Analyzing CAN's Timing under Periodically Authenticated Encryption

With increasing connectivity, it has become easier to remotely access in-vehicle buses like CAN (Controller Area Network). This not only jeopardizes security, but it also exposes CAN's limitations. In particular, to reject replay and spoofing attacks, messages need to be authenticated, i.e., an authentication tag has to be included. As a result, messages become larger and need to be split in at least two frames due to CAN's restrictive payload. This increases the delay on the bus and, thus, some deadlines may start being missed compromising safety. In this paper, we propose a Periodically Authenticated Encryption (PAE) based on the observation that we do not need to send authentication tags with every single message on the bus, but only with a configurable frequency that allows meeting both safety and security requirements. Plausibility checks can then be used to detect whether non-authenticated messages sent in between two authenticated ones have been altered or are being replayed, e.g., the transmitted values exceed a given range or are not in accordance with previous ones. We extend CAN's known schedulability analysis to consider PAE and analyze its timing behavior based on an implementation on real hardware and on extensive simulations.