Group Key Agreement (GKA) protocols enable the participants to derive a key based on each one's contribution over a public network without any central authority. They also provide efficient ways to change the key when the participants change. While some of the proposed GKA protocols are too resource consuming for the constraint devices often present in ad hoc networks, others lack a formal security analysis. In this paper, we propose a simple, efficient and secure GKA protocol well-suited to ad hoc networks and present results of our implementation of the same in a prototype application.