Information-flow security is important to the safety and privacy of cyber-physical systems (CPSs) across many domains: information leakage can both violate user privacy and reveal vulnerabilities to physical attacks. CPSs face the challenge that information can flow both in discrete cyber channels and in continuous real-valued physical channels ranging from time to motion to (e.g.) electrical currents. We call these hybrid-dynamic information flows (HDIFs) and introduce dHL; the first logic for verifying HDIFs in hybrid-dynamical models of CPSs. Our logic extends differential dynamic logic (dL) for hybrid-dynamical systems with hybrid-logical features for explicit program state representation, supporting relational reasoning used for information flow arguments. By verifying HDIFs, we ensure security even under a strong attacker model wherein an attacker can observe time and physical values continuously. We present a Hilbert-style proof calculus for dHL, prove it sound, and compare the expressive power of dHL with dL. We develop a hybrid system model based on the smart electrical grid FREEDM, with which we showcase dHL. We prove that the naïve model has a previously unknown information flow vulnerability, which we verify is resolved in a revised model. This is the first information flow proof both for HDIFs and for a hybrid-dynamical model in general.

62 pages

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