The proliferation of Internet-of-Things devices operating at network edges has driven the demand for secure, low-latency and decentralized communications methods. However, in dynamic and resource-constrained environments, conventional solutions relying on centralized key management servers or multiple handshakes introduce significant complexity and overhead. This chapter describes a keychain-based rekeying function that can be used to devise key management protocols based on chameleon hash functions. It enables lightweight, secure and efficient device-to-device and broadcast communications without relying on a centralized infrastructure. Each message is secured by a unique ephemeral session key derived using a pre-shared master key and chameleon hash, ensuring message authenticity and integrity. The session key can be authenticated and verified easily via chameleon hash collision so that the new session key is securely linked to the previous hash key on the keychain, enabling the verification of the sender and the new session key simultaneously. The protocol avoids expensive handshakes while enabling devices to spontaneously establish secure communications channels. A proof-of-concept implementation over Bluetooth Low Energy demonstrates protocol feasibility and performance. The results indicate that the protocols are particularly suitable for autonomous Internet-of-Things device scenarios, enabling seamless, secure and scalable device-to-device and broadcast communications.