I/O automaton models and proofs for shared-key communication systems

The combination of two security protocols, a simple shared-key communication protocol and the Diffie-Hellman key distribution protocol, is modeled formally and proved correct. The modeling is based on the I/O automaton model for distributed algorithms, and the proofs are based on invariant assertions, simulation relations, and compositional reasoning. Arguments about the cryptosystems are handled separately from arguments about the protocols.