Wireless sensor networks based on highly resource-constrained devices require symmetric cryptography in order to make them secure. Integral to this is the exchange of unique symmetric keys between two devices. In this paper, we propose a novel decentralized key exchange protocol that guarantees the confidentiality of a key exchange even if an attacker has compromised some of the devices in the network. A central objective of the protocol design was to minimize resource consumption on the individual devices. We evaluate the resource requirements of our protocol in terms of memory requirements, CPU usage and network traffic both through theoretical analysis and through simulations.