Patent ID: 7055030

Claim:
A multicast communication system having a multicast server for transmitting data relating to a prescribed data distribution service by multicasting, and a plurality of clients belonging to a multicast group and receiving said data, said multicast server comprising: a data encryption unit for encrypting said data by using a first encryption key; a data transmission unit for transmitting said data encrypted by said data encryption unit to said plurality of clients by multicasting; a key encryption unit for encrypting said first encryption key by using a second encryption key; a key transmission unit for transmitting said first encryption key encrypted by said key encryption unit by unicasting to at least one of the plurality of clients, said at least one client subscribing to said data distribution service; a registration unit for registering one of the plurality of clients, that wishes to subscribe to said data distribution service; a deletion data reception unit for receiving deletion data indicating that the client registered by said registration unit has been deleted at least said first encryption key held by said client itself, said deletion data being transmitted from said client; and an erasure unit for erasing from said registration unit the client that has transmitted said deletion data, when said deletion data reception unit receives said deletion data; and said at least one client comprising: a key reception unit for receiving said encrypted first encryption key transmitted by said transmission unit; a key decryption unit for decrypting said encrypted first encryption key received by said key reception unit, using a decryption key; a data decryption unit for decrypting the encrypted data transmitted by said data transmission unit, using the first encryption key obtained by said decryption unit; a deletion unit for deleting at least said first encryption key held by said client itself in the event of withdrawal from said data distribution service; and a deletion data transmission unit for generating said deletion data and transmitting said deletion data to said multicast server.