Wireless Sensor Networks (WSNs) are easy to deploy and allow flexible installations which have enabled them to be used for numerous applications. Due to these properties, they face distinct information security threats. Security for WSNs is very much needed, because of its sensitive information transmission. Sensor networks are vulnerable to many types of attacks because they are deployed in public environment. So it is necessary to secure sensor networks, this can be achieved by introducing authentication and pair wise key establishment mechanisms to sensor nodes.