Security and Efficiency of Secure Group Messaging Protocols

Overview & Synopsis

Secure Group Messagings are cryptographic protocols that allow safe group conversation within potentially large user groups. They are based on a dedicated type of group key exchange mechanism named Continuous Group Key Agreement (CGKA), which generates a continuously evolving group key in a safe and efficient way. Messaging Layer Security (MLS) is an IETF-standardized secure group messaging protocol which uses a protocol called TreeKEM as its CGKA. We study in this thesis the security and the efficiency of the CGKAs used within secure group messagings. Firstly, we propose a variant of TreeKEM (called Quarantined-TreeKEM) that solves a security issue related to inactive users, whose behavior prevents fundamental security properties to be fulfilled. Moreover, we improve the efficiency of TreeKEM, especially regarding its communication cost, by considering the best way to manage the tree graph used by that CGKA. Finally, we show that in the particular – yet common – use case where only a subset of users is granted the administration rights, we can design a new CGKA whose performances outperform those of regular decentralized CGKAs such as TreeKEM. This thesis thus aims to improve the state-of-the-art in secure group messaging protocols, in a context where strong security and energy sobriety are necessary features.