Patent ID: 7702904

Claim:
A multicast delivery system comprising: a delivery server which enciphers delivery data by using a current use cipher key to generate enciphered data and transmits a multicast packet containing said enciphered data and a current use key identifier, said current use key identifier identifies a pair of said current use cipher key and a current use decipher key as current use keys, said current use cipher key being separate from said current use decipher key; a key management server which is connected with said delivery server through a network, holds as a current use key data, a set of said current use decipher key and said current use key identifier, and transmits a set of said current use decipher key and said current use key identifier as a current use decipherment key data in response to a current use key data request; and a client terminal which is connected with said delivery server and said key management server through said network, receives said multicast packet from said delivery server, issues said current use key data request to said key management server to receive said current use decipherment key data from said key management server, holds a set of said current use decipher key and said current use key identifier, and deciphers said enciphered data contained in said multicast packet by using said current use decipher key when said current use key identifier contained in said multicast packet is coincident with said current use key identifier held in said client terminal.