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A secure communication scheme that uses the random fluctuation of the natural environment of communication channels is studied and an improved scheme for the secret key generation is proposed. We have been developing a novel secret key generation and agreement scheme that uses the fluctuation of channel characteristics with an Espar (Electronically Steerable Parasitic Array Radiator) antenna. This antenna consists of a single central active element surrounded by parasitic elements loaded with variable reactors. Using an Espar antenna, we can increase the fluctuation of the channel characteristics and acquire the undulated RSSI (Received Signal Strength Indicator) profile in a short time to generate a randomized secret key. We acquire more randomized and stronger secret keys generated from the interleaved RSSI values in this scheme. As a result of experiments on the prototype system in which the IEEE802.15.4/ZigBee T M wireless PAN (Personal Area Network) chip is mounted, we confirmed that the probability of success for key generation was estimated to exceed 99.998% when 128-bit secret keys are changed every two seconds and that the generated secret keys were sufficiently random to pass the FIPS (Federal Information Processing Standards) PUB 140-2 statistical test for random numbers.

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SUMMARYWhen the transmission and reception locations of radio waves are different, the correlation of the radio wave propagation characteristics decays quickly. On the other hand, reciprocity of the radio wave propagation holds between two stations that can communicate with each other. Therefore, by making use of the information on the variation of the propagation path, it is possible to generate and share a secret key that cannot be eavesdropped by a third party at a different receiving location. Hence, a secret key generation and agreement system that does not require key management and key distribution can be realized. Also, the ESPAR (Electronically Steerable Parasitic Array Radiator) antenna, under development as a commercial antenna with a variable radiation pattern, can control its directive characteristics by the use of varactor diodes to change the capacitance. Therefore, it is possible to intentionally change the radio propagation characteristics significantly. This paper reports successful test development of a wireless secret key agreement system making use of the fluctuation of the radio propagation characteristics by means of an ESPAR antenna. Further, in a verification experiment using the test system, the key generation success probability was more than 99.998% when a 128-bit secret key was modified every 3 seconds. It is also confirmed by the correlation characteristics with an eavesdropping station that more than 7.19 × 10 4 years would be required for the eavesdropping station to decipher the secret key.