Paper 2019/367

SAID: Reshaping Signal into an Identity-Based Asynchronous Messaging Protocol with Authenticated Ratcheting

Olivier Blazy, Angèle Bossuat, Xavier Bultel, Pierre-Alain Fouque, Cristina Onete, and Elena Pagnin


As messaging applications are becoming increasingly popular, it is of utmost importance to analyze their security and mitigate existing weaknesses. This paper focuses on one of the most acclaimed messaging applications: Signal. Signal is a protocol that provides end-to-end channel security, forward secrecy, and post-compromise security. These features are achieved thanks to a key-ratcheting mechanism that updates the key material at every message. Due to its high security impact, Signal's key-ratcheting has recently been formalized, along with an analysis of its security. In this paper, we revisit Signal, describing some attacks against the original design and proposing SAID: Signal Authenticated and IDentity-based. As the name indicates, our protocol relies on an identity-based setup, which allows us to dispense with Signal's centralized server. We use the identity-based long-term secrets to obtain persistent and explicit authentication, such that SAID achieves higher security guarantees than Signal. We prove the security of SAID not only in the "Authenticated Key Exchange" (AKE) model (as done by previous work), but also in the "Authenticated and Confidential Channel Establishment" (ACCE) model, which we adapted and redefined for SAID and asynchronous messaging protocols in general into a model we call identity-based Multistage Asynchronous Messaging (iMAM). We believe our model to be more faithful in particular to the true security of Signal, whose use of the message keys prevents them from achieving the composable guarantee claimed by previous analysis.

Available format(s)
Cryptographic protocols
Publication info
Published elsewhere. Minor revision. EuroS&P 2019
SignalKey ExchangePost-compromise securityKey-ratcheting
Contact author(s)
xavier bultel @ yahoo fr
angele bossuat @ irisa fr
elena @ cs au dk
olivier blazy @ unilim fr
cristina onete @ gmail com
pa fouque @ gmail com
2019-04-11: received
Short URL
Creative Commons Attribution


      author = {Olivier Blazy and Angèle Bossuat and Xavier Bultel and Pierre-Alain Fouque and Cristina Onete and Elena Pagnin},
      title = {SAID: Reshaping Signal into an Identity-Based Asynchronous Messaging Protocol with Authenticated Ratcheting},
      howpublished = {Cryptology ePrint Archive, Paper 2019/367},
      year = {2019},
      note = {\url{}},
      url = {}
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