Sensor nodes are tiny, low-cost devices equipped with environment sensors and radio for wireless communication. These sensor nodes may constitute the network for monitoring physical phenomena. Such network is called Wireless Sensor Network (WSN). Wireless sensor network consists of high number (102-106) of sensor nodes. Wireless sensor networks can be utilized in a broad variety of applications ranging from battlefield surveillance in military, through remote patient monitoring in medicine to forest fire detection in environmental applications. Majority of WSN applications require at least some level of security.