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1IntroductionRecently, information networks are one of the impor-tant infrastructure for our lives as IT technology develops and spreads rapidly, and we receive many benets from them. On the other hand, it is indispensable to protect condential information by use of encryption techniques because cases of transmission of highly condential infor-mation are increasing such as in online shopping or elec-tronic application to public organizations.e security of present networks is protected by so-called “modern cryptography” such as symmetric key cryptography that ensures condentiality of messages or public key cryptography for key establishment and authen-tication. e algorithms of modern cryptography are open to public, so it can be implemented regardless of physical media such as cables or radio waves. Moreover, the secu-rity of modern cryptography is based on mathematics re-lated to the present technological level. For example, as for public key cryptography, the basis of the security is the diculty in solving certain mathematical problem in a realistic timeline using computers, such as prime decom-position of a large composite number. It is relatively easy to ensure security by such a large amount of calculation. However, the security is threatened by the emergence of ecient decryption algorithms or quantum computers. In order to cope with such kind of threats, countermeasures such as extension of the key size or utilizing mathematical problems that are hard to solve even for quantum comput-ers have been developed year by year. Because of the fea-tures listed above, the present cipher is implemented in various devices and systems, that is a technology to support the base of our present society.On the other hand, an authority of information theory, Wyner, proposes a cipher technique that is based on a quite dierent paradigm from the present cipher. e wiretap channel encoding [1][2] he proposed not only corrects errors that occur during the communication process but also realizes condential communication without sharing a key in advance by utilizing noise occurring at an eaves-dropper. In addition, security against an eavesdropper with any computational ability, that is, information theoretical security [3], is veried basing on the unpredictability of the physical phenomenon of noise. Since then, Maurer [4] and Ahlswede [5] independently proposed secret key agreement that is method of key sharing utilizing physical noise by applying the ideas of Wyner. ese techniques are deemed to operate at the most bottom physical layer, although the present cipher is operated at a higher-level layer in the OSI model. erefore, wiretap channel encoding and secret key agreement are called physical layer cryptography.It is astonishing that wiretap channel encoding [1] proposed by Wyner was already published one year before DH key sharing [6] which is a prototype of public key cryptography. However, these methods require estimating the information leaked against the eavesdropper in design-ing the encoding, which deteriorates convenience much compared with modern cryptography that guarantees its security by the computational cost. Hence, these encoding methods were not established as mainstream techniques to be used with current networks. However, they are valuable from the viewpoint of ensuring information security in 3-3 Channel Estimation Experiment for Physical Layer Cryptography in Free-space Optical CommunicationHiroyuki ENDO, Mikio FUJIWARA, Mitsuo KITAMURA, Orie TSUZUKI, Toshiyuki ITOH, Ryosuke SHIMIZU, Morio TOYOSHIMA, Hideki TAKENAKA, Masahiro TAKEOKA, and Masahide SASAKIPhysical layer cryptography is the cryptographic technique which realizes the information-theoretically secure message transmission and key establishment by exploiting physical properties of transmission channels. In this paper, we give an outline of fundamental model for physical layer cryptography and present the main result of channel estimation experiment toward the physical layer cryptography in free space optical communication which is carried out by Quantum ICT Advanced Developed Center.313 Quantum Key Distribution Network