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The Key Lattice Framework for Concurrent Group Messaging

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Applied Cryptography and Network Security (ACNS 2024)

Part of the book series: Lecture Notes in Computer Science ((LNCS,volume 14584))

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Abstract

Today, two-party secure messaging is well-understood and widely adopted, e.g., Signal and WhatsApp. Multiparty protocols for secure group messaging are less mature and many protocols with different tradeoffs exist. Generally, such protocols require parties to first agree on a shared secret group key and then periodically update it while preserving forward secrecy (FS) and post compromise security (PCS).

We present a new framework, called a key lattice, for managing keys in concurrent group messaging. Our framework can be seen as a “key management” layer that enables concurrent group messaging when secure pairwise channels are available. Security of group messaging protocols defined using the key lattice incorporates both FS and PCS simply and naturally. Our framework combines both FS and PCS into directional variants of the same abstraction, and additionally avoids dependence on time-based epochs.

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Notes

  1. 1.

    https://messaginglayersecurity.rocks/.

  2. 2.

    This approach bears some resemblance to the analysis of Fuchsbauer et al. [24] for public key re-encryption.

  3. 3.

    This tradeoff was similarly explored by [28]; our asynchronous security model specifically accounts for the attacks they describe by withholding some ciphertexts and corrupting a party days later to recover the messages.

  4. 4.

    Some authentication schemes require parties to sign messages with their long-term keys [23] but adapting this to concurrent group messaging is non-trivial, and not the focus of this work.

  5. 5.

    In practice we cannot use the PRF construction because it is not commutative.

  6. 6.

    We remark that the standard definition of one-wayness requires the adversary to find an equivalent pre-image of the function, and not the exact same pre-image.

  7. 7.

    In this work, every graph is a directed acyclic graph.

  8. 8.

    If verification fails due to trying the wrong key from multiple concurrent sessions, return \(\perp \) and process the incoming message via \(\textsf{Recv}\) of a different session.

  9. 9.

    For our construction, this adds all of the edges in \(E_{U,i}\) to \(E^{\textsf{rev}}_{\textsf{sid}}\).

  10. 10.

    This hash function’s purpose is semantic to convert between types. We only require (informally) that if the adversary does not know k then it does not know \(\textsf{H}(k)\). We elide discussion of \(\textsf{H}\) in the proof.

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Acknowledgments

This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) and Space and Naval Warfare Systems Center, Pacific (SSC Pacific) under contract No. FA8750-19-C-0502 (Approved for Public Release, Distribution Unlimited). The first and third author would also like to thank the FWO under an Odysseus project GOH9718N, and by CyberSecurity Research Flanders with reference number VR20192203.

The work of the first author was conducted whilst he was at KU Leuven, the third author whilst he was at SRI International, and the fourth author whilst he was a student at UC Irvine.

Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of any of the funders. The U.S. Government is authorized to reproduce and distribute reprints for governmental purposes notwithstanding any copyright annotation therein.

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Authors and Affiliations

  1. COSIC, KU Leuven, Leuven, Belgium

    Nigel P. Smart

  2. SRI International, Menlo Park, USA

    Karim Eldefrawy & Ben Terner

  3. Zama Inc., Paris, France

    Kelong Cong & Nigel P. Smart

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  1. Kelong Cong
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Correspondence to Nigel P. Smart .

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Editors and Affiliations

  1. New York University Abu Dhabi, Abu Dhabi, United Arab Emirates

    Christina Pöpper

  2. Radboud University Nijmegen, Nijmegen, The Netherlands

    Lejla Batina

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Cong, K., Eldefrawy, K., Smart, N.P., Terner, B. (2024). The Key Lattice Framework for Concurrent Group Messaging. In: Pöpper, C., Batina, L. (eds) Applied Cryptography and Network Security. ACNS 2024. Lecture Notes in Computer Science, vol 14584. Springer, Cham. https://doi.org/10.1007/978-3-031-54773-7_6

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