Abstract: In group key management, the approach based on the key-tree is widely used to achieve logarithmic rekeying costs. However, the key tree may become unbalanced after inserting or deleting members, and has to be kept balanced. In this paper, an improved balanced key tree is presented for dynamic groups. In the scheme based on the proposed tree, the balancing tree after member adding or member deleting does not involve borrowing a child from a sibling, merging with a sibling, and splitting a node. The scheme based on the proposed tree satisfies the desired properties, such as group key secrecy, forward secrecy, backward secrecy, key independence, and has the same worst case rekeying cost as a weight-balanced-tree. Our analysis shows that the rekeying costs of the presented scheme are O(2h) and O(2h²) in the join event and the leave event respectively.

Key words: group key, group key management, key tree, balanced tree, dynamic group