Security protocols are short programs aiming at securing communications over a network. They are widely used in our everyday life. They may achieve various goals depending on the application: confidentiality, authenticity, privacy, anonymity, fairness, etc. Their verification using symbolic models has shown its interest for detecting attacks and proving security properties. A famous example is the Needham-Schroeder protocol [23] on which G. Lowe discovered a flaw 17 years after its publication [20]. Secrecy preservation has been proved to be co-NPcomplete for a bounded number of sessions [24], and decidable for an unbounded number of sessions under some additional restrictions (e.g. [3,12,13,25]). Many tools have also been developed to automatically verify cryptographic protocols like [8,21].