An enhanced definition of implicit key authentication and a secure group key agreement scheme from pairings are presented. This scheme combines the merit of group public key and key trees to achieve a communication-efficient and authenticated group key agreement protocol. Besides, it avoids dependence on signature or MAC by involving member's long-term keys and short-term keys in the group key. Furthermore, the idea behind this design can be employed as a general approach to extend the authenticated two-party Diffie-Hellman protocols to group settings.