Philosophy
Key to understanding the problems that exist in computer security is a recognition that the problems are not new. They are old problems, dating from the beginning of computer security (and, in fact, arising from parallel problems in the noncomputer world). But the locus has changed as the field of computing has changed. Before the mid-1980s, mainframe and mid-level computers dominated the market, and computer security problems and solutions were phrased in terms of securing files or processes on a single system. With the rise of networking and the Internet, the arena has changed. Workstations and servers, and the networking infrastructure that connects them, now dominate the market. Computer security problems and solutions now focus on a networked environment. However, if the workstations and servers, and the supporting network infrastructure, are viewed as a single system, the models, theories, and problem statements developed for systems before the mid-1980s apply equally well to current systems.
As an example, consider the issue of assurance. In the early period, assurance arose in several ways: formal methods and proofs of correctness, validation of policy to requirements, and acquisition of data and programs from trusted sources, to name a few. Those providing assurance analyzed a single system, the code on it, and the sources (vendors and users) from which the code could be acquired to ensure that either the sources could be trusted or the programs could be confined adequately to do minimal damage. In the later period, the same basic principles and techniques apply, except that the scope of some has been greatly expanded (from a single system and a small set of vendors to the world-wide Internet). The work on proof-carrying code, an exciting development in which the proof that a downloadable program module satisfies a stated policy is incorporated into the program itself, is an example of this expansion. It extends the notion of a proof of consistency with a stated policy. It advances the technology of the earlier period into the later period. But in order to understand it properly, one must understand the ideas underlying the concept of proof-carrying code, and these ideas lie in the earlier period.
As another example, consider Saltzer and Schroeder's principles of secure design. Enunciated in 1975, they promote simplicity, confinement, and understanding. When security mechanisms grow too complex, attackers can evade or bypass them. Many programmers and vendors are learning this when attackers break into their systems and servers. The argument that the principles are old, and somehow outdated, rings hollow when the result of their violation is a nonsecure system.
The work from the earlier period is sometimes cast in terms of systems that no longer exist and that differ in many ways from modern systems. This does not vitiate the ideas and concepts, which also underlie the work done today. Once these ideas and concepts are properly understood, applying them in a multiplicity of environments becomes possible. Furthermore, the current mechanisms and technologies will become obsolete and of historical interest themselves as new forms of computing arise, but the underlying principles will live on, to underlie the next generationindeed the next eraof computing.
The philosophy of this book is that certain key concepts underlie all of computer security, and that the study of all parts of computer security enriches the understanding of all parts. Moreover, critical to an understanding of the applications of security-related technologies and methodologies is an understanding of the theory underlying those applications.
Advances in the theory of computer protection have illuminated the foundations of security systems. Issues of abstract modeling, and modeling to meet specific environments, lead to systems designed to achieve a specific and rewarding goal. Theorems about the undecidability of the general security question have indicated the limits of what can be done.
Application of these results has improved the quality of the security of the systems being protected. However, the issue is how compatibly the assumptions of the model (and theory) conform to the environment to which the theory is applied. Although our knowledge of how to apply these abstractions is continually increasing, we still have difficulty correctly transposing the relevant information from a realistic setting to one in which analyses can then proceed. Such abstraction often eliminates vital information. The omitted data may pertain to security in nonobvious ways. Without this information, the analysis is flawed.
Unfortunately, no single work can cover all aspects of computer security, so this book focuses on those parts that are, in the author's opinion, most fundamental and most pervasive. The mechanisms exemplify the applications of these principles.
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