Cryptology ePrint Archive: Report 2016/1013

A Formal Security Analysis of the Signal Messaging Protocol

Katriel Cohn-Gordon and Cas Cremers and Benjamin Dowling and Luke Garratt and Douglas Stebila

Abstract: The Signal protocol is a cryptographic messaging protocol that provides end-to-end encryption for instant messaging in WhatsApp, Wire, and Facebook Messenger among many others, serving well over 1 billion active users. Signal includes several uncommon security properties (such as "future secrecy" or "post-compromise security"), enabled by a novel technique called *ratcheting* in which session keys are updated with every message sent.

We conduct a formal security analysis of Signal's initial extended triple Diffie-Hellman (X3DH) key agreement and Double Ratchet protocols as a multi-stage authenticated key exchange protocol. We extract from the implementation a formal description of the abstract protocol, and define a security model which can capture the ratcheting key update structure as a multi-stage model where there can be a tree of stages, rather than just a sequence. We then prove the security of Signal's key exchange core in our model, demonstrating several standard security properties. We have found no major flaws in the design, and hope that our presentation and results can serve as a foundation for other analyses of this widely adopted protocol.

Category / Keywords: cryptographic protocols / protocols, messaging, post-compromise security, Signal, future secrecy, authenticated key exchange, provable security, multi-stage key exchange

Original Publication (with major differences): IEEE EuroS&P 2017

Date: received 25 Oct 2016, last revised 4 Jul 2019

Contact author: dstebila at uwaterloo ca

Available format(s): PDF | BibTeX Citation

Note: Fix omission in description of initial X3DH handshake, reorganize figures for improved presentation. Full list of changes in Appendix D.

Version: 20190704:165509 (All versions of this report)

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