Cryptology ePrint Archive: Report 2020/1281

Key Agreement for Decentralized Secure Group Messaging with Strong Security Guarantees

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

Abstract: 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.

Category / Keywords: cryptographic protocols / secure messaging, group messaging, decentralization, forward secrecy, post-compromise security

Original Publication (with minor differences): ACM CCS 2021

Date: received 13 Oct 2020, last revised 31 May 2021

Contact author: maweidne at andrew cmu edu, mk428@cst cam ac uk, dh623@cst cam ac uk, arb33@cst cam ac uk

Available format(s): PDF | BibTeX Citation

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

Version: 20210531:162916 (All versions of this report)

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