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In mobile ad hoc networks, the security is the main constraint in message transmission. For secure group based message transmission, we must share the key among users so that we can make the transmission as secure. This paper addresses an interesting security problem in mobile ad hoc networks that is dynamic group key agreement for key establishment. For secure communication, a group key must be shared by all group members. This group key should be updated when the existing group members are leaving the network or new members are entering into the existing network. In this paper, we propose a efficient group key agreement protocol called Key Agreement protocol based on Stability and Power (KASP). Here the idea is to split a large group into several subgroups, each maintaining its subgroup keys to manage the subgroup and managing many subgroups using Elliptic Curve Diffie-Hellman (ECDH) key agreement algorithm. In KASP, we develop two protocols namely, Subgroup Key Generation(SKG) and Group Key Generation (GKG) based on ECDH for subgroups and outer groups respectively. These subgroup keys and group keys should be changed when there are membership changes (such as when the current member leaves or the new member joins). By introducing group-based approach, messages and key updates will be limited within subgroup and outer group. Thus computation load is distributed to many mobile ad hoc nodes. Both theoretical and practical results show that this KASP, a new efficient group key agreement protocol performs well for the key establishment problem in ad hoc network in terms of efficiency and security.