A rewriting-based inference system for the NRL protocol analyzer

The NRL Protocol Analyzer (NPA) is a tool for the formal specification and analysis of cryptographic protocols that has been used with great effect on a number of complex real-life protocols. It probably outranks any of the existing tools in the sheer range of the types of attacks it is able to model and discover. However, the techniques in NPA lack an independent formal specification and model, and instead are closely intertwined with other NPA features. The main contribution of this paper is to rectify this problem by giving for the first time a precise formal specification of one of the main features of the NPA inference system: its grammar-based techniques for invariant generation, as well as a backwards reachability analysis method that captures some of the key features of the NPA. This formal specification is given within the well-known rewriting framework so that the inference system is specified as a set of rewrite rules modulo an equational theory describing the behavior of the cryptographic algorithms involved.