Key History Tree: Efficient Group Key Management with Off-Line Members

Abstract

We present a new approach to deal with off-line members that are part of a secure dynamic group, where all the group members share a secret key, and this key is continuously changed to match current membership. Instead of re-negotiating keys when members become off-line or forcing direct interaction with the key manager, we propose a safe caching mechanism particularly suited for LKH (Logical Key Hierarchy) schemes. The basis of our approach is that in many applications, members that are back on-line just need to know the current key and not all the intermediate keys negotiated while they were off-line. We have devised a compact representation for that purpose called KHT (Key History Tree). A KHT is built using only publicly available information, so it can be safely replicated over the network, and its operation is transparent to clients and key managers. We use as an example of the benefits of our approach a web-based subscription service that anonymizes customer interactions while enforcing membership payments. Extensive simulations show the advantage of our approach over more conventional schemes.