The NavTalks are a series of informal talks given by Navigators members or some special guests.

Title: Practical Semi-Open Chat Groups for Secure Messaging Applications
Speaker: Alex Davidson (LASIGE/Ciências ULisboa)
When: Thursday, October 12, 2026, at 14:00
Where: 6.3.27

Abstract: 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.

Short Bio: Alex Davidson is an Assistant Professor in the Departments of Informatics of the Faculty of Sciences at the University of Lisbon, and an integrated member of the LASIGE research centre. His research is broadly situated within the field of cryptography, the mathematical study of secure communication. His recent research has focused on the development of frameworks for privacy-preserving retrieval of data from online databases, large-scale comparisons and aggregations of sensitive datapoints, and online mechanisms for evaluating reputation of individuals without revealing personally identifiable traits or characteristics. Since completing his PhD studies (Information Security Group, Royal Holloway, University of London, 2014-2019), he has undertaken subsequent periods of scientific investigation in both industry (Cloudflare 2018-2020, Brave 2021-2022) and academia (post-doc @ LIP 2020-2021, Assistant Professor @ NOVA FCT 2023-2024).