| 摘要 |
PROBLEM TO BE SOLVED: To prevent cipher communication from being performed unless using a cryptographic key supplied from a deposit center and also to easily change cryptographic keys. SOLUTION: The deposit center 1 generates an encryption key and a decode key and distributes the keys to a base station 2 and terminal devices 3 and 4. Management information is generated from the encryption key and the decode key and distributed to the station 2. A transmission terminal enciphers a message with the encryption key and transmits the message to the station 2. The station 2 converts the enciphered message with the management information and transmits the message to a receiving terminal. The receiving terminal decodes the converted enciphered message with the decode key. In the case of using any key other than the encryption key and the decode key given from the center 1, the message cannot be decoded even through the message is received because the key does not correspond to the management information. The change of cryptographic keys can dynamically be performed by freely requesting of the center 1 on-line. Because the cryptographic key is stored in the center 1 unconditionally, it is difficult to perform an illegal thing at a terminal side.
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