We present a new model for automated verification of security protocols, permitting the use of an unbounded number of protocol runs. We prove its correctness, completeness and also that it terminates. It has been implemented and its efficiency is clearly shown by the number of protocols successfully studied. In particular, we present an attack previously unreported on the Denning-Sacco symmetric key protocol. || Nous présentons un nouveau modèle pour la vérification automatique de protocoles de sécurité, permettant l'utilisation d'un nombre non borné d'exécutions du protocole. Nous démontrons sa correction, sa complétude et sa terminaison. Il a été implanté et s