Wireless sensor networks are ad hoc networks comprised mainly of small sensor nodes with limited resources, and are rapidly emerging as a technology for large-scale, lowcost, automated sensing and monitoring of different environments of interest. Cluster-based communication has been proposed for these networks for various reasons such as scalability and energy efficiency. In this paper, we investigate the problem of adding security to clusterbased communication protocols for homogeneous wireless sensor networks consisting of sensor nodes with severely limited resources, and propose a security solution for LEACH, a protocol where clusters are formed dynamically and periodically. Our solution uses building blocks from SPINS, a suite of highly optimized security building blocks that rely solely on symmetric-key methods; is lightweight and preserves the core of the original LEACH.