Secure communication is one of the main challenges that are slowing down the development of the Internet of Things (IoT). Key Management (KM) is particularly a difficult security issue, mainly because of the lack of resources of the IoT devices. Most of the existing solutions do not consider the heterogeneous and dynamic nature of the IoT. They do not regard the difference in capability of its components and impose equal loads on them. Moreover, they store keys in device memories before deployment, which makes adding devices difficult afterwards. We propose a novel decentralized blockchain-based KM protocol for the IoT. Our solution balances the loads between nodes according to their capabilities. We prove that this makes it efficient and scalable. Furthermore, our solution securely rekey the network upon a change. To decentralize the KM , we use the blockchain technology and smart contracts. We show that the system continues to operate when an entity fails and that the compromise of an entity does not jeopardize the whole network. We also prove that our solution fulfills the IoT requirements in terms of security and performance. Finally, we propose an implementation on IoT platforms to validate our theoretical analysis and simulation results.