Abstract— Replay attacks on security protocols have been discussed for quite some time in the literature. However, the efforts to address these attacks have been largely incomplete, lacking generality and many times in fact, proven unsuccessful. In this paper we address these issues and prove the efficacy of a simple and general scheme in defending a protocol against these attacks. We believe that our work will be particularly useful in security critical applications and to protocol analyzers that are unable to detect some or all of the attacks in this class.