extension of the previous one, no one is allowed to take out more than five books at a time. In a sophisticated version of this system, a librarian first determines how many books a subject already has out before allowing that subject to take more out. Such a policy requires a check of the subject's identity and current status.
In a library with an even more complex policy, only certain people are allowed to access certain books. The librarian performs a check by name of who is allowed to access which books. This policy frequently involves the development of long lists of names and may evolve toward, in some cases, a negative list, that is, a list of people who should not be able to have access to specific information. In large organizations, determining which users have access to specific information frequently is based on the project they are working on or the level of sensitivity of data for which they are authorized. In each of these cases, there is an access control policy and an enforcement mechanism. The policy defines the access that an individual will have to information contained in the library. The librarian serves as the policy-enforcing mechanism.
The best-known and most widely used formal models of computer security functionality, the Bell and LaPadula model and its variants (Bell and LaPadula, 1976), emphasize confidentiality (protection from unauthorized disclosure of information) as their primary security service. In particular, these models attempt to capture the "mandatory" (what ISO Standard 7498-2 (ISO, 1989) refers to as "administratively directed, label-based") aspects of security policy. This is especially important in providing protection against "Trojan horse" software, a significant concern among those who process classified data. Mandatory controls are typically enforced by operating-system mechanisms at the relatively coarse granularity of processes and files. This state of affairs has resulted from a number of factors, several of which are noted below:
The basic security models were accurately perceived to represent Department of Defense (DOD) security concerns for protecting classified information from disclosure, especially in the face of Trojan horse attacks. Since it was under the auspices of DOD funding that the work in formal security policy models was carried out, it is not surprising that the emphasis was on models that reflected DOD requirements for confidentiality.
The embodiment of the model in the operating system has been