| 摘要 |
PROBLEM TO BE SOLVED: To provide key synchronization in a short time by transmitting a random number generated by a random number generation means to a decoder, nonlinearly or linearly transforming the random number by a key stored in a key storage part and setting it as a cipher key. SOLUTION: A ciphering part 28 appropriately generates the random number of n bits in a random number generation part 29 so as to take the key synchronization, sets it to the head of a data frame and simultaneously outputs it to a nonlinear or linear transformation part 33. The nonlinear or linear transformation part 33 linearly or nonlinearly transforms the random number of n bits by using the key stored in the key storage part 31 and outputs it to a synchronization type key stream cipher part 35. The synchronization type key stream cipher part 35 sets output from the nonlinear or linear transformation part 33 as a ciphering key and ciphers a plain sentence by using the key. The ciphered plain sentence (cipher sentence) is outputted following the random number of n bits of the data frame during m bits until the random number generation part 29 generates the next random number. Thus, the key synchronization is realized by a small hardware scale.
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