Algorithms for dynamic multicast key distribution trees

Many secure group communication systems rely on a group key, which is a secret shared among the members of the group. Secure messages are sent to the group by encrypting them with the group key. Because group membership is dynamic, it becomes necessary to change the group key in an efficient and secure fashion when members join or leave the group. We present a series of algorithms for solving this problem based on 2--3 trees, where each internal node has degree 2 or 3. The algorithms attempt to minimize the worst case communication cost of updating the group key and the auxiliary keys needed by the algorithms. The algorithms are analyzed for the worst case performance and evaluated empirically via simulations. We focus on the trade-off between the communication cost due to the structure of the tree and that due to the restructuring of the tree to maintain its structure.

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