Wireless sensor networks (WSNs) are innovative large-scale wireless networks that consist of distributed, low-power, small-size devices using sensors to cooperatively collect information through infrastructure less ad-hoc wireless network. These small devices used in wireless sensor nodes are called sensor nodes. They are envisioned to play an important role in a wide variety of areas ranging from critical military surveillance applications to forest fire monitoring and building security monitoring in the near future. In these networks, a large number of sensor nodes are deployed to monitor a vast field, where the operational conditions are most often harsh or even hostile. Since these networks are usually deployed in remote places and left unattended, they should be equipped with security mechanisms to defend against attacks such as node capture, physical tampering, eavesdropping, denial of service, etc. Unfortunately, traditional security mechanisms with high overhead are not feasible for resource constrained sensor nodes.