Optimizing Truncated MAC Lengths (24-bit vs 32-bit) for Minimal Bus Load without Sacrificing Security

Abstract: Balancing cryptographic collision resistance against CAN-FD frame payload efficiency in high-frequency drivetrain applications.

1. Probability of False Positive Verification In a truncated MAC system of length L bits, an attacker randomly guessing authentication codes has a success probability of 2^(-L). For L = 24 bits, the probability is 1 in 16.7 million. Combined with rate-limiting diagnostic trouble codes (DTC) that trigger after 3 consecutive verification failures, brute-force injection attacks become practically impossible.

2. Frame Packing Efficiency By right-sizing the MAC to 24 bits and the transmitted freshness snippet to 8 bits, security metadata requires only 4 bytes of the 64-byte CAN-FD payload, leaving 60 bytes for pure application signals and reducing bus utilization overhead to less than 6.5%.