Paper 2020/1281

Key Agreement for Decentralized Secure Group Messaging with Strong Security Guarantees

Matthew Weidner, Martin Kleppmann, Daniel Hugenroth, and Alastair R. Beresford


Secure group messaging protocols, providing end-to-end encryption for group communication, need to handle mobile devices frequently being offline, group members being added or removed, and the possibility of device compromises during long-lived chat sessions. Existing work targets a centralized network model in which all messages are routed through a single server, which is trusted to provide a consistent total order on updates to the group state. In this paper we adapt secure group messaging for decentralized networks that have no central authority. Servers may still optionally be used, but they are trusted less. We define decentralized continuous group key agreement (DCGKA), a new cryptographic primitive encompassing the core of a decentralized secure group messaging protocol; we give a practical construction of a DCGKA protocol and prove its security; and we describe how to construct a full messaging protocol from DCGKA. In the face of device compromise our protocol achieves forward secrecy and post-compromise security. We evaluate the performance of a prototype implementation, and demonstrate that our protocol has practical efficiency.

Note: Includes appendices B and D not present in the CCS version. Relative to previous IACR version, substantially revises the presentation and proofs.

Available format(s)
Cryptographic protocols
Publication info
Published elsewhere. Minor revision. ACM CCS 2021
secure messaginggroup messagingdecentralizationforward secrecypost-compromise security
Contact author(s)
maweidne @ andrew cmu edu
mk428 @ cst cam ac uk
dh623 @ cst cam ac uk
arb33 @ cst cam ac uk
2021-05-31: revised
2020-10-14: received
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Creative Commons Attribution


      author = {Matthew Weidner and Martin Kleppmann and Daniel Hugenroth and Alastair R.  Beresford},
      title = {Key Agreement for Decentralized Secure Group Messaging with Strong Security Guarantees},
      howpublished = {Cryptology ePrint Archive, Paper 2020/1281},
      year = {2020},
      note = {\url{}},
      url = {}
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