Distance-bounding protocol
Encyclopedia
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 a 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.
distance bounding protocol is very sensitive to even the slightest processing delay. 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. This means that even delays on the order of nanoseconds will result in significant errors in the distance bound (a timing error of 1 ns corresponds to a distance error of 15 cm).
Because of the extremely tight timing constraints and the fact that a distance bounding protocol requires that the prover apply an appropriate function to the challenge sent by the verifier, it is not trivial to implement distance bounding in actual physical hardware. Conventional radios have processing times that are orders of magnitudes too big, even if the function applied is a simple XOR.
In 2010, Rasmussen and Capkun devised a way for the prover to apply a function using pure analog components http://www.syssec.ethz.ch/research/freqdb.pdf. The result is a circuit whose processing delay is below 1 nanosecond from receiving a challenge till sending back the response. This processing delay translates into a maximum potential distance error of 15cm.
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, 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.
RF Implementation
The distance bound computed by a Radio frequencyRadio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
distance bounding protocol is very sensitive to even the slightest processing delay. 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. This means that even delays on the order of nanoseconds will result in significant errors in the distance bound (a timing error of 1 ns corresponds to a distance error of 15 cm).
Because of the extremely tight timing constraints and the fact that a distance bounding protocol requires that the prover apply an appropriate function to the challenge sent by the verifier, it is not trivial to implement distance bounding in actual physical hardware. Conventional radios have processing times that are orders of magnitudes too big, even if the function applied is a simple XOR.
In 2010, Rasmussen and Capkun devised a way for the prover to apply a function using pure analog components http://www.syssec.ethz.ch/research/freqdb.pdf. The result is a circuit whose processing delay is below 1 nanosecond from receiving a challenge till sending back the response. This processing delay translates into a maximum potential distance error of 15cm.