Formalized timed automata

Abstract

Timed automata are a widely used formalism for modeling real-time systems, which is employed in a class of successful model checkers such as UPPAAL. These tools can be understood as trust-multipliers: we trust their correctness to deduce trust in the safety of systems checked by these tools. However, mistakes have previously been made. This particularly regards an approximation operation, which is used bymodel-checking algorithms to obtain a finite search space. The use of this operation left a soundness problem in the tools employing it, which was only discovered years after the first model checkers were devised. This work aims to provide certainty to our knowledge of the basic theory via formalization in Isabelle/HOL: we define themain concepts, formalize the classic decidability result for the language emptiness problem, prove correctness of the basic forward analysis operations, and finally outline how both streams of work can be combined to show that forward analysis with the common approximation operation correctly decides emptiness for the class of diagonal-free timed automata.