You don't need to sign all messages. Only those coming from the critical component, like the key fob, and the only parts that need to check signatures are the c

You don't need to sign all messages. Only those coming from the critical component, like the key fob, and the only parts that need to check signatures are the critical ones from the perspective of theft. Once you replace the key fob, you must reprogram your engine ignition. What prevents a thief from reprogramming the engine ignition for some other key? The same thing that prevents the thief from starting ignition manually: it's buried in the car. Notice that in this case, thieves rely on CAN buses running in easily accessible places. If the thief can access your car's ignition directly, you can't stop them anyway.