F7 is an enhanced typechecker for the F# programming language. F7 supports static checking of properties expressed with refinement types. Our motivation is to check various security properties of F# implementation code by typing.
We present the design and implementation of a typechecker for verifying security properties on the source code of cryptographic protocols and access control mechanisms. The underlying type theory is a lambda-calculus equipped with refinement types for expressing pre- and post-conditions within first-order logic. We derive formal cryptographic primitives and represent active adversaries within the type theory. Well-typed programs enjoy assertion-based security properties, with respect to a realistic threat model including key compromise. The implementation amounts to an enhanced typechecker for the general purpose functional language F#; typechecking generates verification conditions that are passed to a SMT solver. We describe a series of checked examples. This is the first tool to verify authentication properties of cryptographic protocols by typechecking their source code.