We consider the proof-based development of cryptographic protocols satisfying security properties. For instance, the model of Dolev- Yao provides a way to integrate a description of possible attacks, when designing a protocol. We use existing protocols and want to provide a systematic way to prove but also to design cryptographic protocols; moreover, we would like to provide proof-based guidelines or patterns for integrating cryptographic elements in an existing protocol. The goal of the paper is to present a first attempt to mix design patterns (as in software engineering) and formal methods (as a verification tool). We illustrate the technique on the well known Needham-Schroeder public key protocol and Blake-Wilson-Menezes key transport protocol. The un- derlying modelling language is Event B and is supported by the RODIN platform, which is used to validate models.