Obfuscated Priority Assignment to CAN-FD Messages with Dependencies: A Swapping-based and Affix-Matching Approach

CAN-FD (CAN with flexible data rate) has been developed to support automated driving as a high-bandwidth version of the conventional CAN (controller area network) bus protocol. Due to the complexity of the emerging automotive functionalities, there exist dependencies between the tasks and thus also between the CAN-FD messages. The current industrial practice is that the same application has exactly the same message transmission flow (i.e., the same ordered sequence of messages to be transmitted) across all vehicles. This renders large-scale attacks possible and potentially leads to millions of vehicles to be recalled, as one vehicle being compromised exposes all the others. To address this issue, an application could have different (obfuscated) message flows on individual vehicles. The challenge is to find a large number of available flows (i.e., flows that respect dependencies and meet application deadlines) within short time. For this purpose, we propose a novel priority assignment approach, which assigns the ordered positions in a flow (named priorities) to the messages. It dynamically generates new valid flows (i.e., flows with only dependencies respected and deadlines not considered) by message swapping, instead of exploring all valid flows as in the existing approaches. We apply pruning through affix-matching to further enhance the efficiency. That is, the prefix, infix, and suffix are all matched when determining whether a certain flow should be discarded without evaluating its availability, aiming for lower false positive rate (FSR) and false negative rate (FNR) than adfix-matching (only prefix and suffix are matched) in the state-of-the-art approach. Experimental results show that the proposed approach dominates the state-of-the-art approach, in the number of available flows found (up to 79x) and time consumption (up to 200x), most notably when the proportion of available flows is small. This work is an important step for obfuscated priority assignment to be deployed on practical CAN-FD messages.