Formal methods and tools are key aspects for the analysis of cryptographic protocols. In this paper, a formal language PEP (principals+environment=protocol) for the specification of cryptographic protocols is proposed. For some cryptographic protocols, their PEP specifications can be translated into finite basic CCS processes, so it is possible to analyze the security properties using CCS-based tools such as CWB (concurrency workbench). The advantage of the mothod proposed in this paper is that the actions of the attacker can be implicitly specified, and if the potential back door of the protocol analyzed exists, the attacking action trace can be explicitly found out by model checker.