Cryptology ePrint Archive: Report 2017/713

More is Less: On the End-to-End Security of Group Chats in Signal, WhatsApp, and Threema

Paul Rösler and Christian Mainka and Jörg Schwenk

Abstract: Secure instant messaging is utilized in two variants: one-to-one communication and group communication. While the first variant has received much attention lately (Frosch et al., EuroS&P16; Cohn-Gordon et al., EuroS&P17; Kobeissi et al., EuroS&P17), little is known about the cryptographic mechanisms and security guarantees of secure group communication in instant messaging.

To approach an investigation of group instant messaging protocols, we first provide a comprehensive and realistic security model. This model combines security and reliability goals from various related literature to capture relevant properties for communication in dynamic groups. Thereby the definitions consider their satisfiability with respect to the instant delivery of messages. To show its applicability, we analyze three widely used real-world protocols: Signal, WhatsApp, and Threema. Since these protocols and their implementations are mostly undocumented for the public and two out of three applications among them are closed source, we describe the group protocols employed in Signal, WhatsApp, and Threema. By applying our model, we reveal several shortcomings with respect to the security definition. Therefore we propose generic countermeasures to enhance the protocols regarding the required security and reliability goals. Our systematic analysis reveals that (1) the communications' integrity – represented by the integrity of all exchanged messages – and (2) the groups' closeness – represented by the members' ability of managing the group – are not end-to-end protected.

We additionally show that strong security properties, such as Future Secrecy which is a core part of the one-to-one communication in the Signal protocol, do not hold for its group communication.

Category / Keywords: applications / End-to-End Encryption, Group Communication, Broadcast, Future Secrecy, Signal, WhatsApp, Threema

Original Publication (with major differences): 3rd IEEE European Symposium on Security and Privacy (EuroS&P 2018)

Date: received 24 Jul 2017, last revised 19 Feb 2018

Contact author: paul roesler at rub de

Available format(s): PDF | BibTeX Citation

Note: Minor revisions, mainly updated format to IEEE EuroS&P format.

Version: 20180219:133524 (All versions of this report)

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