Transition Invariants Revisited: Termination Witnesses and Their Validation

Abstract Whenever automatic software verifiers determine that a program fulfills or violates its specification, they are expected to produce also a witness that justifies the verdict. This allows a third party to independently validate the verdict and the arguments from which it was derived, increasing trust in the results. The current standard exchange format for witnesses in software verification does not support program termination. To fill this gap, we propose an extension of the witness format that is based on transition invariants as a general and effective formalism. We justify this by (a) proving that transition invariants can encode other popular termination arguments, such as ranking functions, and (b) providing three different validation approaches for transition invariants, which together can validate most of the produced witnesses. Our approach based on transition invariants was integrated into version 2.1 of the exchange format for verification witnesses, our experiments show that the new witnesses can be effectively validated and that validation is often more efficient than verification, and the software-verification community has adopted the format already for SV-COMP 2026.