In this paper, we present a mechanism for securing group communications in Wireless Sensor Networks (WSN). First, we derive an extension of Logical Key Hierarchy (LKH). Then we merge the extension with directed diffusion. The resulting protocol, LKHW, combines the advantages of both LKH and directed diffusion: robustness in routing, and security from the tried and tested concepts of secure multicast. In particular, LKHW enforces both backward and forward secrecy, while incurring an energy cost that scales roughly logarithmically with the group size. This is the first security protocol that leverages directed diffusion, and we show how directed diffusion can be extended to incorporate security in an efficient manner.