Pub/Sub Meets MLS: End-to-End Encrypted Group Data Sharing Over Publish-Subscribe

In Cyber-Physical Systems (CPS), numerous devices gather sensor data into the cyber domain, which is then consumed by various services, such as robots and analytic services, in a dynamic and real-time manner. The Publish-Subscribe (Pub/Sub) model is a promising communication paradigm that can easily support many-to-many data sharing. By introducing the intermediate message broker responsible for message forwarding, Pub/Sub can decouple the message publisher and its subscribers and achieve scalable data sharing. However, ensuring the confidentiality of messages against the broker presents a key challenge when distributing private information over Pub/Sub. End-to-end encrypted (E2EE) Pub/Sub protocol is a promising approach to support message confidentiality in Pub/Sub. Within CPS, sensor data is dynamically consumed by multiple services, and thus E2EE Pub/Sub should support dynamic changes in subscribers in a scalable manner. However, existing protocols have scalability issues due to the excessive number of messages on the order of N² for $N$ subscribers during key updates. This paper presents a new E2EE Pub/Sub protocol designed to support dynamic data sharing in CPS, inspired by the state-of-the-art secure group messaging protocol, Messaging Layer Security (MLS). By carefully integrating MLS features into Pub/Sub, our protocol effectively handles dynamic subscriber changes in a scalable manner, updating keys with only (ON)messages.