Never halting growth of the Internet has influenced the development and use of multicast communication which is proving to be an effective method for delivery of data to multiple recipients. Vast number of applications come to benefit from this efficient means of communication. With existing security threats in the Internet it has become imperative to look into multicast security. One of the challenges in securing multicast communication is to efficiently establish and manage shared keys in large and dynamic groups. In this paper we propose very efficient re-keying protocols for multicast encryption. One of our protocols has complexity at most logarithmic in all measures considered in the literature, namely, communication, number of keys stored by the user and by the center, and time complexity per update. We then analyze the performance of the family of tree-based re-keying protocols for multicast encryption, with respect to a set of multiple up-date operations, and show that for a particular class of such updates, we can modify these schemes so that they guarantee essentially optimal performance. Specifically, while performing m update operations each one at a time would guarantee a complexity of O(m log n), we show that for a specific (but large) class of instances, we can modify the schemes so that they guarantee O(long n) communication complexity only, by keeping the user storage complexity O(log n), n being the maximum size of the multicast group.
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