Reception terminal, key management apparatus, and key updating method for public key cryptosystem

A method, for use in a data distribution system having a key management center, a distribution station, and a reception terminal, for updating a pair of a distribution public key and a distribution secret key which conform to a public key cryptosystem and are both unique to the reception terminal, the distribution public key being used to encrypt data to be distributed to the reception terminal, the distribution secret key being used to decrypt the distributed encrypted data, the method comprising:

• an update secret key acquiring step in which the reception terminal acquires an update secret key prior to a data distribution;
• an update public key acquiring step in which the key management center acquires an update public key that makes a pair with the update secret key, prior to the data distribution;
• a key generating step in which the key management center generates a new pair of a distribution public key and a distribution secret key for the reception terminal;
• an encrypting step in which the key management center generates an encrypted secret key by encrypting the new distribution secret key using the update public key;
• a transmission step in which the key management center transmits the encrypted secret key to the reception terminal;
• a distribution public key updating step in which, after the transmission step, the key management center updates the distribution public key having been used so far by the distribution station in data distributions to the new distribution public key;
• a reception step in which the reception terminal receives the encrypted secret key;
• and
• a distribution secret key updating step in which the reception terminal restores the new distribution secret key as necessary by decrypting the encrypted secret key using the update secret key, and updates the distribution secret key having been used so far to the restored new distribution secret key.