Computer security [16,5] is usually defined as ensuring integrity, confidentiality, and availability requirements even in the presence of a determined, malicious opponent. Sensitive data must be modified and consulted by authorized users only (integrity, confidentiality); moreover, the system should resist “denial of service” attacks that attempt to render it unusable (availability). In more colorful language, computer security has been described as “programming Satan’s computer” [6]: the implementor must assume that every weakness that can be exploited will be.