Concealing CAN Message Sequences to Prevent Schedule-based Bus-off Attacks

Abstract: This work focuses on eliminating timing-side channels in real-time safety-critical cyber-physical network protocols like Controller Area Networks (CAN). Automotive Electronic Control Units (ECUs) implement predictable scheduling decisions based on task level response time estimation. Such levels of determinism exposes timing information about task executions and therefore corresponding message transmissions via the network buses (that connect the ECUs and actuators). With proper analysis, such timing side channels can be utilized to launch several schedule-based attacks that can lead to eventual denial-of-service or man-in-the-middle-type attacks. To eliminate this determinism, we propose a novel schedule obfuscation strategy by skipping certain control task executions and related data transmissions along with random shifting of the victim task instance. While doing this, our strategy contemplates the performance of the control task as well by bounding the number of control execution skips. We analytically demonstrate how the attack success probability (ASP) is reduced under this proposed attack-aware skipping and randomization. We also demonstrate the efficacy and real-time applicability of our attack-aware schedule obfuscation strategy Hide-n-Seek by applying it to synthesized automotive task sets in a real-time Hardware-in-loop (HIL) setup.