Formal Security Analysis with Interacting State Machines

We introduce the ISM approach, a framework for modeling and verifying reactive systems in a formal, even machine-checked, way. The framework has been developed for applications in security analysis. It is based on the notion of Interacting State Machines (ISMs), kind of high-level Input/Output Automata. The ISM framework is used to define system models and present them graphically with the AutoFocus tool, to let them be checked for consistency and translated to a representation within the theorem prover Isabelle/HOL (or alternatively to define them directly as Isabelle theory sections), and finally to employ the theorem prover for performing any kind of syntactic and semantic checks, in particular semi-automatic verification. We demonstrate that the framework can be fruitfully applied for formal system analysis by two classical application examples: the LKW model of the Infineon SLE 66 smart card chip and Lowe’s fix of the Needham-Schroeder Public-Key Protocol.