Wireless sensor networks (WSN) consist of small nodes with sensing, computation and wireless communication capabilities. Increasing computing and wireless communication capabilities have expanded the role of sensor from mere information dissemination to more demanding task as sensor fusion, classification, collaborative target tracking etc. WSN facilitate monitoring and controlling of physical environment from remote location with better accuracy. Recent advances in WSN have lead to many new protocols specially designed for sensor networks. Many routing protocols have been proposed, energy awareness is an essential consideration but none of them have been designed with security as goal. In particular, we believe that some kind of security is necessary for WSN. In this paper we describe a simple secure algorithm that is an enhancement to the functionality of cluster based routing protocol.