JET: Dynamic Join-Exit-Tree Amortization and Scheduling for Contributory Key Management

In secure group communications, the time cost associated with key updates in the events of member join and departure is an important aspect of quality of service, especially in large groups with highly dynamic membership. To achieve better time efficiency, we propose a join-exit-tree (JET) key management framework. First, a special key tree topology with join and exit subtrees is introduced to handle key updates for dynamic membership. Then, optimization techniques are employed to determine the capacities of join and exit subtrees for achieving the best time efficiency, and algorithms are designed to dynamically update the join and exit trees. We show that, on average, the asymptotic time cost for each member join/departure event is reduced to O(log(logn)) from the previous cost of O(logn), where n is the group size. Our experimental results based on simulated user activities as well as the real MBone data demonstrate that the proposed JET scheme can significantly improve the time efficiency, while maintaining low communication and computation cost, of tree-based contributory key management