Paper 2026/010

Third-Party Moderation of Abuse Reports for End-to-End Encrypted Messaging with Multiple Moderators

Abstract

Abuse reporting tools like message franking allow end-to-end encrypted (E2EE) messaging platforms to verify user-generated abuse reports as part of their platform content moderation policies. While the lightweight message franking protocol deployed by WhatsApp and Meta's Messenger is designed with the assumption that the moderator and the platform processing messages are the same entity, proposals for other message franking-style protocols allow for a separation between the platform and moderator, albeit at a higher cost. This paper introduces new techniques for third-party moderation on E2EE messaging platforms. First, we develop a simple scheme with near-optimal performance that matches the cost of the deployed scheme on the client side, and we demonstrate the inherent necessity of increased server-side costs. Next, we introduce new techniques that allow E2EE messaging platforms to work with multiple vetted moderators, giving users options in their choice of moderators for messages they send to their friends. Verifiable abuse reporting in a multi-moderator setting requires new security notions to capture deniability requirements with respect to the platform and other moderators, as well as new privacy requirements with respect to users' choices of moderator(s). We comprehensively study these requirements and propose protocols for verifiable abuse reporting in this setting, offering a range of security and performance tradeoffs for different deployment scenarios. We evaluate the performance of our proposed schemes, showing that in many cases they match or exceed the performance of prior schemes that only support a single moderator.