| 摘要 |
PROBLEM TO BE SOLVED: To realize key synchronization, where hazards in key decoding is avoided without increasing a transfer data capacity by generating an enciphering key, based on key generating information generated from a cipher sentence packet and from a master key. SOLUTION: When the previously transmitted sentence packet is inputted to a first key generating information generating function part 15 of an enciphering part 13, the part 15 draws a data bit at a prescribed position in the cipher sentence packet and generates key generating information. When a master key is inputted to a key generating arithmetic part 17, together with key generating information, the arithmetic part 17 generates the enciphering key and outputs it to a synchronous key stream enciphering part 19. The part 19 enciphers a sentence inputted, based on the enciphering key and transmits it as the ciphered sentence packet. Therefore, key synchronization is obtained, without increasing the transfer data capacity and also key information with not transmitted as is, so that hazards in key decoding is eliminated.
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