Paper 2025/469
Practical Semi-Open Chat Groups for Secure Messaging Applications
, LASIGE, Universidade de Lisboa, Aarhus University, University of SurreyAbstract
Secure messaging groups in applications such as Signal, Telegram, and Whatsapp are nowadays used for rapid and widespread dissemination of information to large groups of people. This is common even in sensitive contexts, involving the organisation of protests, activist groups, and internal company dialogues, for instance. Manual administration of who has access to such groups quickly becomes infeasible, in the presence of even hundreds of members. We construct a practical, privacy-preserving reputation protocol, that automates the approval of new group members based on their reputation amongst the existing membership. We prove security against malicious adversaries in a single-server model, with no further trust assumptions required, while supporting arbitrary reputation calculations even when almost all group members are offline (as is likely). We demonstrate the practicality of the approach experimentally: for groups of size 50 (resp. 500), admitting a user that received 40 (resp. 80) scores requires 1312.2 KiB (resp. 13086.3 KiB) of communication, and 3.4 s (resp. 42.3 s) of single-threaded computation. While our protocol design matches existing secure messaging applications, we believe it can have value in distributed reputation computation beyond this problem setting.
Note: To appear in PoPETs 2026, Issue 3
Metadata
- Available format(s)
-
PDF
- Category
- Cryptographic protocols
- Publication info
- Published elsewhere. PETS 2026
- Keywords
- Secure MessagingPrivacy-preserving reputationE-voting
- Contact author(s)
-
alex davidson @ fc ul pt
reisbs luiza @ gmail com
f virdia @ surrey ac uk - History
- 2026-04-22: last of 2 revisions
- 2025-03-12: received
- See all versions
- Short URL
- https://ia.cr/2025/469
- License
-
CC BY
BibTeX
@misc{cryptoeprint:2025/469,
author = {Alex Davidson and Luiza Soezima and Fernando Virdia},
title = {Practical Semi-Open Chat Groups for Secure Messaging Applications},
howpublished = {Cryptology {ePrint} Archive, Paper 2025/469},
year = {2025},
url = {https://eprint.iacr.org/2025/469}
}
