Distance bounding protocols are cryptographic protocols that enable a verifier V to establish an upper bound on the physical distance to a prover P.[1] They are based on timing the delay between sending out challenge bits and receiving back the corresponding response bits.
This is because any delay introduced, anywhere in the system, will be multiplied by approximately 299,792,458 m/s (the speed of light) in order to convert time into distance.
Conventional radios have processing times that are orders of magnitudes too big, even if the function applied is a simple XOR.
[3] The result is a circuit whose processing delay is below 1 nanosecond from receiving a challenge till sending back the response.
In 2015, the same protocol was modified, prototyped and practically evaluated for ten indoor and outdoor locations.