Adaptive Rekeying for Secure Multicast∗

In this paper, we focus on the problem of secure multicast in dynamic groups. In this problem, a group of users communicate using a shared group key. Due to the dynamic nature of these groups, to preserve secrecy, it is necessary to change the group key whenever the group membership changes. While the group key is being changed, the group communication needs to be interrupted until the rekeying is complete. This interruption is especially necessary if the rekeying is done because a user has left (or is removed). We split the rekeying cost into two parts: the cost of the critical path —where each user receives the new group key, and the cost of the non-critical path —where each user receives any other keys that it needs to obtain. We present a family of algorithms that show the tradeoff between the cost of the critical path and the cost of the noncritical path. Our solutions allow the group controller to choose the appropriate algorithm for key distribution by considering the requirements on critical and non-critical cost. In our solutions, the group controller can dynamically change the algorithm for key distribution to adapt to changing application requirements. Moreover, we argue that our solutions allow the group controller to effectively manage heterogeneous groups where users have different requirements/capabilities. key words: Group Communication, Secure Multicast, Critical Path, Adaptive Rekeying