Popular messaging applications now enable end-to-end-encryption (E2EE) by default, and E2EE data storage is becoming common. These important advances for security and privacy create new content moderation challenges for online services, because services can no longer directly access plaintext content. While ongoing public policy debates about E2EE and content moderation in the United States and European Union emphasize child sexual abuse material and misinformation in messaging and storage, we identify and synthesize a wealth of scholarship that goes far beyond those topics. We bridge literature that is diverse in both content moderation subject matter, such as malware, spam, hate speech, terrorist content, and enterprise policy compliance, as well as intended deployments, including not only privacy-preserving content moderation for messaging, email, and cloud storage, but also private introspection of encrypted web traffic by middleboxes. In this work, we systematize the study of content moderation in E2EE settings. We set out a process pipeline for content moderation, drawing on a broad interdisciplinary literature that is not specific to E2EE. We examine cryptography and policy design choices at all stages of this pipeline, and we suggest areas of future research to fill gaps in literature and better understand possible paths forward.

翻译：主流即时通讯应用现已默认启用端到端加密（E2EE），且E2EE数据存储逐渐普及。这些安全与隐私领域的重大进展为在线服务带来了全新的内容审核挑战——服务方无法再直接访问明文内容。尽管美国和欧盟围绕E2EE与内容审核的公共政策辩论主要聚焦于即时通讯和存储中的儿童性虐待材料及虚假信息，但我们识别并整合了大量超越这些议题的学术研究。我们桥接了内容审核主题（如恶意软件、垃圾邮件、仇恨言论、恐怖主义内容与企业政策合规）及应用场景（不仅涵盖面向即时通讯、电子邮件和云存储的隐私保护型内容审核，还包括中间设备对加密网络流量的私密审查）均呈现多样性的文献体系。在本研究中，我们系统梳理了E2EE环境下的内容审核研究，基于广泛跨学科文献（非E2EE特定场景）构建了内容审核流程管道，考察了该流程各阶段中密码学与策略设计的选择，并提出未来研究方向以填补文献空白、探索可行发展路径。