This paper presents techniques developed to check program equivalences in the context of cryptographic software development, where specifications are typically reference implementations. The techniques allow for the integration of interactive proof techniques (required given the difficulty and generality of the results sought) in a verification infrastructure that is capable of discharging many verification conditions automatically. To this end, the difficult results in the verification process (to be proved interactively) are isolated as a set of lemmas. The fundamental notion of natural invariant is used to link the specification level and the interactive proof construction process.