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number sequence than that from Alice because indepen-dent noise was added to the former. Actually, the random number sequences of Alice and Eve expressed by Bob’s random number sequence are xn = yn ⊕ en and zn = yn ⊕ en ⊕ dn. at is, even if Eve wiretaps under a lower noise condition than that of Bob’s, it is possible to make the condition disadvantageous for Eve by reverse information reconciliation. Finally, the random number sequence equivalent to the information leaked against Eve is com-pressed in privacy amplication [33][34]. is process re-moves the amount of information leaked to Eve. For this purpose, a trap door function is applied so that input in-formation is hard to estimate from the output.Here, the sharable key rate by secret key agreement can be derived utilizing the security analysis that Renner conducted in the context of QKD. Renner [35] showed that the sharable key rate using reverse reconciliation with any techniques and privacy amplication based on the Universal 2 hash function [36] could be derived as follows:)];();([max)(ZYIYXICxPKX .Here, I(X;Y) is the information amount that can be shared in information reconciliation, and I(Y;Z) represents leaked information to be removed. It shows that it is pos-sible to compose a practical secret key agreement protocol using practical error correction code such as LDPC or the Universal 2 hash function.4Experiment of channel estimation using free space optical communication testbedAs we discussed above, the performance of physical layer cryptography is measured by the secrecy capacity for wiretap channel coding and by (lower limit) of the secret key capacity for secret key agreement. us, in order to design appropriate code, Alice and Bob estimate the prob-ability distribution of the channel in advance, then estimate and reconstruct the probability model from the data, and calculate these capacities. However, there exist some di-culties. First of all, the refractive index of the atmosphere temporally changes as temperature varies. is eect is called atmospheric uctuation and can cause variation of the received intensity on a time scale of several millimeter seconds and shi of beam direction. Hence, performance evaluation considering the eect of atmospheric uctuation is dicult.In addition, there is a diculty in the estimation of the leaked information against Eve.erefore, in order to experimentally solve the problem, the Quantum ICT Advanced Development Center estab-lished a free space optical communication testbed of 7.8 km in length, connecting the University of Electro-Communications (UEC) and NICT as shown in Fig. 3. In this testbed conguration, Alice is a dome on the top of a building of the UEC, Bob is the receiving system installed on the 6th oor of a building of NICT, and Eve is a termi-nal set on the rooop of the same building.In this article, we report on the experiment imple-mented on November 17, 2015 [37][38], with the purpose of revealing the relation between the eect of weather conditions and physical layer cryptography, by evaluating the transmission performance of physical layer cryptogra-phy under various weather conditions. In this experiment, Alice transmitted a pseudorandom number sequence with a cycle of 215-1 by on-o keying. e light source was an eye-safe laser of wavelength 1,550 nm and output power 100 mW. As the beam diverging angle of the transmission lens was about 1 mrad, the radius of the beam at NICT became about 8 m. Due to the conguration constraint of the experiment, the center of the beam was adjusted so that Eve could receive the light by approaching the beam center from Bob to Eve by about 1 m. Bob and Eve indepen-dently measure the power of light detected by a PIN photodiode detector and avalanche photodiode detector using a telescope of about 100 mm diameter to collect light beams. In addition to the dierence in sensitivity of the FiF3Schematic view of Tokyo FSO Testbed [37]. ©OpenStreetMap contributors, CC-BY-SA.(Koganei)EveBob(Chofu)353-3 Channel Estimation Experiment for Physical Layer Cryptography in Free-space Optical Communication