Optimizing Rekeying Costs in Group Key Agreement

In this chapter, we describe two contributory schemes that employ novel tree structures and rekeying algorithms, with the aim to achieve the low bound of rekeying cost. Particular, the first method, referred to as JET [60, 63], uses a special join-tree/exit-tree topology and takes advantage of cost amortization. This method can significantly reduce the rekeying cost for user join. The second method is consist of a new key tree structure, called PFMH, and a key agreement protocol, called PACK [64, 65]. This method only needs O(1) rounds of two-party DH upon any single user join event and O(log n) rounds of two-party DH upon any single user leave event, which achieves the lower bound described in the previous paragraph.