Sensor networks offer economically viable solutions for a wide variety of monitoring applications. In surveillance of critical infrastructure such as airports by sensor networks, security becomes a major concern. To resist against malicious attacks, secure communication between severely resource-constrained sensor nodes is necessary while maintaining scalability and flexibility to topology changes. A robust security solution for such networks must facilitate authentication of sensor nodes and the establishment of secret keys among nodes In this paper, we propose a decentralized authentication and key management framework for hierarchical ad hoc sensor networks. This scheme is light weight and energy aware and reduces the communication overhead.