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On the Possibility of Key Agreement Using Variable Directional Antenna

This paper evaluates security of the key agreement system for wireless networks proposed recently by Aono et al. This system exploiting the reciprocal property of radio communication channel shows a high potential for providing unconditional security where protection is provided even against an eavesdropper with unlimited computing power. However, a rigorous security analysis was missing there. In this work, we move towards it: we define and compute the information measures characterizing for the legitimate parties and the eavesdropper their uncertainty about the generated key. Furthermore, we show a method for choosing parameters of this system such that the parties succeed in generating a common key while the adversary is ignorant about it with high probability. We also point out that the system needs strengthening using privacy amplification in order to achieve unconditional security under some reasonable (as we shall argue) assumptions: a) that an eavesdropper did not intercept more information on the key candidates than he is expected to; and b) truly random number generation. To illustrate our results, we provide a number of experimental measurements (performed in the typical wireless network environment) which allow us to conclude that using this system one can achieve unconditionally secure key agreement as long as the eavesdropper is not located too close (at the order of tens of centimeters) to the players.