Goals

This book has three goals. The first is to show the importance of theory to practice and of practice to theory. All too often, practitioners regard theory as irrelevant and theoreticians think of practice as trivial. In reality, theory and practice are symbiotic. For example, the theory of covert channels, in which the goal is to limit the ability of processes to communicate through shared resources, provides a mechanism for evaluating the effectiveness of mechanisms that confine processes, such as sandboxes and firewalls. Similarly, business practices in the commercial world led to the development of several security policy models such as the Clark-Wilson model and the Chinese Wall model. These models in turn help the designers of security policies better understand and evaluate the mechanisms and procedures needed to secure their sites.

The second goal is to emphasize that computer security and cryptography are different. Although cryptography is an essential component of computer security, it is by no means the only component. Cryptography provides a mechanism for performing specific functions, such as preventing unauthorized people from reading and altering messages on a network. However, unless developers understand the context in which they are using cryptography, and unless the assumptions underlying the protocol and the cryptographic mechanisms apply to the context, the cryptography may not add to the security of the system. The canonical example is the use of cryptography to secure communications between two low-security systems. If only trusted users can access the two systems, cryptography protects messages in transit. But if untrusted users can access either system (through authorized accounts or, more likely, by breaking in), the cryptography is not sufficient to protect the messages. The attackers can read the messages at either endpoint.

The third goal is to demonstrate that computer security is not just a science but also an art. It is an art because no system can be considered secure without an examination of how it is to be used. The definition of a "secure computer" necessitates a statement of requirements and an expression of those requirements in the form of authorized actions and authorized users. (A computer engaged in work at a university may be considered "secure" for the purposes of the work done at the university. When moved to a military installation, that same system may not provide sufficient control to be deemed "secure" for the purposes of the work done at that installation.) How will people, as well as other computers, interact with the computer system? How clear and restrictive an interface can a designer create without rendering the system unusable while trying to prevent unauthorized use or access to the data or resources on the system?

Just as an artist paints his view of the world onto canvas, so does a designer of security features articulate his view of the world of human/machine interaction in the security policy and mechanisms of the system. Two designers may use entirely different designs to achieve the same creation, just as two artists may use different subjects to achieve the same concept.

Computer security is also a science. Its theory is based on mathematical constructions, analyses, and proofs. Its systems are built in accordance with the accepted practices of engineering. It uses inductive and deductive reasoning to examine the security of systems from key axioms and to discover underlying principles. These scientific principles can then be applied to untraditional situations and new theories, policies, and mechanisms.