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theory, and thus research is underway to apply this physi-cal layer cryptography to certain communication systems. For example, for radio wave wireless communication [7]–[12], there could be secret key agreement that extracts the key from temporal random variation of the received intensity caused by multipath reection. On the other hand, in the case of quantum key distribution (QKD) [13]–[15] pro-posed before Maurer proposed secret key agreement, the feasibility of detecting the existence of an eavesdropper and estimating the amount of leaked information is proved mathematically and physically from the error rate, by transmitting a random number bit aer being coded to photons in a special quantum state. erefore, QKD allows establishing of secure keys that will prevent eavesdroppers with high computational ability from accessing communi-cations assuming that devices have no defect leading to information leakage. QKD is the only physical layer cryp-tography that is realized as is 2017 for which verication experiments for ground optical ber network have been demonstrated[16]–[20] and which is commercially avail-able [21]. On the other hand, there are severe limitations such that the transmissible distance / key generation rate is 50 km and 1 Mbps, respectively, for optical ber [22]. us, there still remain some technical problems in ap-plication.Now, at the Quantum ICT Advanced Development Center, we are promoting research on physical layer cryp-tography in classical free space optical communication as a complementary technique to solve the problem of throughput in QKD mentioned above. As free space optical communication is conducted in line-of-sight within a nar-row beam extent, an eavesdropper is obliged to wiretap at the edge of the beam. Hence, the upper limit of the amount of information leaked to the eavesdropper can be esti-mated based on the physical condition, and utilization of the physical layer is therefore potentially feasible. Moreover, realization of condential communication over a long distance at high speed of around several Gbps from the property of free space optical communication can be ex-pected. erefore, physical layer cryptography is expected to be applied to satellite communication where high-speed key generation is dicult by present QKD, drones that require safety technology, and various kinds of IoT devices.On the other hand, as for the security of the physical layer in free space optical communication and the method of estimating the amount of leaked information, denitive discussion has not emerged yet, which is dierent from the case of QKD whose robust security has been proved math-ematically and physically. Actually, on the contrary to sucient theoretical studies [23]–[28], we do not know of any practical research in which the practical composition of devices and attacks to be supposed are fully discussed.us, considering the present situation, we have been researching the realization of physical layer cryptography for free space optical communication by an experimental approach such as a communication channel estimation experiment where the communication channel and amount of leaked information are estimated based on data obtained from the free space optical communication testbed between the University of Electro-Communications (UEC) and NICT established by NICT in collaboration with UEC.e purpose of this article is to explain the outline of basic technical items of physical layer cryptography, then to roughly explain the knowledge obtained from experi-ments we have carried out. First, in Sections 2 and 3, we explain the principle of the wiretap channel coding and secret key agreement, both of which are representative models of physical layer cryptography. en, in Section 4, we explain the channel estimation experiment that NICT carried out.2Confidential communication by wiretap channel coding2.1Channel codinge purpose of this section is to explain wiretap chan-nel coding that is the most basic model in physical layer cryptography. We start with explaining the channel coding. In this paper, information is represented by a sequence of 0 and 1 bits, and the amount (length) of information is expressed in terms of bits.Here, we assume that an error where a certain bit changes to another bit occurs during communication from the sender (Alice) to the receiver (Bob) via wireless or a communication channel. Alice needs to take countermea-sure to transmit the correct information to Bob. To this end, Alice adds redundant information to a message to be sent in order that Bob can retrieve the original message using the redundant information. For example, in the case where Alice transmits each bit adding two copies of the bit (0 → 000 or 1 → 111), Bob can correct the error by a major-ity decision even when one of the three serial bits changed. us, the process of adding redundant information is called channel encoding and the bit sequence of a message with redundant information is called a code word. e operation to retrieve a message from the bit sequence that Bob re-3 Quantum Key Distribution Network32　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)