Abstract : Cryptographic protocols are crucial for securing electronic transactions. The conﬁdence in these protocols can be increased by the formal analysis of their security properties. Although many works have been dedicated to standard protocols like Needham-Schroeder, very few address the class of group protocols whose main characteristics are : the speciﬁc security properties that
they must satisfy, and the arbitrary number of participants they imply.
This thesis provides two main contributions. The ﬁrst one deals with the ﬁrst characteristic of group protocols. For that, we deﬁned a model called the services model which we used to propose a strategy for ﬂaws detection based on constraints solving. The suggested approach allows us to ﬁnd known attacks and new ones on some group protocols. Some attacks have been also generalized to cover the case of n participants. The second main contribution of
this thesis consists in deﬁning a synchronous model, that eneralizes standard protocol models by permitting unbounded lists inside messages. This is ensured by the introduction of a new operator called mpair which represents a list built on the same pattern. In this extended model, we have proposed a decision procedure for a particular class of group protocols called the class of well-tagged protocols with autonomous keys, in presence of an active intruder and with composed keys.