The last ten years, since the seminal work on the BAN logic [6], have seen the rapid development of formal methods for the analysis of security protocols. But security protocols have also
developed rapidly, becoming more and more complex. Protocols for electronic commerce are the béte noir: six pages were enough to describe the Needham-Schroeder protocol in 1978 [15], six hundred pages were not enough to describe the SET protocol of VISA and Mastercard [11, 12, 13] twenty years later.