Patent ID: 7961887

Claim:
A content distribution system comprising: a content encryption unit configured to encrypt a content by using a plurality of different session keys s j (j=1, 2, . . . , n), to obtain a plurality of encrypted contents; a session key encryption unit configured to encrypt each of the plurality of different session keys with a public key corresponding to a decryption key unique to each user system, to obtain a plurality of encrypted session keys; a header information generation unit configured to generate, for a group including a plurality of user identification information items for respectively identifying a plurality of user systems to which the plurality of different session keys are assigned, header information which allows decryption of each encrypted session key assigned to each user system by using the decryption key unique to each user system belonging to the group, the header information including (a) the encrypted session keys, and (b) a first vector (L 0 , L 1 , L 2 , . . . , L k ) which corresponds to a session key of the plurality of different session keys and is assigned to arbitrary user identification information item u in the group, the first vector being set such that an inner product of the first vector and a second vector (1, u, u 2 , . . . , u k ) concerning the arbitrary user identification information item u becomes equal to z j u v (where k is a predetermined positive integer, z j is a constant value which is one of a plurality of different constant values corresponding to the plurality of different session keys and corresponds to a session key s j assigned to the arbitrary user identification information item u, and v is a group identification information item assigned to the group and is an integer not less than “0” and not more than k ); and a hardware transmitter to transmit the header information and at least one of the plurality of encrypted contents to the plurality of user systems.