I think the battery/memory/computation issues work out fine. Remember, the normal alternative is heavy-duty crypto like AES or a public-private key pair. This i

I think the battery/memory/computation issues work out fine. Remember, the normal alternative is heavy-duty crypto like AES or a public-private key pair. This is just XOR. Write memory wearout is not an issue. You can do this with write-once memory, so you don't even need flash. Manufacture the device with FF bytes, load keys into it by clearing bits, and then dispose of keys by clearing the rest of the bits. The same could apply to the car, or that could be different. Billions of keys could fit on a key fob. Wearing out the car door hinges will happen long before wearing out the key fob storage. Timing... well I don't know, but common ethernet chips have registers that tell the OS how long the cable is. Cars also ship with radar that has much tighter time measurement requirements.