The design and verification of cryptographic protocols is a notoriously difficult task, even in abstract Dolev-Yao models. This is mainly due to several sources of unboundedness (size of messages, number of sessions, ...). In this paper, we characterize a class of protocols for which secrecy for an unbounded number of sessions is decidable. More precisely, we present a simple transformation which maps a protocol that is secure for a single protocol session (a decidable problem) to a protocol that is secure for an unbounded number of sessions.

Our result provides an effective strategy to design secure protocols: (i) design a protocol intended to be secure for one protocol session (this can be verified with existing automated tools); (ii) apply our transformation and obtain a protocol which is secure for an unbounded number of sessions. The proof of our result is closely tied to a particular constraint solving procedure by Comon-Lundh et al.