Recent developments in low-rate wide-area wireless technologies will enable the deployment of publish–subscribe architectures that can span larger geographical areas in the smart grid. In such a setting, typically, there is a lot of data exchange due to the increased number of multicasts/broadcasts among any device. Thus, using technologies such as long range wide area network (LoRaWAN) will leave very limited bandwidth to perform critical security management operations such as key creation and updates. If security management overhead cannot be minimized, eventually, it may risk meeting the real-time requirements for critical control messages and weakening the resilience of smart grid systems. In this article, we propose a new protocol for group key generation and renewal that not only reduces the number of messages but also minimizes the total delay of the process. The approach integrates classical Diffie–Hellman (DH) key exchange with Shamir’s secret sharing to generate a group key by initiating random pairing points and applying Lagrange interpolation. The hash-chain concept is employed to renew the group key without requiring further message exchanges, essentially achieving key renewal in a single message. The evaluation results show that our approach reduces the number of messages and the setup delay significantly compared to the alternative approaches.