Abstract. We have constructed a mechanically verified prototype implementation of a model checker for timed automata, a popular formalism for modeling real-time systems. Our goal is two-fold: first, we want to provide a reference implementation that is fast enough to check other model checkers against it on reasonably sized benchmarks; second, we strive for maximal feature compatibility with the state-of-the-art tool Uppaal. The starting point of our work is an existing highly abstract formalization of reachability checking of timed automata. We reduce checking of Uppaal-style models to the problem of model checking a single automaton in this abstract formalization, while retaining the ability to perform on the fly model-checking. Using the Isabelle Refinement Framework, the abstract specification of the model checker is refined, via multiple intermediate steps, to an actual imperative implementation in Standard ML. The resulting tool is evaluated on a set of standard benchmarks to demonstrate its practical usability.

Prior research has shown how to construct a mechanically verified model checker for timed automata, a popular formalism for modeling real-time systems. In this paper, we shift the focus from verified model checking to certifying unreachability. This allows us to benefit from better approximation operations for symbolic states, and reduces execution time by exploring fewer states and by exploiting parallelism. Moreover, this gives us the ability to audit results of unverified model checkers that implement a range of further optimizations, including certificate compression. The resulting tool is evaluated on a set of standard benchmarks to demonstrate its practicality, using a new unverified model checker implementation in Standard ML to construct the certificates.

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