The issues of privacy and security have important role in widely-used wireless communication systems. Traditionally, security is recognized as an issue of cryptographic protocols (e.g. RSA and AES) on upper layer. On the other hand, there exist theoretical and practical contributions that utilize some properties of the physical layer for a security measure. They exploit the randomness of communication channels for secret key generation and secret communication. These techniques have information theoretical security [1, 2]. Wireless communication is a suitable field of application because of its fading phenomenon. Thus secret key generation methods utilizing the fluctuation of channel characteristics are proposed in [3, 4, 5, 6]. They exploit the reciprocal property of the radio communication channel which provides similar channel-properties for two communicating entities while it is difficult for an eavesdropper at distant location to obtain similar channel response. It is an excellent approach to solve the problems of the key-distribution since it can provide an one-time key whenever it is needed. Security can also be improved by a hybrid cryptography which uses generated secret keys in the traditional cryptographies.
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