The Application of Correctness Preserving Transformations to Software Maintenance

The size and complexity of hardware and software systems continues to grow, making the introduction of subtle errors a more likely possibility. A major goal of software engineering is to enable developers t o construct systems that operate reliably despite increased size and complexity. One approach to achieving this goal is through formal methods: mathematically based languages, techniques and tools for specifying and verifying complex software systems. In this paper, we apply a theoretical tool that is supported by many formal methods, the correctness preserving transformation (CPT), t o a real software engineering problem: the need for optimization during the maintenance of code. We present four program transformations and a model that forms a framework for proof of correctness. We prove the transformations correct and then apply them to a cryptography application implemented in CS+. Our experience shows that CPTs can facilitate generation of more efficient code while guaranteeing the preservation of original behavior. KeywordsReverse engineering, formal methods, public key cryptography, correctness preserving transformation, code optimization.