Decryption-key distribution method and authentication apparatus

A decryption key for decrypting data from an access node is distributed to an access terminal intending to receive the data. An authentication unit receives a message for terminal authentication including a terminal identifier from the terminal and authenticates the terminal. The authentication unit refers to a content registration table having stored in advance the content type of a content which the terminal can receive, in association with the terminal identifier, according to the received terminal identifier to obtain a corresponding content type. The authentication unit refers to a decryption data base having stored in advance a decryption key and its valid period in association with a content type, according to the obtained content type to obtain a corresponding decryption key and valid period. The authentication unit sends an authentication result and the decryption key and valid period to the terminal or to a packet control unit.