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does not require resending the data and the next received packet is decrypted abandoning the previous lost packet in order not to miss the movement of the monitored target while continuing to receive the data from the camera drone.5Open-air field experimentsWe conducted an experiment on Wednesday, February 22, 2017, at an open-air test eld in the suburbs of Toyota city in Aichi Prefecture. We simulated a monitoring mis-sion from the sky of a video of a suspicious person (monitored target) taken by a camera drone which is re-ceived by a ground station via a relay drone (Fig. 4).e experiment was conducted several times, for about 15 minutes each time. e camera drone ew around the patrol area behind trees where it could not be seen from the ground station. en, the camera drone transmitted encrypted video data to the relay drone waiting in the air 50-100 meters away from the camera drone. e relay drone received the video data with a directional antenna and relayed the data to the ground station placed about 10 meters away from the relay drone. e ground station decrypted and reproduced the video by using the same encryption key as that used in the camera drone. e ex-periments were conducted assuming that the video data taken by a monitoring camera is transmitted with perfect secrecy.To simulate disconnection of relay, the relay drone moved (le end in Fig. 4) so that communication between the drones was interrupted by the forest when the camera drone moved to the boundary of the experiment area (up-per edge in Fig. 4). Before the experiment, we received permission from Toyota city in Aichi Pref. who managed the area (as for controls of the drone, we assigned one operator for one drone so that any unexpected incidents such as ights out of direct line of sight or unforeseen crashes could be prevented by manual operation at any time).We observed the communication status between the ground station and the camera drone, by taking a video of reproduced video by a handheld video camera and by re-cording the response time of PING*4 sent every one second from the ground station to the video camera. In addition to the reproduced video, the whole view and actual condi-tions of the experiment were recorded by a handheld video camera (see NICT’s press release “Development of relaying video data by a drone with perfect secrecy” (only in Japanese): http://www.nict.go.jp/press/2017/03/22-1.html)e videos were reproduced smoothly in real time at the ground station without drop-frames (loss of image data). Hence, the function of video relay, encryption and decryption of the video data in this component system were veried to work without problems at this experiment. When the relay drone moved to hide behind the forest (coverage hole) to simulate communication disruption, video reproduction at the ground station froze and the PING response disappeared completely. But the PING re-sponse reappeared and the video reproduction restarted again skipping the video taken when the communication was disrupted aer the relay drone moved back to the original position for relaying. From the above, it was conrmed that encrypted data was decrypted by matching the encryption key at both the sender and the receiver with key synchronization code inserted into the video data pe-riodically, even though the transmission of encrypted video data was disrupted.We succeeded in relaying video data without any seri-ous problems, but a phenomenon occurred where repro-duction of the video paused for about 5 seconds when the FiF4Experiment of simulation of monitoring mission using drone implementing data relay system with perfect secrecy. Configuration of ground station — relay drone — camera drone. The inserted image is a received video at the ground station.Figure4.Experiment of simulation of monitoring mission using drone implementing data relay system with perfect secrecy. Configuration of ground station –relay drone –camera drone. The inserted image is a received video at the ground station.Chasing the objectRunning away from camera droneForest areaRelay droneSuspicious person(monitoring target)10mReceived video at ground stationCamera droneVehicleGround stationImage excerpted from CCB20073-C4-58 issued by the Geospatial Information Authority of Japan, and processed.*4PING is standard software installed to verify the reachability of a node on the IP (Internet Protocol) network. Network connection can be verified by sending a packet of a certain size and confirming its response. As it shows response speed, the speed of the network can also be confirmed. The name stems from the echo (“ping”) of active sonar that submarine sounds in underwater, because the behavior is like sonar.3 Quantum Key Distribution Network44　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)