Abstract: In order to offer security for multicast applications，the group key has to be changed whenever a user joins or leaves the group.Batch rekeying alleviates the problems of inefficient and out-of-sync which are brought by individual rekeying.But the batch re-keying ignores the change probability of members.Even in an update cycle，no membership changes or some members change only in a sub-tree，the group manager must update all the keys，therefore will increase the overhead of group rekeying and communication costs.The paper proposes the triggered batch re-keying based on the six-probability-
optimization key-tree which is constructed according to the users’ change probabilities.The paper also proves that this approach further reduces the overhead of group re-keying and communication costs，improves scalability of group members and adapts better to the dynamic changes in multicast group membership through performance analysis and simulation experiments.

Key words: secure multicast, rekeying, batch rekeying, multi-level-key, probabilistic organization

摘要： 为了保证多播通信的安全性，当有成员加入或离开时必须更新组密钥。独立密钥更新和批量密钥更新是目前常用的两种密钥更新策略。其中，批量密钥更新方式较好解决了独立密钥更新方法中存在的低效和失序问题，但忽略了成员变化的动态性。即使在一个更新周期内无成员变动或者只有一个子树上的成员发生变动时，组管理器都要更新所有的密钥，因此加大了组密钥更新次数和通信开销。构建了一棵六级概率优化树，即把不同变动概率的组成员放置于不同级别的概率优化子树中，并为此组织结构设置触发式的批量更新策略。通过性能分析和仿真实验证明这种更新策略不仅有效降低了组密钥更新次数和通信开销，而且提高了密钥管理的可扩展性，较适合于大型的动态多播组。

关键词: 安全多播, 密钥更新, 批量更新, 多级密钥树, 概率组织