Symmetric key encryption techniques are the base of providing security in Mobile ad-hoc network (MANET) communication. The performance of existing schemes is not providing complete security in many respects. Their level of security may vary depending on several factors like network topology, routing mechanism, presence of malicious nodes, etc. A novel key generation scheme for MANETs deploying one of the truly dynamic system parameters -traffic volume is proposed in this scheme. The efficiency and performance of the technique is analyzed for routing protocol build on reactive methods. The system proposed here is simulated with two different routing environments: DSR and AODV. These two are similar in their on-demand nature of routing but differ in many ways. The complexity of the secret key generation procedure, the packet loss with and without malicious nodes, and detection probability of active intruders are the parameters evaluated through simulation. The analysis proves the applicability and efficiency of the technique to the reactive routing based networks. Also the typical characteristics like node mobility are not affecting the key generation.