Modular Verification of Security Protocol Code by Typing

We propose a method for verifying the security of protocol implementations. Our method is based on declaring and enforcing invariants on the usage of cryptography. We develop cryptographic libraries that embed a logic model of their cryptographic structures and that specify preconditions and postconditions on their functions so as to maintain their invariants. We present a theory to justify the soundness of modular code verification via our method. We implement the method for protocols coded in F# and verified using F7, our SMT-based typechecker for refinement types, that is, types carrying formulas to record invariants. As illustrated by a series of programming examples, our method can flexibly deal with a range of different cryptographic constructions and protocols. We evaluate the method on a series of larger case studies of protocol code, previously checked using whole-program analyses based on ProVerif, a leading verifier for cryptographic protocols. Our results indicate that compositional verification by typechecking with refinement types is more scalable than the best domain-specific analysis currently available for cryptographic code.