Anonymous End to End Encryption Group Messaging Protocol Based on Asynchronous Ratchet Tree

摘要

Double ratchet protocol was first proposed and used in Signal's end to end encryption and later widely applied by WhatsApp, Face-book and other popular applications. Asynchronous Ratchet Tree (ART) is the new group messaging protocol based on ratchet and is the first protocol that applied forward secrecy (FS) and post-compromised-security (PCS) in group key exchange. However, anonymity is not considered which is crucial for privacy preserving solutions. Thus, it is meaningful to provide anonymous features while applying FS and PCS. In this paper we propose "Anonymous Asynchronous Ratchet Tree (AART)" to improve the structure of ART to achieve anonymity in group messaging while retaining FS and PCS. Also, we formalize the definitions of anonymity as Internal Group Anonymity (IGA) and External Group Anonymity (EGA). We prove that our AART satisfies IGA and EGA as well as FS and PCS.