  In this paper, we assess existing technical proposals for content moderation
in End-to-End Encryption (E2EE) services. First, we explain the various tools
in the content moderation toolbox, how they are used, and the different phases
of the moderation cycle, including detection of unwanted content. We then lay
out a definition of encryption and E2EE, which includes privacy and security
guarantees for end-users, before assessing current technical proposals for the
detection of unwanted content in E2EE services against those guarantees.
  We find that technical approaches for user-reporting and meta-data analysis
are the most likely to preserve privacy and security guarantees for end-users.
Both provide effective tools that can detect significant amounts of different
types of problematic content on E2EE services, including abusive and harassing
messages, spam, mis- and disinformation, and CSAM, although more research is
required to improve these tools and better measure their effectiveness.
Conversely, we find that other techniques that purport to facilitate content
detection in E2EE systems have the effect of undermining key security
guarantees of E2EE systems.
