Anybody up to date on distance bounding protocols? Is there a well studied implementation that is anywhere near practical?
Given that the speed of light is ~1ns per foot then a total response time greater than (2d + p) where d = max distance in feet, and p = processing time within the keyfob in nanoseconds would provide a bound.
I suspect however that making the keyfob response time consistent might be the hardest part of the check, closely followed by an accurate timing facility within the car.
There is a startup from ETH Zurich [1] which develops technology to make relay attacks impossible. In short, they are developing a method to make the key proove its within a certain distance of the car. Some of the tech behind it: https://arxiv.org/abs/1404.4435
The key fob can't invalidate signals with the car once it's sent since it's too far away, so even if you have distance-bounding on the key fob, it can't tell the car to ignore an otherwise valid signal.