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4Communication packet structuree video data is comprised of 15 frames per second (standard frame size is 320 × 240 pixel) compressed with H.264 (one of the video compression standards of MPEG). e rate of data transmission is relatively high at 12 Mbps and the duration of shooting and transmission per experi-ment is about 15 minutes. erefore, the necessary size of the true random numbers per experiment is about 11 Gbits.As the channel property of the drone communications easily uctuates and loss of data occurs frequently, a system to synchronize many true random number sequences and update the encryption key to conceal transmitted video data perfectly was needed. So, we developed a technology to transmit key synchronization codes at an optimized packet interval depending on the channel properties to cope with such packet losses during the data transmission. Also, as a real-time capture of the monitored target is re-quired for monitoring systems by video cameras, UDP (User Datagram Protocol) as a protocol in the fourth layer (transport layer) of the OSI layer model was used in this experiment not to retransmit lost data but to reproduce the video in real time as possible by synchronizing with the key of the next packet. Also, even if a loss of long-time (one-second) data occurs, this system enables guaranteeing the key synchronization between the sender and the re-ceiver by transmitting a key synchronization code at an optimized bit interval (32 Mbit in this experiment).A rough structure of the packets is shown in Fig. 3. Each segment, into which the video data is divided at 2,048 bits, is XORed with a true random number sequence of the same size as that of the segment, and is prexed with an 8-bit ID header. Here, a key synchronization code of 32 bits is inserted into the video data at every 32 Mbits (identica-tion ID header is also added to this code). ese are pay-loads which are packetized below a transport layer. Here, a packet is a Media Access Control Frame*3 according to the Ethernet frame format. e MAC header is removed by Ethernet converter when transmitted by wireless to be encapsulated according to IEEE 802.11. Data loss due to packet loss in wireless transmission is detected by 32-bit FCS (Frame Check Sequence) added at encapsulation.e video data from the camera drone is sent at low latency with accurate OTP encryption, while suppressing deterioration of data transmission eciency by correcting the time gap of synchronization of the true random num-ber sequences of the above key synchronization code. Also, in case that a loss of data is detected, the ground station FiF3　Outline of OTP encryption for video data in case of data lossData IDVideo dataFCSData IDKey synchro-nizationbitData IDVideo data・・・・・・Detection of Data loss802.11hd・・・802.11hdKey synchronization while suppressing deterioration of data transmission efficiency to the minimumVideo dataEncrypted dataEncrypted data・・・・・・Encrypted dataEncrypted data・・・・・・Encrypted data・・・・・・Encrypted dataVideo dataFCSEncryption by XORing packet of video data with encryption keyTransmitting of a key synchronization code at an optimized packet interval (every 32 Mbits) depending on the channel propertiesConstituting a frame format optimized for wireless data transmission2,048bits2,048 bits2,048 bits8 bits32 bits8 bitsPayload containing encrypted dataMAChd・・・Payload containing encrypted dataMAChd12,000 bitsConverting header and FCS into those of IEEE 802.11 frame format by Ethernet converterOSI layer modelThe fifth layerand overWired（Ethernet）Wireless LAN（IEEE 802.11）The first layerPayload containing encrypted dataPhys.hdPhys.hdFCSPayload containing encrypted dataPhys.hdFCSPhys.hd32 bits*Data ID:Data identifier*Phys. hd: Physical header*MAC hd:MACheader*802.11 hd: 802.11 header*3Media Access Control (MAC) frame is a small data group to which control information such as destination is added in Ethernet. Data is exchanged by frame in Ethernet of which the size range is 514-12,144 bits.433-4 Secure Transmission of Video Data Relayed by UAV