Batch rekeying for secure group communications

Many emerging web and Internet applications are based on a group communications model. Thus, securing group communications is an important Internet design issue. The key graph approach has been proposed for group key management. Key tree and key star are two important types of key graphs. Previous work has been focused on individual rekeying, i.e., rekeying after each join or leave request. In this paper, we first identify two problems with individual rekeying: inefficiency and an out-of-sync problem between keys and data. We then propose the use of periodic batch rekeying which can improve efficiency and alleviate the out-of-sync problem. We devise a marking algorithm to process a batch of join and leave requests. We then analyze the key server’s processing cost for batch rekeying. Our results show that batch rekeying, compared to individual rekeying, saves server cost substantially. We also show that when the number of requests in a batch is not large, the best key tree degree is four; otherwise, key star (a special key tree with root degree equal to group size) outperforms small-degree key trees.