Mobile ad-hoc networking (MANET) is a collection of mobile nodes that want to communicate to each others, but has no fixed links like wireless infrastructure networks. Each node acts as a router and is responsible for dynamically discovering other nodes it can directly communicate with. Due to the network topology of an ad hoc network changes frequently and unpredictable, so the security of multicast routing becomes more challenging than the traditional networks. In this paper, we describe how any user in the multicast group can compose the group keys and use the group Diffie Hellman key-exchange protocol (GDH) to securely multicast data from the multicast source to the rest of the multicast group in wireless ad hoc networks. Then, according to the measures of GPS units, it allows the multicast source to structure representation of network topology, and computes a multicast tree. Therefore the multicast source can utilize the group set information with Prufer number of the multicast tree for multicast header. As a result, we deliver the group key based on the network topology and the multicast routing. It not only provides the multicast routing information, but also fits the robustness of the wireless networks and reduces the overhead for the security management.