Programming with rewrite rules and strategies has been already used for describing several computational logics. In this paper is described the way the Needham-Schroeder Public-Key protocol is specified in ELAN, the system developed in Nancy for executing rewrite programs. The protocol aims to establish a mutual authentication between an initiator and a responder that communicate via an insecure network. The protocol has been shown vulnerable and a correction has been proposed by G. Lowe. The behavior of the agents and of the intruders as well as the security invariants the protocol should verify are naturally described by conditional rewrite rules. The application of the rewrite rules is controlled by strategies. Similar attacks to those already described in the literature have been discovered. We show how different strategies using the same set of rewrite rules can improve the efficiency in finding the attacks and we compare our results to existing approaches.