Using composition to design secure, fault-tolerant systems

Complex systems must be analyzed in smaller pieces. Analysis must support both bottom-up (composition) and top-down (refinement) development, and it must support the consideration of several critical properties, e.g. functional correctness, fault tolerance and security, as appropriate. We describe a mathematical framework, called the CSS (Composability for Secure Systems) framework, for performing composition and refinement analysis and discuss some lessons learned from its application. The framework is written and verified in the PVS specification language.