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number sequences are indispensable to implement infor-mation theoretically secure communications.In OTP encryption, the same size of true random number sequences as that of the transmitted data is neces-sary. However, it is suciently possible to minimize the amount of random number sequences enough to imple-ment the secure communication despite a small capacity of memory, because the ight time of drones is so limited. Also, the delay time in calculation process is very small in encryption and decryption because only XOR between the key and data or cryptogram is applied, so it is possible to implement a very light physical circuit constitution with a fast encryption process at low cost. Stealing encryption keys by unauthorized drone can be eliminated by a device authentication using true random number sequences, though the true random number sequences should be shared with communication terminals in advance.is device authentication is an application of message authentication where a device is authenticated as a client when the Message Authentication Codes*1 between com-munication devices derived from the shared message (a true random number sequence) by using the Hash Function*2 coincide (Wegman-Carter authentication method [4]). Moreover, both the shared message and the Hash Function are generated by true random number se-quences and the device authentications are conducted by both devices to implement a strongly secure function to prevent spoong.3Video data relaysIn a camera drone equipped with a video camera, a cryptogram of a packet (a bit sequence) of video data from the camera is generated by XOR with the encryption key and transmitted to a relay drone. e relay drone receives the encrypted video data with a directional antenna and relays the data to a ground station without decryption. e received cryptogram is decrypted by the encryption key at the ground station (Fig. 1).Low-cost commercial Wi-Fi devices (Air Station Pro WAPS-AG300 H: directional sectored-antenna WLE-HG-DA/AG with horizontal pattern 60 ± 5°, vertical pattern 65 ± 5°, is attached: comply with IEEE 802.11 b/g, made by Bualo), which does not require a radio license, are used for this system. Implementation of secret video data trans-mission in an area out of radio wave attainment range using an inexpensive and easy method with the general outdoor Wi-Fi frequency (2.4 GHz band) (Fig. 2).e camera drone for taking video and the ground station share a true random number sequence as the en-cryption key in advance to provide communication with perfect secrecy by using OTP encryption for each packet of control communication or data communication. As a loss of data occurs frequently in drone communications due to large variation of properties of channels, a new system is necessary to synchronize a large amount of en-cryption keys accurately between the drone and the ground station and to update it. We developed a technology to transmit a key synchronization code at an optimized packet interval depending on the channel properties. is technology makes it possible to continue to transmit new video data taken by the camera with low latency, while suppressing deterioration of data transmission eciency and implementing a key synchronization.FiF1　Relaying data by a drone with perfect secrecyFiF2　Components of the system(Left: Camera drone, Center: Relay drone, Right: Ground station)Relay droneGround stationCamera droneImage3Encryption keyPhysical random number generator00001111Table of true random numbers101100110011100110101101011…..101100110011100110101101011…..Video dataImage 1Image2Image 3⊕10110011=Cryptogram10111100PlaintextImage1Image3Image 2Image 1Data loss00001111⊕10110011=10111100-Detectionofdataloss-KeysyncronizationImage2Image 3 is surely reproduced by key synchronization,even if there is a data loss in image 2.Table of true random numbersCryptogramEncryption keyPlaintext(Directional) antenna for communication between dronesWi-Fi router for communication between dronesTransmittingvideo data to relay droneTransmitting video data to ground stationCamera droneRelay droneGround stationOpposite SideWi-Fi router/antennafor communication between relay drone and ground stationCCD camera(Directional) antenna for communication between dronesWi-Fi router for communication between drones*1Message Authentication Code (MAC) is a small fixed length code to authenti-cate that the transmitted message is not falsified. MAC is obtained by calculat-ing a message using the Hash Function.*2A Hash Function is a one-directional function that outputs code (bit sequence) of a small fixed length from input data. It always outputs the same code if the input data is the same.3 Quantum Key Distribution Network42　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)