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this post was submitted on 23 May 2024
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The only thing I can think of is having incredibly tight timing on a challenge/response. With ~10 nanosecond level precision, it's not physically possible for em waves to travel more a few meters before the time is up.
Potentially better idea, add a gyroscope to the key fob, and stop broadcasting after the fob is perfectly still for some threshold. That way when you set it down inside it can’t be relayed, but if it’s in your pocket, it won’t remain perfectly still, and will start transmitting. Could also add an IR blaster to detect if you set it down in the car. Battery life would start to become a bigger issue, but I think solutions to these problems could be engineered.
Problem with that is that it really only covers the keys sitting on a nightstand situation. You could still get your car stolen while you're shopping or in a restaurant.
Relaying a key signal 20 ft when you know the key is there isn’t too tricky, like when you’re home. But I would propose that trying to relay a signal across hundreds of feet, like a busy mall or store, when you’re not even sure the owner is there is quite another thing. You can also require that the IR blaster is in the car before starting. There’s also a technology Google has been using for a while now where the device (car) would emit a constant ultrasonic signal for the other device (key) to pick up on to determine if they are close to each other. Something that could be done through clothing, but not easily relayed.
How about a simple faraday shield for the key fob?
Might as well have a push button instead. Having it work from your pocket without interaction is what makes a fob different and should be a design requirement.
How about making the signal so weak that you have to put the key inside a hole in the car for it to work?