Paper 2025/469

Practical Semi-Open Chat Groups for Secure Messaging Applications

Abstract

Chat groups in secure messaging 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, associated with the organisation of protests, activist groups, and internal company dialogues. Manual administration of who has access to such groups quickly becomes infeasible, in the presence of hundreds or thousands of members. We construct a practical, privacy-preserving reputation system, that automates the approval of new group members based on their reputation amongst the existing membership. We demonstrate security against malicious adversaries in a single-server model, with no further trust assumptions required. Furthermore, our protocol supports arbitrary reputation calculations while almost all group members are offline (as is likely). In addition, we demonstrate the practicality of the approach via an open-source implementation. For groups of size 50 (resp. 200), an admission process on a user that received 40 (resp. 80) scores requires 1312.2 KiB (resp. 5239.4 KiB) of communication, and 3.3s (resp. 16.3s) of overall computation on a single core. While our protocol design matches existing secure messaging applications, we believe it can have value in distributed reputation computation beyond this problem setting.