The firmware must be able to create and sign a document with written the signature of the keys inside so that the car's owner can check the result (the car's ma

The firmware must be able to create and sign a document with written the signature of the keys inside so that the car's owner can check the result (the car's maker would publish his public keys). If the hardware of the firmware is tamper resistant you can pretty much be sure that it's unmodified. If the attacker can modify tamper resistant hardware I don't think that the car is your first concern.