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Chapter: Security in Computing : Designing Trusted Operating Systems


Most system consumers (that is, users or system purchasers) are not security experts.



Most system consumers (that is, users or system purchasers) are not security experts. They need the security functions, but they are not usually capable of verifying the accuracy or adequacy of test coverage, checking the validity of a proof of correctness, or determining in any other way that a system correctly implements a security policy. Thus, it is useful (and sometimes essential) to have an independent third party evaluate an operating system's security. Independent experts can review the requirements, design, implementation, and evidence of assurance for a system. Because it is helpful to have a standard approach for an evaluation, several schemes have been devised for structuring an independent review. In this section, we examine three different approaches: from the United States, from Europe, and a scheme that combines several known approaches.


U.S. "Orange Book" Evaluation


In the late 1970s, the U.S. Department of Defense (DoD) defined a set of distinct, hierarchical levels of trust in operating systems. Published in a document [DOD85] that has become known informally as the "Orange Book," the Trusted Computer System Evaluation Criteria (TCSEC) provides the criteria for an independent evaluation. The National Computer Security Center (NCSC), an organization within the National Security Agency, guided and sanctioned the actual evaluations.


The levels of trust are described as four divisions, A, B, C, and D, where A has the most comprehensive degree of security. Within a division, additional distinctions are denoted with numbers; the higher numbers indicate tighter security requirements. Thus, the complete set of ratings ranging from lowest to highest assurance is D, C1, C2, B1, B2, B3, and A1. Table 5 -7 (from Appendix D of [DOD85]) shows the security requirements for each of the seven evaluated classes of NCSC certification. (Class D has no requirements because it denotes minimal protection.)


  D, with no requirements


  C1/C2/B1, requiring security features common to many commercial operating systems


  B2, requiring a precise proof of security of the underlying model and a narrative specification of the trusted computing base


  B3/A1, requiring more precisely proven descriptive and formal designs of the trusted computing base


These clusters do not imply that classes C1, C2, and B1 are equivalent. However, there are substantial increases of stringency between B1 and B2, and between B2 and B3 (especially in the assurance area). To see why, consider the requirements for C1, C2, and B1. An operating system developer might be able to add security measures to an existing operating system in order to qualify for these ratings. However, security must be included in the design of the operating system for a B2 rating. Furthermore, the design of a B3 or A1 system must begin with construction and proof of a formal model of security. Thus, the distinctions between B1 and B2 and between B2 and B3 are significant.


Let us look at each class of security described in the TCSEC. In our descriptions, terms in quotation marks have been taken directly from the Orange Book to convey the spirit of the evaluation criteria.


Class D: Minimal Protection


This class is applied to systems that have been evaluated for a higher category but have failed the evaluation. No security characteristics are needed for a D rating.


Class C1: Discretionary Security Protection


C1 is intended for an environment of cooperating users processing data at the same level of sensitivity. A system evaluated as C1 separates users from data. Controls must seemingly be sufficient to implement access limitation, to allow users to protect their own data. The controls of a C1 system may not have been stringently evaluated; the evaluation may be based more on the presence of certain features. To qualify for a C1 rating, a system must have a domain that includes security functions and that is protected against tampering. A keyword in the classification is "discretionary." A user is "allowed" to decide when the controls apply, when they do not, and which named individuals or groups are allowed access.


Class C2: Controlled Access Protection


A C2 system still implements discretionary access control, although the granularity of control is finer. The audit trail must be capable of tracking each individual's access (or attempted access) to each object.


Class B1: Labeled Security Protection


All certifications in the B division include nondiscretionary access control. At the B1 level, each controlled subject and object must be assigned a security level. (For class B1, the protection system does not need to control every object.)


Each controlled object must be individually labeled for security level, and these labels must be used as the basis for access control decisions. The access control must be based on a model employing both hierarchical levels and nonhierarchical categories. (The military model is an example of a system with hierarchical levelsunclassified, classified, secret, top secretand nonhierarchical categories, need-to-know category sets.) The mandatory access policy is the BellLa Padula model. Thus, a B1 system must implement BellLa Padula controls for all accesses, with user discretionary access controls to further limit access.


Class B2: Structured Protection


The major enhancement for B2 is a design requirement: The design and implementation of a B2 system must enable a more thorough testing and review. A verifiable top-level design must be presented, and testing must confirm that the system implements this design. The system must be internally structured into "well-defined largely independent modules." The principle of least privilege is to be enforced in the design. Access control policies must be enforced on all objects and subjects, including devices. Analysis of covert channels is required.


Class B3: Security Domains


The security functions of a B3 system must be small enough for extensive testing. A high-level design must be complete and conceptually simple, and a "convincing argument" must exist that the system implements this design. The implementation of the design must "incorporate significant use of layering, abstraction, and information hiding."


The security functions must be tamperproof. Furthermore, the system must be "highly resistant to penetration." There is also a requirement that the system audit facility be able to identify when a violation of security is imminent.


Class A1: Verified Design


Class A1 requires a formally verified system design. The capabilities of the system are the same as for class B3. But in addition there are five important criteria for class A1 certification: (1) a formal model of the protection system and a proof of its consistency and adequacy,

(2) a formal top-level specification of the protection system, (3) a demonstration that the top-level specification corresponds to the model,

(4) an implementation "informally" shown to be consistent with the specification, and (5) formal analysis of covert channels.


European ITSEC Evaluation


The TCSEC was developed in the United States, but representatives from several European countries also recognized the need for criteria and a methodology for evaluating security-enforcing products. The European efforts culminated in the ITSEC, the Information Technology Security Evaluation Criteria [ITS91b].


Origins of the ITSEC


England, Germany, and France independently began work on evaluation criteria at approximately the same time. Both England and Germany published their first drafts in 1989; France had its criteria in limited review when these three nations, joined by the Netherlands, decided to work together to develop a common criteria document. We examine Britain and Germany's efforts separately, followed by their combined output.


German Green Book


The (then West) German Information Security Agency (GISA) produced a catalog of criteria [GIS88] five years after the first use of the U.S. TCSEC. Keeping with tradition, the security community began to call the document the German Green Book because of its green cover. The German criteria identified eight basic security functions, deemed sufficient to enforce a broad spectrum of security policies:


  identification and authentication: unique and certain association of an identity with a subject or object


  administration of rights: the ability to control the assignment and revocation of access rights between subjects and objects


  verification of rights: mediation of the attempt of a subject to exercise rights with respect to an object


  audit: a record of information on the successful or attempted unsuccessful exercise of rights


  object reuse: reusable resources reset in such a way that no information flow occurs in contradiction to the security policy


  error recovery: identification of situations from which recovery is necessary and invocation of an appropriate action


  continuity of service: identification of functionality that must be available in the system and what degree of delay or loss (if any) can be tolerated


  data communication security: peer entity authentication, control of access to communications resources, data confidentiality, data integrity, data origin authentication, and nonrepudiation


Note that the first five of these eight functions closely resemble the U.S. TCSEC, but the last three move into entirely new areas: integrity of data, availability, and a range of communications concerns.


Like the U.S. DoD, GISA did not expect ordinary users (that is, those who were not security experts) to select appropriate sets of security functions, so ten functional classes were defined. Classes F1 through F5 corresponded closely to the functionality requirements of U.S. classes C1 through B3. (Recall that the functionality requirements of class A1 are identical to those of B3.) Class F6 was for high data and program integrity requirements, class F7 was appropriate for high availability, and classes F8 through F10 relate to data communications situations. The German method addressed assurance by defining eight quality levels, Q0 through Q7, corresponding roughly to the assurance requirements of U.S. TCSEC levels D through A1, respectively. For example,


The evaluation of a Q1 system is merely intended to ensure that the implementation more or less enforces the security policy and that no major errors exist.

  The goal of a Q3 evaluation is to show that the system is largely resistant to simple penetration attempts.


  To achieve assurance level Q6, it must be formally proven that the highest specification level meets all the requirements of the formal security policy model. In addition, the source code is analyzed precisely.


These functionality classes and assurance levels can be combined in any way, producing potentially 80 different evaluation results, as shown in Table 5-8. The region in the upper-right portion of the table represents requirements in excess of U.S. TCSEC requirements, showing higher assurance requirements for a given functionality class. Even though assurance and functionality can be combined in any way, there may be limited applicability for a low -assurance, multilevel system (for example, F5, Q1) in usage. The Germans did not assert that all possibilities would necessarily be useful, however.



Another significant contribution of the German approach was to support evaluations by independent, commercial evaluation facilities.


British Criteria


The British criteria development was a joint activity between the U.K. Department of Trade and Industry (DTI) and the Ministry of Defence (MoD). The first public version, published in 1989 [DTI89a], was issued in several volumes.


The original U.K. criteria were based on the "claims" language, a metalanguage by which a vendor could make claims about functionality in a product. The claims language consisted of lists of action phrases and target phrases with parameters. For example, a typical action phrase might look like this:


This product can [not] determine … [using the mechanism described in paragraph n of this document] …


The parameters product and n are, obviously, replaced with specific references to the product to be evaluated. An example of a target phrase is


… the access-type granted to a [user, process] in respect of a(n) object.


These two phrases can be combined and parameters replaced to produce a claim about a product.


This access control subsystem can determine the read access granted to all subjects in respect to system files.


The claims language was intended to provide an open -ended structure by which a vendor could assert qualities of a product and independent evaluators could verify the truth of those claims. Because of the generality of the claims language, there was no direct correlation of U.K. and U.S. evaluation levels.


In addition to the claims language, there were six levels of assurance evaluation, numbered L1 through L6, corresponding roughly to U.S. assurance C1 through A1 or German Q1 through Q6.



The claims language was intentionally open-ended because the British felt it was impossible to predict which functionality manufacturers would choose to put in their products. In this regard, the British differed from Germany and the United States, who thought manufacturers needed to be guided to include specific functions with precise functionality requirements. The British envisioned certain popular groups of claims being combined into bundles that could be reused by many manufacturers.


The British defined and documented a scheme for Commercial Licensed Evaluation Facilities (CLEFs) [DTI89b], with precise requirements for the conduct and process of evaluation by independent commercial organizations.


Other Activities


As if these two efforts were not enough, Canada, Australia, and France were also working on evaluation criteria. The similarities among these efforts were far greater than their differences. It was as if each profited by building upon the predecessors' successes.


Three difficulties, which were really different aspects of the same problem, became immediately apparent.


  Comparability. It was not clear how the different evaluation criteria related. A German F2/E2 evaluation was structurally quite similar to a U.S. C2 evaluation, but an F4/E7 or F6/E3 evaluation had no direct U.S. counterpart. It was not obvious which U.K. claims would correspond to a particular U.S. evaluation level.


  Transferability. Would a vendor get credit for a German F2/E2 evaluation in a context requiring a U.S. C2? Would the stronger F2/E3 or F3/E2 be accepted?


  Marketability. Could a vendor be expected to have a product evaluated independently in the United States, Germany, Britain, Canada, and Australia? How many evaluations would a vendor support? (Many vendors suggested that they would be interested in at most one because the evaluations were costly and time consuming.)


For reasons including these problems, Britain, Germany, France, and the Netherlands decided to pool their knowledge and synthesize their work.


ITSEC: Information Technology Security Evaluation Criteria


In 1991 the Commission of the European Communities sponsored the work of these four nations to produce a harmonized version for use by all European Union member nations. The result was a good amalgamation.


The ITSEC preserved the German functionality classes F1F10, while allowing the flexibility of the British claims language. There is similarly an effectiveness component to the evaluation, corresponding roughly to the U.S. notion of assurance and to the German E0E7 effectiveness levels.


A vendor (or other "sponsor" of an evaluation) has to define a target of evaluation (TOE), the item that is the evaluation's focus. The TOE is considered in the context of an operational environment (that is, an expected set of threats) and security enforcement requirements. An evaluation can address either a product (in general distribution for use in a variety of environments) or a system (designed and built for use in a specified setting). The sponsor or vendor states the following information:


  system security policy or rationale: why this product (or system) was built


  specification of security-enforcing functions: security properties of the product (or system)


  definition of the mechanisms of the product (or system) by which security is enforced


  a claim about the strength of the mechanisms


  the target evaluation level in terms of functionality and effectiveness


The evaluation proceeds to determine the following aspects:


  suitability of functionality: whether the chosen functions implement the desired security features

  binding of functionality: whether the chosen functions work together synergistically


  vulnerabilities: whether vulnerabilities exist either in the construction of the TOE or how it will work in its intended environment


  ease of use


  strength of mechanism: the ability of the TOE to withstand direct attack


The results of these subjective evaluations determine whether the evaluators agree that the product or system deserves its proposed functionality and effectiveness rating.


Significant Departures from the Orange Book


The European ITSEC offers the following significant changes compared with the Orange Book. These variations have both advantages and disadvantages, as listed in Table 5-9.



U.S. Combined Federal Criteria


In 1992, partly in response to other international criteria efforts, the United States began a successor to the TCSEC, which had been written over a decade earlier. This successor, the Combined Federal Criteria [NSA92], was produced jointly by the National Institute for Standards and Technology (NIST) and the National Security Agency (NSA) (which formerly handled criteria and evaluations through its National Computer Security Center, the NCSC).


The team creating the Combined Federal Criteria was strongly influenced by Canada's criteria [CSS93], released in draft status just before the combined criteria effort began. Although many of the issues addressed by other countries' criteria were the same for the United States, there was a compatibility issue that did not affect the Europeans, namely, the need to be fair to vendors that had already passed U.S. evaluations at a particular level or that were planning for or in the middle of evaluations. Within that context, the new U.S. evaluation model was significantly different from the TCSEC. The combined criteria draft resembled the European model, with some separation between features and assurance.

The Combined Federal Criteria introduced the notions of security target (not to be confused with a target of evaluation, or TOE) and protection profile. A user would generate a protection profile to detail the protection needs, both functional and assurance, for a specific situation or a generic scenario. This user might be a government sponsor, a commercial user, an organization representing many similar users, a product vendor's marketing representative, or a product inventor. The protection profile would be an abstract specification of the security aspects needed in an information technology (IT) product. The protection profile would contain the elements listed in Table 5-10.


In response to a protection profile, a vendor might produce a product that, the vendor would assert, met the requirements of the profile. The vendor would then map the requirements of the protection profile in the context of the specific product onto a statement called a security target. As shown in Table 5-11, the security target matches the elements of the protection profile.


The security target then becomes the basis for the evaluation. The target details which threats are countered by which features, to what degree of assurance and using which mechanisms. The security target outlines the convincing argument that the product satisfies the requirements of the protection profile. Whereas the protection profile is an abstract description of requirements, the security target is a detailed specification of how each of those requirements is met in the specific product.


The criteria document also included long lists of potential requirements (a subset of which could be selected for a particular protection profile), covering topics from object reuse to accountability and from covert channel analysis to fault tolerance. Much of the work in specifying precise requirement statements came from the draft version of the Canadian criteria.


The U.S. Combined Federal Criteria was issued only once, in initial draft form. After receiving a round of comments, the editorial team announced that the United States had decided to join forces with the Canadians and the editorial board from the ITSEC to produce the Common Criteria for the entire world.


Common Criteria


The Common Criteria [CCE94, CCE98] approach closely resembles the U.S. Federal Criteria (which, of course, was heavily influenced by the ITSEC and Canadian efforts). It preserves the concepts of security targets and protections profiles. The U.S. Federal Criteria were intended to have packages of protection requirements that were complete and consistent for a particular type of application, such as a network communications switch, a local area network, or a stand-alone operating system. The example packages received special attention in the Common Criteria.


The Common Criteria defined topics of interest to security, shown in Table 5-12. Under each of these classes, they defined families of functions or assurance needs, and from those families, they defined individual components, as shown in Figure 5-24.


Individual components were then combined into packages of components that met some comprehensive requirement (for functionality) or some level of trust (for assurance), as shown in Figure 5-25.



Summary of Evaluation Criteria


The criteria were intended to provide independent security assessments in which we could have some confidence. Have the criteria development efforts been successful? For some, it is too soon to tell. For others, the answer lies in the number and kinds of products that have passed evaluation and how well the products have been accepted in the marketplace.


Evaluation Process


We can examine the evaluation process itself, using our own set of objective criteria. For instance, it is fair to say that there are several desirable qualities we would like to see in an evaluation, including the following:


  Extensibility. Can the evaluation be extended as the product is enhanced?


  Granularity. Does the evaluation look at the product at the right level of detail?


  Speed. Can the evaluation be done quickly enough to allow the product to compete in the marketplace?


  Thoroughness. Does the evaluation look at all relevant aspects of the product?


  Objectivity. Is the evaluation independent of the reviewer's opinions? That is, will two different reviewers give the same rating to a product?


  Portability. Does the evaluation apply to the product no matter what platform the product runs on?


  Consistency. Do similar products receive similar ratings? Would one product evaluated by different teams receive the same results?


  Compatibility. Could a product be evaluated similarly under different criteria? That is, does one evaluation have aspects that are not examined in another?


  Exportability. Could an evaluation under one scheme be accepted as meeting all or certain requirements of another scheme?


Using these characteristics, we can see that the applicability and extensibility of the TCSEC are somewhat limited. Compatibility is being addressed by combination of criteria, although the experience with the ITSEC has shown that simply combining the words of criteria documents does not necessarily produce a consistent understanding of them. Consistency has been an important issue, too. It was unacceptable for a vendor to receive different results after bringing the same product to two different evaluation facilities or to one facility at two different times. For this reason, the British criteria documents stressed consistency of evaluation results; this characteristic was carried through to the ITSEC and its companion evaluation methodology, the ITSEM. Even though speed, thoroughness, and objectivity are considered to be three essential qualities, in reality evaluations still take a long time relative to a commercial computer product delivery cycle of 6 to 18 months.


Criteria Development Activities


Evaluation criteria continue to be developed and refined. If you are interested in doing evaluations, in buying an evaluated product, or in submitting a product for evaluation, you should follow events closely in the evaluation community. You can use the evaluation goals listed above to help you decide whether an evaluation is appropriate and which kind of evaluation it should be.


It is instructive to look back at the evolution of evaluation criteria documents, too. Figure 5-27 shows the timeline for different criteria publications; remember that the writing preceded the publication by one or more years. The figure begins with Anderson's original Security Technology Planning Study [AND72], calling for methodical, independent evaluation. To see whether progress is being made, look at the dates when different criteria documents were published; earlier documents influenced the contents and philosophy of later ones.

The criteria development activities have made significant progress since 1983. The U.S. TCSEC was based on the state of best practice known around 1980. For this reason, it draws heavily from the structured programming paradigm that was popular throughout the 1970s. Its major difficulty was its prescriptive manner; it forced its model on all developments and all types of products. The TCSEC applied most naturally to monolithic, stand-alone, multiuser operating systems, not to the heterogeneous, distributed, networked environment based largely on individual intelligent workstations that followed in the next decade.


Experience with Evaluation Criteria


To date, criteria efforts have been paid attention to by the military, but those efforts have not led to much commercial acceptance of trusted products. The computer security research community is heavily dominated by defense needs because much of the funding for security research is derived from defense departments. Ware [WAR95] points out the following about the initial TCSEC:


  It was driven by the U.S. Department of Defense.


  It focused on threat as perceived by the U.S. Department of Defense.


  It was based on a U.S. Department of Defense concept of operations, including cleared personnel, strong respect for authority and management, and generally secure physical environments.


  It had little relevance to networks, LANs, WANs, Internets, client-server distributed architectures, and other more recent modes of computing.


When the TCSEC was introduced, there was an implicit contract between the U.S. government and vendors, saying that if vendors built products and had them evaluated, the government would buy them. Anderson [AND82] warned how important it was for the government to keep its end of this bargain. The vendors did their part by building numerous products: KSOS, PSOS, Scomp, KVM, and Multics. But unfortunately, the products are now only of historical interest because the U.S. government did not follow through and create the market that would encourage those vendors to continue and other vendors to join. Had many evaluated products been on the market, support and usability would have been more adequately addressed, and the chance for commercial adoption would have been good. Without government support or perceived commercial need, almost no commercial acceptance of any of these products has occurred, even though they have been developed to some of the highest quality standards.


Schaefer [SCH04a] gives a thorough description of the development and use of the TCSEC. In his paper he explains how the higher evaluation classes became virtually unreachable for several reasons, and thus the world has been left with less trustworthy systems than before the start of the evaluation process. The TCSEC's almost exclusive focus on confidentiality would have permitted serious integrity failures (as obliquely described in [SCH89b]).


On the other hand, some major vendors are actively embracing low and moderate assurance evaluations: As of May 2006, there are 78 products at EAL2, 22 at EAL3, 36 at EAL4, 2 at EAL5 and 1 at EAL7. Product types include operating systems, firewalls, antivirus software, printers, and intrusion detection products. (The current list of completed evaluations (worldwide) is maintained at www.commoncriteriaportal.org.) Some vendors have announced corporate commitments to evaluation, noting that independent evaluation is a mark of quality that will always be a stronger selling point than so-called emphatic assertion (when a vendor makes loud claims about the strength of a product, with no independent evidence to substantiate those claims). Current efforts in criteria-writing support objectives, such as integrity and availability, as strongly as confidentiality. This approach can allow a vendor to identify a market niche and build a product for it, rather than building a product for a paper need (that is, the dictates of the evaluation criteria) not matched by purchases. Thus, there is reason for optimism regarding criteria and evaluations. But realism requires everyone to accept that the marketnot a criteria documentwill dictate what is desired and delivered. As Sidebar 5 -7 describes, secure systems are sometimes seen as a marketing niche: not part of the mainstream product line, and that can only be bad for security.


It is generally believed that the market will eventually choose quality products. The evaluation principles described above were derived over time; empirical evidence shows us that they can produce high-quality, reliable products deserving our confidence. Thus, evaluation criteria and related efforts have not been in vain, especially as we see dramatic increases in security threats and the corresponding increased need for trusted products. However, it is often easier and cheaper for product proponents to speak loudly than to present clear evidence of trust. We caution you to look for solid support for the trust you seek, whether that support be in test and review results, evaluation ratings, or specialized assessment.


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