SAID: Reshaping signal into an identity-based asynchronous messaging protocol with authenticated ratcheting

Research output: Contribution to book/anthology/report/proceedingArticle in proceedingsResearchpeer-review

  • Olivier Blazy, Universite de Limoges
  • ,
  • Angele Bossuat, Department of Integrated Structural Biology, IGBMC (Institute of Genetics and of Molecular and Cellular Biology), Centre National de la Recherche Scientifique (CNRS) UMR 7104/Institut National de la Santé de la Recherche Médicale (INSERM) U964/Université de Strasbourg
  • ,
  • Xavier Bultel, Department of Integrated Structural Biology, IGBMC (Institute of Genetics and of Molecular and Cellular Biology), Centre National de la Recherche Scientifique (CNRS) UMR 7104/Institut National de la Santé de la Recherche Médicale (INSERM) U964/Université de Strasbourg
  • ,
  • Pierre Alain Fouque, Department of Integrated Structural Biology, IGBMC (Institute of Genetics and of Molecular and Cellular Biology), Centre National de la Recherche Scientifique (CNRS) UMR 7104/Institut National de la Santé de la Recherche Médicale (INSERM) U964/Université de Strasbourg
  • ,
  • Cristina Onete, Universite de Limoges
  • ,
  • 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.

Original languageEnglish
Title of host publicationProceedings - 4th IEEE European Symposium on Security and Privacy, EURO S and P 2019
Number of pages16
PublisherIEEE
Publication year2019
Pages294-309
Article number8806729
ISBN (Electronic)9781728111476
DOIs
Publication statusPublished - 2019
Event4th IEEE European Symposium on Security and Privacy, EURO S and P 2019 - Stockholm, Sweden
Duration: 17 Jun 201919 Jun 2019

Conference

Conference4th IEEE European Symposium on Security and Privacy, EURO S and P 2019
LandSweden
ByStockholm
Periode17/06/201919/06/2019

    Research areas

  • Key Exchange, Key ratcheting, Post compromise security, Signal

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