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A spread of short-range and infrastructure-less wireless communication devices increases opportunities for temporarily exchanging information in a face-to-face style, called wireless face-to-face communication. In such wireless communication, a critical issue with secure information exchange is encryption key sharing without the aid of any centralized administration. In this paper, we propose a novel key sharing technique for wireless face-to-face communications. The main idea is to generate the same encryption key on a communication pair using received signal strength indicators (RSSI) from observable wireless LAN appliances, rather than distribute a prepared key between the pair. The generation for the same key is realized by utilizing the similarity of RSSI observed within a small area. Furthermore, we examine the successful ratio for generating the same key and consider the security level for the proposed method.