Protocol design for scalable and reliable group rekeying

Zhang, Xincheng
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In secure group communications, group users share a symmetric key, called group key. The group key is used for encrypting data traffic among group users or restricting access to resources intended for group users only. A key server needs to change the group key after users join and leave (called group rekeying), by composing a rekey message that consists of encrypted new keys (encryptions, in short) and delivering it to all users. When group size is large, it becomes infeasible to rekey per user join or leave because of its high processing and bandwidth overheads. Instead, the key server changes the group key per rekey interval, the length of which indicates how tight the group access control is. It is desired to reduce the overhead of group rekeying as much as possible in order to allow frequent rekeying. To address the scalability issue of the key server, Wong, Gouda, and Lam proposed the key tree approach in 1998. The same idea also appears in vi RFC 2627. The scalability of rekey transport, however, was not addressed. The objective of this dissertation is to design a scalable and reliable rekey transport protocol and evaluate its performance. Rekey transport differs from data transport because rekey messages require scalable, reliable, and realtime delivery. Furthermore, each user needs only a small subset of all of the encryptions in a rekey message. We have proposed a scalable and reliable rekey transport protocol; its efficiency benefits from the special properties of rekey transport. The protocol runs in two steps: a multicast step followed by a unicast recovery step. Proactive forward error correction (FEC) is used in multicast to reduce delivery latency and limit the number of users who need unicast recovery. The unicast recovery step provides eventual reliability; it also reduces the worst-case delivery latency as well as user bandwidth overhead. In the protocol design, various technical issues are addressed. First, a key identification scheme is proposed for each user to identify the subset of new keys that it needs. The communication cost of this scheme is only several bytes per packet. Second, we investigate how to space the sending times of packets to make proactive FEC resilient to burst loss. Lastly, an adaptive FEC scheme is proposed to make the number of users who need unicast recovery controlled around a small target value under dynamic network conditions. This scheme also makes FEC bandwidth overhead and rekey interval close to the the feasible minima. Application-layer multicast (ALM) offers new opportunities to do naming and routing. In the dissertation, we have studied how to use ALM to support concurrent rekey and data transport in secure group communications. Rekey traffic is bursty and requires fast delivery. It is desired to reduce rekey bandwidth overhead as much as possible since it competes for available bandwidth with data traffic. Towards this goal, we propose a multicast scheme that exploits proximity in the underlying network. We further propose a rekey message splitting scheme to significantly reduce rekey bandwidth overhead at each user access link and network link. We formulate and prove correctness properties for the multicast scheme and rekey message splitting scheme. Simulation results show that our approach can reduce rekey bandwidth overhead from several thousand encryptions to less than ten encryptions for more than 90% of users in a group of 1024 users.