| 摘要 |
PROBLEM TO BE SOLVED: To provide a key management method that is stiff against attack by a 3rd party. SOLUTION: A number of available times is respectively set to a plurality of encryption keys. An A terminal 10 uses any encryption key to generate encryption data, counts number of times of using the encryption key, selects other encryption key when the accumulated count reaches the available number of times and informs a B terminal 20 of switching information of the encryption key. The B terminal 20 uses any of a plurality of decoding keys corresponding to each of a plurality of the encryption keys to decode the encryption data, and replaces the decoding key having been used with other decoding key corresponding to the new encryption key on the opportunity of reception of the key switching information from the A terminal 10.
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