The technology of generating and sharing the key as the representative application of smart antennas is introduced. This scheme is based on the reciprocity theorem of radio wave propagation between the two communication parties. The random and intentional change of antenna directivity that is electrically changed by using such an ESPAR antenna as variable directional antenna is more effective for this scheme, because the propagation environment can be undulated intentionally and the reproducibility of the propagation environment can be decreased. In this chapter, experimental results carried out at many environments are described. From these results, this system has a potential to achieve the “unconditional security.”
Key Generation System Using The Fluctuation Of Radio Waves
In this section, to achieve “information-theoretic (unconditional) security” in encrypted wireless communication, adopting the key generation method that the fluctuation of the radio wave channel response is used is the most suitable.
Smart antenna like an ESPAR antenna (Ohira & Cheng, 2004) is effective to an intentional fluctuation of the radio waves. This fluctuation of radio waves and the reciprocity theorem of radio wave propagation are the key technology to achieve security.