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Distance bounding protocols are cryptographic protocols that enable a verifier V to establish an upper bound on the physical distance to a prover P.
They are based on timing the delay between sending out challenge bits and receiving back the corresponding response bits. The delay time for responses enables V to compute an upper-bound on the distance, as the round trip delay time divided into twice the speed of light. The computation is based on the fact that electro-magnetic waves travel nearly at the speed of light, but cannot travel faster.
Distance bounding protocols can have different applications. For example, when a person conducts a cryptographic identification protocol at an entrance to a building, the access control computer in the building would like to be ensured that the person giving the responses is no more than a few meters away.