I don't think there's any reason to rely on a one-way channel. If the keyfob is in range to transmit, then it is definitely in range to receive (due to reciproc

I don't think there's any reason to rely on a one-way channel. If the keyfob is in range to transmit, then it is definitely in range to receive (due to reciprocity); plus the car has a lot more electricity to spare. There is fundamentally no reason why there can't be full authenticated PKI for keyfobs. It certainly isn't going to happen as part of a recall of current models, though.