Wireless Sensor Networks (WSNs) are composed of a large, yet limited number of sensing devices called sensors, communicating over a wireless media. Wireless Sensor Networks (WSNs) find prospective applications in fields like environmental monitoring, healthcare, battlefield surveillance, and homeland security. A much broader spectrum of future applications is likely to follow. Deployment of WSN in hostile environments, unattended operation, and openness of communication and resource constraints e.g. limited memory, energy and computational capabilities exposes WSN to a number of security attacks. The resource constrained nature of WSN makes conventional security mechanisms incongruous to apply.