Application of Wireless Sensor Network (WSN) is increasing in a rapid speed. As sensor networks may interact with sensitive data and operate in hostile unattended environments, it is imperative that security concern be addressed from the beginning of the system. But sensor networks also introduce severe resource constraints due to their lack of data storage and power. Data encryption is widely used to ensure security in open networks such as the internet. The capabilities of cryptosystems such as of RSA and Diffie-Hellman are inadequate due the requirement of large number of bits.