CART: Concurrent Asynchronous Ratchet Tree for Group Messaging

In this paper, we propose the Concurrent Asynchronous Ratchet Tree (CART), a novel Continuous Group Key Agreement (CGKA) protocol, specifically designed to facilitate smooth concurrent group key updates. Our approach innovatively leverages a chameleon hash collision-based trapdoor function as a strategic substitution for the single-step DH function traditionally utilized in the Asynchronous Ratchet Tree (ART) framework. This innovation effectively facilitates the resolution of conflicts arising during simultaneous updates, thereby ensuring the consistency of keys from the perspective of each participant. We have executed comprehensive simulations involving random concurrent updates across large group settings and performed comparative analyses against other CGKA methodologies, particularly The CoCoA protocol. Our empirical results indicate that CART presents a reduction in computational requirements at various scales, particularly noting that its advantages are magnified when there is a decrease in the proportion of updating participants. Moreover, our examination of communication overheads unequivocally demonstrates CART’s superior efficiency. This not only underscores its feasibility but also positions CART as a more resource-efficient solution, rendering it ideally suited for practical implementation in real-world group communication settings.