In next-generation mobile communication, multicast service will be a key application for supporting a large group of subscribers simultaneously. Because multicast transmits data to the group simultaneously, it reduces the communication cost significantly. Since multicast may be vulnerable to an overhearing attack, this efficiency can be achieved only when security or access control is guaranteed; only authorized group members can read the data properly. Many commercial applications such as PayTV, vehicular ad hocnetwork (VANET), and group signature require that only legal users have access authority . Nowadays, because smart phones are becoming popular, many mobile applications which need group communication such as DMB, video conference, and online game have emerged. The model of the proposed framework consists of a group controller(GC), a server, and a mobile user group. The GC manages all key types that are used to encrypt data or other keys, and transmits the keys that are updated upon changes in the mobile user group. A server sends group members data that is encrypted with the group key. a new GKM framework consisting of algorithms for initial construction of a keytree, and for optimizing the key-tree after the group membership changes. This paper proposes a new GKM framework for the environment with dynamic mobile subscribers. Unlike other tree-based GKM schemes, ours assumes no special conditions: users have unequal leaving probabilities, and the key-tree is unbalanced and incomplete. We first compute the exact communication cost for the general key-tree. On the basis of the accurate communication cost, we propose two algorithms: one for initial keytree construction, and the other for optimizing the keytree when the group membership changes. The first algorithm generates a more optimal key-tree structure than that in previous works, and the second algorithm maintains its optimality.
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