Sensor networks are finding multiple applications and are being increasingly deployed in the real world. These sensors are fragile with limited computational and storage resources and communicate with each other and a base station through routing protocols. Routing protocols in sensor networks seek to minimize energy consumption and do not take security into consideration, leaving the network vulnerable to malicious outside attacks. Due to the resource limitations of sensors, the standard security protocols cannot be applied. A number of key management protocols for sensors have been proposed. However, these key management protocols do not consider the routing problem. In this paper we modify an existing key management protocol called LEAP and integrate it into the directed diffusion sensor routing protocol to produce a secure routing protocol for sensor networks. We show that the proposed protocol can protect the network from malicious outside attacks. Simulation results also show that there is a slight overhead in terms of energy expenditure for the proposed protocol. The other overhead is a slightly increased packet size.