Abstract : Cryptographic protocols need to meet numerous goals: algorithms to use, properties to enforce, identification tools, etc. New protocols appear often. Despite the apparent simplicity, building a cryptographic protocol is difficult and can lead to errors. For some protocols, attacks have been found many years after their conception. Most existing works are based on the Dolev Yao intruder and cannot easily be extended to another intruder with more power. We present here a deduction system taking the intruder power as input. Moreover, protocol rules are added as a supplemental capacity of the intruder. The main result is a proof normalization theorem allowing to reduce the search space for attacks.