JET: Join-Exit Tree for Time Efficient Contributory Group Key Management

In secure group communications, the time cost associated with key updates for 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. A special key tree topology with side trees is first introduced to handle member join and departure, then optimization techniques are employed to manage the key tree structure for time efficiency, and algorithms are designed to dynamically update the join and exit trees. We show that on average, the asymptotic time cost per event is reduced to O(log (log n)) from the previous cost of O(log n), where an event is defined as either a member join or departure and n is the group size. Our experimental results on both simulated user activities and the real MBone data also show that the proposed scheme greatly improves the time-efficiency of the existing tree-based schemes for key management.