Freshness Value Management (FVM): Anti-Replay Counter Synchronization over CAN-FD

A cryptographic MAC alone cannot prevent replay attacks: an attacker recording a valid 'Unlock Door' frame could retransmit it at will.

The Freshness Value Manager (FVM) defeats replay attempts by ensuring that every transmitted frame incorporates a unique, monotonically increasing counter. The FVM structure typically deploys a 64-bit counter divided into: • Trip Counter (incremented on ignition cycles). • Reset Counter (periodically broadcast by a master gateway). • Message Counter (incremented on each frame transmission). Only the lowest bits are transmitted over the bus; the receiver reconstructs the full 64-bit value using local state tracking, ensuring zero vulnerability to stale or repeated frame injections.