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realizes a secrecy outrage probability of 0 aer sunset when atmospheric uctuations are stable. is fact agrees with the discussion on time variation of the secrecy rate just before and aer sunset shown in Fig. 4.Such discussion based on experimental data where at-mospheric uctuation aects physical layer cryptography is, as far as we know, unprecedented. We have obtained important knowledge for future studies on physical layer cryptography in free space optical communication and development of protocols.5SummaryIn this paper, we presented the outline of a technique for physical layer cryptography from the information theo-retical aspect and the research on the eect of various at-mospheric conditions on the physical layer cipher using experimental data. is study has been undertaken by the Quantum ICT Advanced Development Center to put the technique into practical use. Now, we are engaged in de-velopment of a secret key agreement based on the technique of QKD we experienced and in study on a novel key dis-tribution protocol utilizing the properties of optical com-munication.e promising application eld for the technology is laser communication between satellites and earth terminals in which key generation at practical speed is dicult to realize by QKD. Also, an application eld that requires low-cost and high-speed condential communication such as last-mile communication that connects vehicles or basic networks and users would be an important candidate. Moreover, a multi-layer security protocol and exible cryptography system that can switch to QKD according to user needs could be supplied by combining modern cryp-tography operated on a dierent OSI layer. As mentioned in the introduction, physical layer cryptography in free space optical communication has not been veried and reported yet. So, it is very important to verify such technol-ogy for the rst time in the world not only for development of communication systems but also for academic research.However, there are still many problems to be solved. Although the beam used for free space optical communica-tion is narrow, the wireless communication is still easily accessible to eavesdroppers. erefore, Eve can take various wiretapping measures, such as wiretapping or detection of reected or scattered light in a position away from the center of the beam or wiretapping from a small instrument. It is not an exaggeration that prevention of wiretapping by Eve by monitoring, etc. is the rst priority in our tasks because there is no denitive and eective measure for estimation of the worst value of the amount of leaked in-formation. At present, we are endeavoring to solve this problem involving physical layer cryptography in free space optical communication, by studying methods to estimate the ability of an eavesdropper with certain detection sys-tems, as well as developing the practical protocol mentioned above.Acknowledgmentsis study was commissioned from Impulsing Paradigm Change rough Disruptive Technologies Program (ImPACT) designed by the Council for Science, Technology and Innovation, via JST. We thank Prof. Takao AOKI, Department of Applied Physics, Waseda University, Assoc. Prof. Ryutaro MATSUMOTO, School of Engineering, Tokyo Institute of Technology, and Prof. Yoshihisa TAKAYAMA, Information and Telecommunication Engineering, Tokai University, for their kind cooperation.ReferenceR1A. D. Wyner, “The wire-tap channel,” Bell Syst. Tech. J., vol.54, no.8, pp.1355–1387, Oct. 1975. 2I. Csiszár and J. Körner, “Broadcast channels with confidential messages,” IEEE Trans. on Inform. 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