New Decidability Results for Fragments of First-Order Logic and Application to Cryptographic Protocols

We consider a new extension of the Skolem class for first-order logic and prove its decidability by resolution techniques. We then extend this class including the built-in equational theory of exclusive or. Again, we prove the decidability of the class by resolution techniques. Considering such fragments of first-order logic is motivated by the automatic verification of cryptographic protocols, for an arbitrary number of sessions; the first-order formalization is an approximation of the set of possible traces, for instance relaxing the nonce freshness assumption. As a consequence, we get some new decidability results for the verification of cryptographic protocols with exclusive or.