SUMMARYAs a measure for preventing eavesdropping on landmobile communication, this paper studies an application of a private key agreement based on the propagation characteristics of channel to a practical system without key delivery. The paper proposes a private key agreement based on the time-varying frequency characteristics suitable for OFDM. In the proposed method, the time-varying frequency characteristics are alternately measured on the basis of the channel reciprocity and generation of the key and key agreement is attempted. Also, in the proposed method, for prevention of errors in key generation, measurement time difference compensation of the channel, noise reduction by the synchronous addition process, and key disagreement correction by an algebraic decoding method are applied. Further, in order to evaluate the safety of the key, a method for evaluation of independence of the key is discussed. As a system mode for evaluation of the performance of the proposed method, a model is constructed for study that is based on the specification of the IEEE802.11a wireless LAN using the OFDM, so that verification of an operation can be carried out in a more realistic environment. Numerical simulation indicates that it is possible to share the private key in a realistic setting with an SN ratio of 15 dB by means of various measures for prevention of key disagreement. Also, an evaluation confirms that independence is sufficiently maintained.

Schemes for secret key agreement based on the radio transmission path characteristics between authorized stations have been proposed as anti-eavesdropping measures in land mobile radio. In this paper, we propose a new scheme for secret key agreement. It compares the strengths of the signals received at two antennas to produce a bit of the keys. It does not use a threshold which is used in the conventional schemes. We evaluate the performance of the proposed scheme by computer simulation.

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