This paper presents the design, implementation and evaluation of an automated method for distributing symmetric cryptographic keys in a Wireless Sensor Network (WSN). Unlike previous methods for key distribution in WSNs, we do not rely on sensitive knowledge to be stored in program memory prior to network deployment. Additionally, the protocol proposed uses dominant security primitives to ensure strong security and interoperability with existing networks (such as the Internet), while operating independent of the network layer protocol. Through both hardware experimentation and simulation, we show that this protocol can provide strong confidentiality, integrity and authenticity protection to the symmetric keys as they are distributed throughout a network, while maintaining the ability to scale to large-size networks and remain energy efficient.