself, and to follow up with an implementation evaluation, focusing on field experience and incident reporting and tracking. Incident reporting and tracking could have the added effect of documenting vendor attentiveness to security, educating customers, and even illuminating potential sources of legal liability. Over time,

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Computers at Risk: Safe Computing in the Information Age the following steps might be anticipated: If GSSP were instituted, prudent consumers would demand GSSP-conforming systems as a part of normal practice. GSSP would drive field evaluation. If vendors perceived field evaluation as helping them in the marketplace or reducing their liability, they would come to support the process, and perhaps even argue for a stronger implementation evaluation as a means to obtain a higher assurance rating for systems. Thus GSSP could combine with market forces to promote development of systems evaluated as having relatively high assurance (analogous to the higher levels of the current Orange Book), a level of assurance that today does not seem to be justified in the eyes of many vendors and consumers. For this chain of events to unfold, GSSP must be embraced by vendors and users. To stimulate the development of GSSP, the committee recommends basing the initial set of GSSP on the Orange Book (specifically, the committee recommends building from C2 and B1 criteria) and possibly making conformance to GSSP mandatory in some significant applications, such as medical equipment or other life-critical systems. Trade-offs in Grouping of Criteria In developing product criteria, one of the primary trade-offs involves the extent to which security characteristics are grouped together. As noted above, aspects of security can be divided into two broad types: functionality and assurance. Some criteria, for example, the Orange Book and the TNI, tend to ''bundle" together functionality and assurance characteristics to define a small set of system security ratings. Other criteria, for example, the proposed West German (ZSI) set, group characteristics of each type into evaluation classes but keep the two types independent, yielding a somewhat larger set of possible ratings. At the extreme, the originally proposed British (DTI) criteria (a new evaluation scheme for both government and commercial systems has since been developed (U.K. CESG/DTI, 1990)) are completely unbundled, defining security controls and security objectives and a language in which to formulate claims for how a system uses controls to achieve the objectives. Comparisons with the successor harmonized criteria, the ITSEC, which builds on both the ZSI and DTI schemes, are amplified in the section below titled "Comparing National Criteria Sets." One argument in favor of bundling criteria is that it makes life easier for evaluators, users, and vendors. When a product is submitted for evaluation, a claim is made that it implements a set of security functions with the requisite level of assurance for a given rating. The job of an evaluator is made easier if the security functions and assurance techniques against which a product is evaluated have been bundled

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Computers at Risk: Safe Computing in the Information Age into a small number of ratings (e.g., six, as in the Orange Book). Because evaluators are likely to see many systems that have been submitted for the same rating, they gain experience that can be applied to later evaluations, thus reducing the time required to perform an evaluation. When completely unbundled criteria are used (e.g., the proposed DTI set), the evaluators may have to examine anew the collection of security features claimed for each product, since there may not have been previously evaluated products with the same set of features. In this sense, evaluation associated with unbundled criteria would probably become more time consuming and more difficult (for a system with comparable functionality and assurance characteristics) than evaluation against bundled criteria. Bundled criteria define what their authors believe are appropriate combinations of security functions and assurance techniques that will yield useful products. This signaling of appropriate combinations is an especially important activity if users and vendors are not competent to define such combinations on their own. Bundled criteria play a very powerful role in shaping the marketplace for secure systems, because they tend to dictate what mechanisms and assurances most users will specify in requests for proposals and what vendors will build (in order to match the ratings). A small number of evaluation ratings helps channel user demands for security to systems that fall into one of a few rated slots. If user demands are not focused in this fashion, development and evaluation costs cannot be amortized over a large enough customer base. Vendors can then be faced with the prospect of building custom-designed secure systems products, which can be prohibitively expensive (and thus diminish demand). Bundled criteria enable a vendor to direct product development to a very small number of rating targets. A concern often cited for unbundled criteria is that it is possible in principle to specify groupings of security features that might, in toto, yield "nonsecure" systems. For example, a system that includes sophisticated access control features but omits all audit facilities might represent an inappropriate combination of features. If vendors and users of secure systems were to become significantly more sophisticated, the need to impose such guidance through bundled criteria would become less crucial. However, there will always be users and vendors who lack the necessary knowledge and skills to understand how trustworthy a system may be. The question is whether it is wise to rely on vendors to select "good" combinations of security features for systems and to rely on users to be knowledgeable in requesting appropriate groupings if unbundled criteria are adopted.

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Computers at Risk: Safe Computing in the Information Age While bundled criteria may protect the naive vendor, they may also limit the sophisticated vendor, because they do not reward the development of systems with security functionality or assurance outside of that prescribed by the ratings. For example, recent work on security models (Clark and Wilson, 1987) suggests that many security practices in the commercial sector are not well matched to the security models that underlie the Orange Book. A computer system designed expressly to support the Clark-Wilson model of security, and thus well suited to typical commercial security requirements, might not qualify under evaluation based on the Orange Book. A system that did qualify for an Orange Book rating and had added functions for integrity to support the Clark-Wilson model would receive no special recognition for the added functionality since that functionality, notably relating to integrity, is outside the scope of the Orange Book.3 The government-funded LOCK project (see Appendix B), for example, is one attempt to provide both security functionality and assurance beyond that called for by the highest rating (A1) of the Orange Book. But because this project's security characteristics exceed those specified in the ratings scale, LOCK (like other attempts to go beyond A1) cannot be "rewarded" for these capabilities within the rating scheme. It can be argued that if LOCK were not government funded it would not have been developed, since a vendor would have no means within the evaluation process of substantiating claims of superior security and users would have no means of specifying these capabilities (e.g., in requests for proposals) relative to the criteria (Orange Book). Bundled criteria make it difficult to modify the criteria to adapt to changing technology or modes of use. Changing computer technology imposes the requirement that security criteria must evolve. The advent of networking represents a key example of this need. For example, as this report is prepared, none of the computers rated by the NCSC includes network interface software in the evaluated product, despite the fact that many of these systems will be connected to networks. This may be indicative, in part, of the greater complexity associated with securing a computer attached to a network, but it also illustrates how criteria can become disconnected from developments in the workplace. For some of these computers, the inclusion of network interface software will not only formally void the evaluation but will also introduce unevaluated, security-critical software. This experience argues strongly that evaluation criteria must be able to accommodate technological evolution so that fielded products remain true to their evaluations. The discussion and examples given above demonstrate that constraints on the evolving marketplace can occur unless evaluation criteria can

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Computers at Risk: Safe Computing in the Information Age be extended to accommodate new paradigms in security functionality or assurance. Such problems could arise with unbundled criteria, but criteria like the Orange Book set seem especially vulnerable to paradigm shifts because their hierarchic, bundled nature makes them more difficult to extend. Based on these considerations, the committee concludes that in the future a somewhat less bundled set of security criteria will best serve the needs of the user and vendor communities. It is essential to provide for evolution of the criteria to address new functions and new assurance techniques. The committee also believes that naive users are not well served by bundled criteria, but rather are misled to believe that complex security problems can be solved by merely selecting an appropriately rated product. If naive users or vendors need protection from the possibility of selecting incompatible features from the criteria, this can be made available by providing guidelines, which can suggest collections of features that, while useful, are not mandatory, as bundled criteria would be. Comparing National Criteria Sets The Orange Book and its Trusted Network Interpretation, the Red Book, establish ratings that span four hierarchical divisions: D, C, B, and A, in ascending order. The "D" rating is given to products with negligible or no security; the "C," "B," and "A'' ratings reflect specific, increasing provision of security. Each division includes one or more classes, numbered from 1 (that is, stronger ratings correlate with higher numbers), that provide finer-granularity ratings. Thus an evaluated system is assigned a digraph, for example, C2 or A1, that places it in a class in a division. At present, the following classes exist, in ascending order: C1, C2, B1, B2, B3, and A1. A summary of criteria for each class, reproduced from the Orange Book's Appendix C, can be found in Appendix A of this report. There are significant, security functionality distinctions between division-C and division-B systems. In particular, the C division provides for discretionary access control, while the B division adds mandatory access control. A1 systems, the only class today within the A division, add assurance, drawing on formal design specification and verification, but no functionality, to B3 systems. Assurance requirements increase from one division to the next and from one class to the next within a division. The Orange Book describes B2 systems as relatively resistant, and B3 as highly resistant, to penetration. The robustness of these and higher systems comes from their added requirements for functionality and/or assurance, which in turn drive greater attention to security, beginning

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Computers at Risk: Safe Computing in the Information Age in the early stages of development. That is, more effort must be made to build security in, as opposed to adding it on, to achieve a B2 or higher rating. In these U.S. criteria, both the language for expressing security characteristics and the basis for evaluation are thus embodied in the requirements for each division and class. This represents a highly "bundled" approach to criteria in that each rating, for example, B2, is a combination of a set of security functions and security assurance attributes. The Information Technology Security Evaluation Criteria (ITSEC)—the harmonized criteria of France, Germany, the Netherlands, and the United Kingdom (Federal Republic of Germany, 1990)—represents an effort to establish a comprehensive set of security requirements for widespread international use. ITSEC is generally intended as a superset of TCSEC, with ITSEC ratings mappable onto the TCSEC evaluation classes (see below). Historically, ITSEC represents a remarkably easily attained evolutionary grafting together of evaluation classes of the German (light) Green Book (GISA, 1989) and the "claims language" of the British (dark) Green Books (U.K. DTI, 1989). ITSEC unbundles functional criteria (F1 to F10) and correctness criteria (E0 as the degenerate case, and E1 to E6), which are evaluated independently. The functional criteria F1 to F5 are of generally increasing merit and correspond roughly to the functionality of TCSEC evaluation classes C1, C2, B1, B2, and B3, respectively. The remaining functionality criteria address data and program integrity (F6), system availability (F7), data integrity in communication (F8), data confidentiality in communication (F9), and network security, including confidentiality and integrity (F10). F6 to F10 may in principle be evaluated orthogonally to each other and to the chosen base level, F1, F2, F3, F4, or F5. The correctness criteria are intended to provide increased assurance. To a first approximation, the correctness criteria cumulatively require testing (E1), configuration control and controlled distribution (E2), access to the detailed design and source code (E3), rigorous vulnerability analysis (E4), demonstrable correspondence between detailed design and source code (E5), and formal models, formal descriptions, and formal correspondences between them (E6). E2 through E6 correspond roughly to the assurance aspects of TCSEC evaluation classes C2, B1, B2, B3, and A1, respectively. ITSEC's unbundling has advantages and disadvantages. On the whole it is a meritorious concept, as long as assurance does not become a victim of commercial expediency, and if the plethora of rating combinations does not cause confusion. A particular concern with the ITSEC is that it does not mandate

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Computers at Risk: Safe Computing in the Information Age any particular modularity with respect to system architecture. In particular, it does not require that the security-relevant parts of the system be isolated into a trusted computing base, or TCB. It is of course possible to evaluate an entire system according to ITSEC without reference to its composability (e.g., as an application on top of a TCB), but this complicates the evaluation and fails to take advantage of other related product evaluations. The effectiveness of this approach remains to be seen. The initial ITSEC draft was published and circulated for comment in 1990. Hundreds of comments were submitted by individuals and organizations from several countries, including the United States, and a special meeting of interested parties was held in Brussels in September 1990. In view of the volume and range of comments submitted, plus the introduction of a different proposal by EUROBIT, a European computer manufacturers' trade association, a revised draft is not expected before mid-1991. The dynamic situation calls for vigilance and participation, to the extent possible, by U.S. interests. At present, the National Institute of Standards and Technology (NIST) is coordinating U.S. inputs, although corporations and individuals are also contributing directly. It is likely that the complete process of establishing harmonized criteria, associated evaluation mechanisms, and related standards will take some time and will, after establishment, continue to evolve. Because the European initiatives are based in part on a reaction to the narrowness of the TCSEC, and because NIST's resources are severely constrained, the committee recommends that GSSP and a new organization to spearhead GSSP, the Information Security Foundation, provide a focus for future U.S. participation in international criteria and evaluation initiatives. Reciprocity Among Criteria Sets A question naturally arises with regard to comparability and reciprocity of the ratings of different systems. Even though ratings under one criteria set may be mappable to roughly comparable ratings under a different criteria set, the mapping is likely to be imprecise and not symmetric; for example, the mappings may be many-to-one. Even if there is a reasonable mapping between some ratings in different criteria, one country may refuse to recognize the results of an evaluation performed by an organization in another country, for political, as well as technical, reasons. The subjective nature of the ratings process makes it difficult, if not impossible, to ensure consistency among evaluations performed at different facilities, by different evaluators, in different countries, especially when one adds the differences in the

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Computers at Risk: Safe Computing in the Information Age criteria themselves. In such circumstances it is not hard to imagine how security evaluation criteria can become the basis for erecting barriers to international trade in computer systems, much as some have argued that international standards have become (Frenkel, 1990). Reciprocity has been a thorny problem in the comparatively simpler area of rating conformance to interoperability standards, where testing and certification are increasingly in demand, and there is every indication it will be a major problem for secure systems. Multinational vendors of computer systems do not wish to incur the costs and delay to market associated with multiple evaluations under different national criteria sets. Equally important, they may not be willing to reveal to foreign evaluators details of their system design and their development process, which they may view as highly proprietary. The major U.S. computer system vendors derive a significant fraction of their revenue from foreign sales and thus are especially vulnerable to proliferating, foreign evaluation criteria. At the same time, the NCSC has interpreted its charter as not encompassing evaluation of systems submitted by foreign vendors. This has stimulated the development of foreign criteria and thus has contributed to the potential conflicts among criteria on an international scale. Analyses indicate that one can map any of the Orange Book ratings onto an ITSEC rating. A reverse mapping (from ITSEC to Orange Book ratings) is also possible, although some combinations of assurance and functionality are not well represented, and thus the evaluated product may be "underrated." However, the ITSEC claims language may tend to complicate comparisons of ITSEC ratings with one another. Products evaluated under the Orange Book could be granted ITSEC ratings and ratings under other criteria that are relatively unbundled. This should be good news for U.S. vendors, if rating reciprocity agreements are enacted between the United States and foreign governments. Of course, a U.S. vendor could not use reciprocity to achieve the full range of ratings available to vendors who undergo ITSEC evaluation directly. Even when there are correspondences between ratings under different criteria, there is the question of confidence in the evaluation process as carried out in different countries.4 Discussions with NCSC and NSA staff suggest that reciprocity may be feasible at lower levels of the Orange Book, perhaps B1 and below, but not at the higher levels (committee briefings; personal communications). In part this sort of limitation reflects the subjective nature of the evaluation process. It may also indicate a reluctance to rely on "outside" evaluation for systems that would be used to separate multiple levels of DOD classified data. If other countries were to take a similar approach for

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Computers at Risk: Safe Computing in the Information Age high assurance levels under their criteria, then reciprocity agreements would be of limited value over time (as more systems attain higher ratings). Another likely consequence would be a divergence between criteria and evaluations for systems intended for use in defense applications and those intended for use in commercial applications. SYSTEM CERTIFICATION VS. PRODUCT EVALUATION The discussion above has addressed security evaluation criteria that focus on computer and network products. These criteria do not address all of the security concerns that arise when one actually deploys a system, whether it consists of a single computer or is composed of multiple computer and network products from different vendors. Procedural and physical safeguards, and others for personnel and emanations, enter into overall system security, and these are not addressed by product criteria. Overall system security is addressed by performing a thorough analysis of the system in question, taking into account not only the ratings of products that might be used to construct the system, but also the threats directed against the system and the concerns addressed by the other safeguards noted above, and producing a security architecture that address all of these security concerns. The simple ratings scheme embodied in the Orange Book and the TNI have led many users to think in terms of product ratings for entire systems. Thus it is not uncommon to hear a user state that his system, which consists of numerous computers linked by various networks, all from different vendors, needs to be, for example, B1. This statement arises from a naive attempt to apply the environment guidelines developed for the Orange Book to entire systems of much greater complexity and diversity. It leads to discussions of whether a network connecting several computers with the same rating is itself rated at or below the level of the connected computers. Such discussions, by adopting designations developed for product evaluation, tend to obscure the complexity of characterizing the security requirements for real systems and the difficulty of designing system security solutions. In fact, the term "evaluation" is often reserved for products, not deployed systems. Instead, at least in the DOD and intelligence communities, systems are certified for use in a particular environment with data of a specified sensitivity.5 Unfortunately, the certification process tends to be more subjective and less technically rigorous than the product evaluation process. Certification of systems historically preceded Orange Book-style product evaluation, and certification criteria are typically less uniform, that is, varying from agency to agency.

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Computers at Risk: Safe Computing in the Information Age Nonetheless, certification does attempt to take into account the full set of security disciplines noted above and thus is more an attempt at a systems approach to security than it is product evaluation. Certified systems are not rated with concise designations, and standards for certification are less uniform than those for product evaluation, so that users cannot use the results of a certification applied to an existing system to simply specify security requirements for a new system. Unlike that from product evaluations, the experience gained from certifying systems is not so easily codified and transferred for use in certifying other systems. To approach the level of rigor and uniformity comparable to that involved in product evaluation, a system certifier would probably have to be more extensively trained than his counterpart who evaluates products. After all, certifiers must be competent in more security disciplines and be able to understand the security implications of combining various evaluated and unevaluated components to construct a system. A user attempting to characterize the security requirements for a system he is to acquire will find applying system certification methodology a priori a much more complex process than specifying a concise product rating based on a reading of the TCSEC environment guidelines (Yellow Book; U.S. DOD, 1985b). Formulating the security architecture for a system and selecting products to realize that architecture are intrinsically complex tasks that require expertise most users do not possess. Rather than attempting to cast system security requirements in the very concise language of a product ratings scheme such as the Orange Book, users must accept the complexity associated with system security and accept that developing and specifying such requirements are nontrivial tasks best performed by highly trained security specialists.6 In large organizations the task of system certification may be handled by internal staff. Smaller organizations will probably need to enlist the services of external specialists to aid in the certification of systems, much as structural engineers are called in as consultants. In either case system certifiers will need to be better trained to deal with increasingly complex systems with increased rigor. A combination of formal training and real-world experience are appropriate prerequisites for certifiers, and licensing (including formal examination) of consulting certifiers may also be appropriate. Increasingly, computers are becoming connected via networks and are being organized into distributed systems. In such environments a much more thorough system security analysis is required, and the product rating associated with each of the individual computers is in no way a sufficient basis for evaluating the security of the system as a whole. This suggests that it will become increasingly important to

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Computers at Risk: Safe Computing in the Information Age develop methodologies for ascertaining the security of networked systems, not just evaluations for individual computers. Product evaluations are not applicable to whole systems in general, and as "open systems" that can be interconnected relatively easily become more the rule, the need for system security evaluation, as distinct from product evaluation, will become even more critical. Many of the complexities of system security become apparent in the context of networks, and the TNI (which is undergoing revision) actually incorporates several distinct criteria in its attempt to address these varied concerns. Part I of the TNI provides product evaluation criteria for networks, but since networks are seldom homogeneous products this portion of the TNI seems to have relatively little direct applicability to real networks. Part II and Appendix A of the TNI espouse an unbundled approach to evaluation of network components, something that seems especially appropriate for such devices and that is similar to the ITSEC F9 and F10 functionality classes. However, many of the ratings specified in Part II and Appendix A of the TNI are fairly crude; for example, for some features only "none" or "present" ratings may be granted. More precise ratings, accompanied by better characterizations of requirements for such ratings, must be provided for these portions of the TNI to become really useful. Appendix C of the TNI attempts to provide generic rules to guide users through the complex process of connecting rated products together to form trusted systems, but it has not proven to be very useful. This is clearly a topic suitable for further research (see Chapter 8). RECOMMENDATIONS FOR PRODUCT EVALUATION AND SYSTEM CERTIFICATION CRITERIA The U.S. computer industry has made a significant investment in developing operating systems that comply with the Orange Book. This reality argues against any recommendation that would undercut that investment or undermine industry confidence in the stability of security evaluation criteria. Yet there are compelling arguments in favor of establishing less-bundled criteria to address some of the shortcomings cited above. This situation suggests a compromise approach in which elements from the Orange Book are retained but additional criteria, extensions of the TCSEC, are developed to address some of these arguments. This tack is consistent with the recommendations for GSSP made in Chapter 1, which would accommodate security facilities generally regarded as useful but outside the scope of the current criteria, for example, those supporting the model for Clark-Wilson integrity (Clark and Wilson, 1987).

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Computers at Risk: Safe Computing in the Information Age The importance of maintaining the momentum generated by the Orange Book process and planning for some future reciprocity or harmonization of international criteria sets makes modernization of the Orange Book necessary, although the committee anticipates a convergence between this process and the process of developing GSSP. In both instances, the intent is to reward vendors who wish to provide additional security functionality and/or greater security assurance than is currently accommodated by the Orange Book criteria. The TNI should be restructured to be more analogous to the ITSEC (i.e., with less emphasis on Parts I and II and more on a refined Appendix A). The TNI is new enough so as not to have acquired a large industry investment, and it is now undergoing revision anyway. Thus it should be politically feasible to modify the TNI at this stage. The ITSEC effort represents a serious attempt to transcend some of the limitations in the TCSEC, including the criteria for integrity and availability. However, it must be recognized that neither TCSEC nor ITSEC provides the ultimate answer, and thus ongoing efforts are vital. For example, a weakness of ITSEC is that its extended functional criteria F6 through F10 are independently assessable monolithic requirements. It might be more appropriate if integrity and availability criteria were graded similarly to criteria Fl through F5 for confidentiality, with their own hierarchies of ratings. (The draft Canadian criteria work in that direction.) There is also a need to address broader system security concerns in a manner that recognizes the heterogeneity of integrated or conglomerate systems. This is a matter more akin to certification than to product evaluation. To better address requirements for overall system security, it will be necessary to institute more objective, uniform, rigorous standards for system certification. The committee recommends that GSSP include relevant guidelines to illuminate such standards. To begin, a guide for system certification should be prepared, to provide a more uniform basis for certification. A committee should be established to examine existing system certification guidelines and related documentation—for example, password management standards—from government and industry as input to these guidelines. An attempt should be made to formalize the process of certifying a conglomerate system composed of evaluated systems, recognizing that this problem is very complex and may require a high degree of training and experience in the certifier. Development and evaluation of heterogeneous systems remain crucial research issues. For systems where classified information must be protected, a further kind of criteria development is implied, notably development of an

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Computers at Risk: Safe Computing in the Information Age additional assurance class within the A division, for example, A2 (this is primarily for government, not commercial, users),7 as well as functionality extensions for all divisions of the Orange Book. The committee's conclusions and specific recommendations, which are restated in Chapter 1 under recommendation 1, are as follows: A new generation of evaluation criteria is required and should be established, to deal with an expanded set of functional requirements for security and to respond to the evolution of computer technology, for example, networking. These criteria can incorporate the security functions of the existing TCSEC (at the C2 or B1 level) and thus preserve the present industry investment in Orange Book-rated systems. The committee's proposed GSSP are intended to meet this need. The new generation of criteria should be somewhat unbundled, compared to the current TCSEC, both to permit the addition of new functions and to permit some flexibility in the assurance methodology used. Guidelines should be prepared to prevent naive users from specifying incompatible sets of requirements. The ITSEC represents a reasonable example of the desirable degree of unbundled specification. Systems designed to conform to GSSP should undergo explicit evaluation for conformance to the GSSP criteria. Design evaluation should be performed by an independent team of evaluators. Implementation evaluation should include a combination of explicit system audit, field experience, and organized reporting of security faults. Such a process, which should be less costly and less onerous than the current NCSC process, is more likely to be cost-effective to the vendor and user, and is more likely to gain acceptance in the market. Effort should be expended to develop and improve the organized methods and criteria for dealing with complete systems, as opposed to products. This applies particularly to distributed systems, in which various different products are connected by a network. NOTES 1.   In the current environment, in which evaluations have been conducted by the NCSC, commercial system developers may face a greater challenge than those with defense contracting experience, who may have both cleared personnel and a working understanding of the documentation requirements. This practical problem underscores the need for a more effective interface between the commercial and the national security or classified worlds. 2.   Based on information obtained in a briefing from NCSC officials, the NCSC evaluation process consists of five phases, including: (1) Pre-review Phase, (2) Vendor

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Computers at Risk: Safe Computing in the Information Age     Assistance Phase (VAP), (3) Design Analysis Phase, (4) Formal Evaluation Phase, and (5) Rating Maintenance Phase (RAMP). In the Pre-review Phase vendors present the NCSC with a proposal defining the goals they expect to achieve and the basic technical approach being used. The pre-review proposal is used to determine the amount of NCSC resources needed to perform any subsequent evaluation. The Vendor Assistance Phase, which can begin at any stage of product development, consists primarily of monitoring and providing comments. During this phase, the NCSC makes a conscious effort not to "advise" the vendors (for legal reasons and because it is interested in evolution, not research and development). The Vendor Assistance Phase usually ends six to eight months before a product is released. The Design Analysis Phase takes an in-depth look at the design and implementation of a product using analytic tools. During this phase the Initial Product Analysis Report (IPAR) is produced, and the product is usually released for Beta testing. The Formal Evaluation Phase includes both performance and penetration testing of the actual product being produced. Products that pass these tests are added to the Evaluated Products List (EPL) at the appropriate level. Usually vendors begin shipping their product to normal customers during this phase. The Rating Maintenance Phase (RAMP), which takes place after products are shipped and pertains to enhancements (e.g., movement from one version of a product to another), is intended for C2 and B1 systems, to enable vendors to improve their product without undergoing a complete recertification. 3.   The NCSC has argued that it is premature to adopt criteria that address security features that support Clark-Wilson integrity because formal models for such security policies do not yet exist. In this way they justify the present bundled structure of the TCSEC (committee briefing by NSA). The NCSC continues to view integrity and assured service as research topics, citing a lack of formal policy models for these security services. However, it is worth noting that the Orange Book does not require a system to demonstrate correspondence to a formal security policy model until class B2, and the preponderance of rated systems in use in the commercial sector are below this level, for example, at the C2 level. Thus the NCSC argument against unbundling the TCSEC to include integrity and availability requirements in the criteria, at least at these lower levels of assurance, does not appear to be consistent. 4.   In the future software tools that capture key development steps may facilitate evaluation and cross-checks on evaluations by others. 5.   In the DOD environment the term "accreditation" refers to formal approval to use a system in a specified environment as granted by a designated approval authority. The term "certification" refers to the technical process that underlies the formal accreditation. 6.   The claims language of the ITSEC may be more amenable to system security specification. However, product evaluation and system certification are still different processes and should not be confused, even if the ratings terminology can be shared between the two. 7.   Proposals for an A2 class have been made before with no results, but LOCK and other projects suggest that it may now be time to extend the criteria to provide a higher assurance class. This class could apply formal specification and verification technology to a greater degree, require more stringent control on the development process (compare to the ITSEC E6 and E7), and/or call for stronger security mechanisms (e.g., the LOCK SIDEARM and BED technology, described in Appendix B of this report). The choice of which additional assurance features might be included in A2 requires further study.