Abstract: Wireless sensor networks consist of sensor nodes with radio, processor, memory, battery and sensor hardware. With widespread deployment of these devices one can precisely monitor the environment. Sensor nodes are resource constrained in terms of radio range, processor speed, memory and power. WSNs are mostly unguarded and the wireless medium is broadcast. This makes them more vulnerable to attacks. Without proper security measures, an enemy can launch various kinds of attacks in hostile environments. These attacks can disrupt the normal working of WSNs and can defeat the purpose of their deployment. Therefore security is an important aspect of these networks. WSNs are often deployed to monitor important emergency events, such as forest fire and battlefield monitoring.