Exploiting refactoring in formal verification

In previous work, we introduced Echo, a new approach to the formal verification of the functional correctness of software. Part of what makes Echo practical is a technique called verification refactoring. The program to be verified is mechanically refactored specifically to facilitate verification. After refactoring, the program is documented with low-level annotations, and a specification is extracted mechanically. Proofs that the semantics of the refactored program are equivalent to those of the original program, that the code conforms to the annotations, and that the extracted specification implies the program's original specification constitute the verification argument. In this paper, we discuss verification refactoring and illustrate it with a case study of the verification of an optimized implementation of the Advanced Encryption Standard (AES) against its official specification. We compare the practicality of verification using refactoring with traditional correctness proofs and refinement, and we assess its efficacy using seeded defects.