On the Formalization of Decentralized Contact Tracing Protocols

We present a preliminary formalization based on transition systems of decentralized contact tracing protocols for smart devices equipped with Bluetooth trasmitters. In our model the behaviour of individual users, via their app, is modelled as a timed automata with a local unbounded memory. Protocol configurations consist of the current state of a shared server and a finite set of local states containing the states of individual users. The transition system models the interaction between devices in the same physical location and between a sigle device and the shared server. In the paper we address different research directions concerning semi-automated verification based on automated reasoning tools of the considered class of protocols, theoretical issues related to the expressiveness of the resulting class of formal models, and data-driven analysis of the logs collected on the server as well as on user devices.