Developing Security Protocols by Refinement

We propose a development method for security protocols based on stepwise refinement. Our refinement strategy guides the transformation of abstract security goals into protocols that are secure when operating over an insecure channel controlled by a Dolev-Yao-style intruder. The refinement steps successively introduce local states, an intruder, communication channels with security properties, and cryptographic operations realizing these channels. The abstractions used provide insights on how the protocols work and foster the development of families of protocols sharing a common structure and properties. In contrast to post-hoc verification methods, protocols are developed together with their correctness proofs. We have implemented our method in Isabelle/HOL and used it to develop different entity authentication and key transport protocols.