Patent ID: 8054973

Claim:
An encryption key management method of an encryption key management system for a broadcast encryption (BE) of a data transmission system, comprising: generating a plurality of subgroups, each subgroup consisting of at least one of a plurality of nodes, and forming layers according to a number of nodes which form the subgroup; assigning prime numbers to the subgroups not to overlap one another, and connecting a subgroup at an upper layer with a subgroup, which includes nodes forming the subgroup of the upper layer, at a lower layer; and re-assigning a prime number assigned to a subgroup at an upper layer, a prime number assigned to an unconnected subgroup at the same layer, and a prime number assigned to an unconnected subgroup at a lower layer, to each of the subgroups, and assigning an encryption key which is generated from the re-assigned prime numbers only except for the prime numbers assigned by the assigning to each of the subgroups, wherein, if a revoked node is included in the nodes, a session key is encrypted using an encryption key which is assigned to a subgroup consisting of nodes other than the revoked node, thereby the revoked node is not capable of obtaining the session key.