The naive solution would be to make it illegal to modify the communication system of your car, but wouldn't just making it illegal to broadcast false data be enough? And then if an accident is caused because of that, you can tack on some other charge like an autonomous version of reckless driving.

A better solution would be to provision each car with its own key pair and the car maker's root CA just before it rolls off the factory floor, that way it can constantly beam 2 signals:

- a signed (regulator-mandated) signal for cars from other manufacturers;

- an encrypted signal intended for cars from the same manufacturer.

The first signal would be signed by the root CA of the car maker to indicate that its communications can be trusted.

The encrypted signal would of course carry an information-rich payload for coordinating a convoy to travel at the same speed, brake, signal a turn, etc simultaneously, for instance.

Signed messages only ensure that they weren't modified by a MITM. If the attack controls the sending vehicle, the signature doesn't help.

Tamper proof modules that store the key? Like Secure Enclave?