An algorithm for strengthening state invariants generated from requirements specifications

In earlier work (Jeffords and Heitmeyer, 1998) we developed a fixpoint algorithm for automatically generating state invariants, properties that hold in each reachable state of a state machine model, from state-based requirements specifications. Such invariants are useful both in validating requirements specifications and as auxiliary lemmas in proofs that a requirements specification satisfies other invariant properties. This paper describes a new related algorithm that strengthens state invariants generated by our initial algorithm and demonstrates the new algorithm on a simplified version of an automobile cruise control system. The paper concludes by describing how the two algorithms were used to generate state invariants from a requirements specification of a cryptographic device and how the invariants in conjunction with a theorem prover were used to prove formally that the device satisfies a set of critical security properties