Patent ID: 8122246

Claim:
A content distribution system which distributes an encrypted content and header information to a plurality of user systems, comprising: a first encryption unit configured to encrypt a content such that the content is adapted to be decrypted with a session key, to obtain the encrypted content; a second encryption unit connected to the first encryption unit and configured to encrypt the session key with a public key corresponding to a plurality of decryption keys respectively assigned to the user systems, to obtain an encrypted session key; a generation unit connected to the first and second encryption units and configured to generate the header information which includes the encrypted session key and allows the encrypted session key to be decrypted by using one of a plurality of decryption keys unique to each user system which is permitted to decrypt the encrypted session key; a memory to store the one of the plurality of decryption keys unique to each user system; and a transmission unit connected to the first encryption and generation units and configured to transmit the encrypted content and the header information to each user system; wherein the one of the plurality of decryption keys unique to each user system is a value obtained by (a) assigning different individual key generation polynomials to a root, a plurality of nodes, and a plurality of leaves of a tree structure, respectively, (b) assigning the different leaves on the tree structure a plurality of subgroups obtained by dividing a group of a plurality of user identification information items which are for individually identifying the user systems, and (c) substituting the user identification information item of the each user system into one of the individual key generation polynomials which corresponds to one of leaves assigned to one of the subgroups to which the user identification information item corresponding to the each user system belongs or an ancestor node of the one of the leaves, and substituting the user identification information item of the each user system into a common key generation polynomial common to the root, the nodes, and the leaves.