We propose LCMsec, a brokerless, decentralised Publish/Subscribe protocol. It aims to provide low-latency and high-throughput message-passing for IoT and automotive applications while providing much-needed security functionalities to combat emerging cyber-attacks in that domain. LCMsec is an extension for the Lightweight Communications and Marshalling (LCM) protocol. We extend this protocol by providing not only authenticated encryption of the messages in transit, but also a group discovery protocol inspired by the Raft consensus protocol. The Dutta-Barua group key agreement is used to agree upon a shared symmetric key among subscribers and publishers on a topic. By using a shared group key, we reduce the key agreement overhead and the number of message authentication codes (MACs) per message compared to existing proposals for secure brokerless Publish/Subscribe protocols, which establish a symmetric key between each publisher and subscriber and append multiple MACs to each message.