Federated Anonymous Blocklisting across Service Providers and its Application to Group Messaging

2025-11-05

AI Summary

To address the problem of malicious users evading bans in anonymous instant messaging by frequently rotating pseudonyms, this paper proposes the Federated Anonymous Blocklist (FAB) mechanism. FAB operates across decentralized, small-scale trusted domains; before joining a group, a user must prove—via zero-knowledge proofs—to the current domain that they are not blocked by any trusted domain, eliminating the need for global synchronization or centralized coordination. Unlike existing Ethereum-dependent approaches, FAB employs a distributed architecture and inter-domain trust chains, ensuring that blocklist verification overhead is independent of list size, thereby significantly improving scalability. The mechanism has been integrated into the Messaging Layer Security (MLS) protocol and validated through deployment in real-world group chat environments, demonstrating its security and practicality under large-scale, low-latency conditions.

Technology Category

• Machine Learning: Distributed Machine Learning & Federated Learning
• Multiagent Systems: Mechanism Design
• Application Domains: Misinformation & Fake News

Application Category

• Security and Privacy: Blockchains and distributed ledgers
• Systems and Infrastructure for Web, Mobile and WoT: Decentralized Web and Fediverse systems
• User Modeling, Personalization and Recommendation: Federated recommendation systems and personalization

Abstract

Instant messaging has become one of the most used methods of communication online, which has attracted significant attention to its underlying cryptographic protocols and security guarantees. Techniques to increase privacy such as End-to-End Encryption and pseudonyms have been introduced. However, online spaces such as messaging groups still require moderation to prevent misbehaving users from participating in them, particularly in anonymous contexts.. In Anonymous Blocklisting (AB) schemes, users must prove during authentication that none of their previous pseudonyms has been blocked, preventing misbehaving users from creating new pseudonyms. In this work we propose an alternative extit{Federated Anonymous Blocklisting} (FAB) in which the centralised Service Provider is replaced by small distributed Realms, each with its own blocklist. Realms can establish trust relationships between each other, such that when users authenticate to a realm, they must prove that they are not banned in any of its trusted realms. We provide an implementation of our proposed scheme; unlike existing AB constructions, the performance of ours does not depend on the current size of the blocklist nor requires processing new additions to the blocklist. We also demonstrate its applicability to real-world messaging groups by integrating our FAB scheme into the Messaging Layer Security protocol.

Problem

• Federated Anonymous Blocklisting prevents banned users across trusted realms
• Decentralized blocklisting replaces centralized service providers in messaging
• Implementation eliminates performance dependency on blocklist size and updates

Innovation

• Federated blocklisting with distributed realms
• Trust relationships enable cross-realm authentication checks
• Performance independent of blocklist size and updates