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Program Security

This chapter is from the book

This chapter is from the book


Software is increasingly used in systems which, should the software malfunction, may threaten life, health, national security, the environment, or the economy. This situation means that developers, regulators, and users place an increasing priority on high-confidence software: software for which compelling evidence is required that it delivers a specified set of services in a manner that satisfies specified critical properties. For this reason, we look to software engineering research to help us build software that is not only more secure but also is of generally higher quality than the software we build and use today. Thus, the security field can leverage work being done in other domains on high-confidence software development.

The software engineering practices that offer us the most benefit can involve processes, products, or resources. When software has tight quality constraints, we do not want to wait until the system is developed to see if they are met. Rather, we want to see some evidence during development that the completed system is likely to meet the quality requirements we have imposed. We want to have confidence, based on compelling and objective evidence, that the risk associated with using a system conforms with our willingness to tolerate that risk.

An assurance argument lays out the evidence, not only in terms of software properties but also in terms of steps taken, resources used, and any other relevant issue that may have bearing on our confidence in the software's quality. The Common Criteria (studied in Chapter 5) require such an assurance case for security-critical systems. A framework for assurance arguments includes a description of what assurance is required for the system, how the case will be made that the required confidence is justified, what evidence is to be gathered, and how the evidence will be combined and evaluated. Some such frameworks exist and are being used. However, assurance argument frameworks suffer from several deficiencies:

  • They are strong on organization and layout but weak on process.

  • They emphasize repeated but narrow measurements instead of offering a broad perspective.

  • They offer no guidance on assurance for evolving systems.

Researchers at RAND and MITRE are addressing these issues. MITRE is mapping existing assurance arguments to a common, machine-processable form, using two kinds of notations: Toulmin structures, developed as a general framework for presenting and analyzing arguments in legal and regulatory contexts, and Goal Structuring Notation, developed in the U.K.'s safety-critical software community for structuring safety arguments. RAND researchers are examining questions of confidence and assurance, particularly about how bodies of evidence and constructions of arguments support confidence in the assurance case. In particular, RAND is determining how assurance activities and techniques, such as reliability modeling and design-by-contract, fit into the larger picture of providing an assurance argument.

At the same time, researchers are examining ways to make code self-stabilizing or self-healing. Such systems can sense when they reach an illegitimate state—that is, an insecure one—and can automatically return to a legitimate, secure state. The self-healing process is not so simple as realizing a failure and correcting it in one step. Imagine, instead, that you awaken one morning and discover that you are in poor physical shape, overweight, and bored. A program of exercise, nutrition, and mental stimulation can gradually bring you back, but there may be some missteps along the way. Similarly a program may realize that it has allowed many program extensions—some perhaps malicious—to become integrated into the system and wants to return gradually to a secure configuration. Dijkstra [DIJ74] introduced this concept, and Lamport

  • [LAM84] publicized it; it is closely related to the Byzantine generals problem

  • [LAM82] that has been studied in many similar contexts.

In looking to the future it is important not to forget the past. Every student of computer security should know the foundational literature of computer security, including the works of Saltzer and Schroeder [SAL75] and Lampson [LAM71]. Other historical papers of interest are listed in the "To Learn More" section.

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