Many applications in distributed computing systems, such as IP telephony, teleconferencing, collaborative workspaces, interactive chats and multi-user games, involve dynamic peer groups. In order to secure communications in dynamic peer groups, group key agreement protocols are needed. In this paper, we come up with a new group key agreement protocol, composed of a basic protocol and a dynamic protocol, for large-scale dynamic peer groups. Our protocols are natural extensions of one round tripartite Diffie-Hellman key agreement protocol. In view of it, our protocols are believed to be more efficient than
those group key agreement protocols built on two-party Diffie- Hellman key agreement protocol. In addition, our protocols have the properties of group key secrecy, forward and backward secrecy, and key independence.