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Messaging between two parties and in the group setting has enjoyed widespreadattention both in practice, and, more recently, from the cryptographiccommunity. One of the main challenges in the area is constructing secure(end-to-end encrypted) and efficient messaging protocols for groupconversations. The popular messaging applications WhatsApp and Signal utilise aprotocol in which, instead of sharing a single group key, members haveindividual \textit{sender keys}, which are shared with all other group members.The Sender Keys protocol is claimed to offer forward security guarantees.However, despite its broad adoption in practice, it has never been studiedformally in the cryptographic literature.  In this paper we present the first analysis of the Sender Keys protocol alongwith some prospective improvements. To this end, we introduce a newcryptographic primitive, develop a game-based security model, present asecurity analysis in the passive and active settings, and propose severalimprovements to the protocol.

Analysis and Improvements of the Sender Keys Protocol for Group Messaging