> Am I missing something or is the fix a little computer in the fob to cryptographically sign a one-time challenge sent by the car? I mean, RSA isn't that hard, is it?
The problem has nothing to do with cryptography. Given radio signals can be relayed at will, and there's no way of knowing so (short of a way to measure quite small latencies), there needs to be a proper way to bind the cryptographic exchange to the person pulling the door open. Relay attacks (which do nothing to exploit cryptographic exchanges -- just relay messages fast) exploit that lack of binding.
The problem has nothing to do with cryptography. Given radio signals can be relayed at will, and there's no way of knowing so (short of a way to measure quite small latencies), there needs to be a proper way to bind the cryptographic exchange to the person pulling the door open. Relay attacks (which do nothing to exploit cryptographic exchanges -- just relay messages fast) exploit that lack of binding.