Content Moderation for End-to-End Encrypted Messaging

I aim to demonstrate these clarifications by formalizing specific content moderation properties for end-to-end encrypted messaging, then offering at least one possible protocol design for each property. User Reporting: If a user receives a message that he or she believes contains harmful content, can the user report that message to the service provider? Known Content Detection: Can the service provider automatically detect when a user shares content that has previously been labeled as harmful? Classifier-based Content Detection: Can the service provider detect when a user shares new content that has not been previously identified as harmful, but that an automated classifier predicts may be harmful? Content Tracing: If the service provider identifies a message that contains harmful content, and the message has been forwarded by a sequence of users, can the service provider trace which users forwarded the message? Popular Content Collection: Can the service provider curate a set of content that has been shared by a large number of users, without knowing which users shared the content?