End-to-End Encrypted Collaborative Documents

Collaborative documents (e.g., Google Docs, Microsoft 365) often contain sensitive information such as personal or financial data. In this work, we extend the protection of E2EE encryption, currently (mostly) restricted to the use case of messaging, to collaborative documents. We elicit and formalize the security and functional requirements of End-to-End Encrypted Collaborative Documents (E2EE-CD). We then put forth a generic framework to realize E2EE-CD, by combining an end-to-end encrypted asynchronous broadcast channel with any edit reconciliation mechanism which ensures globally consistent views of a document. We give formal proofs that directly relate the security of our E2EE-CD solution to the security of the underlying end-to-end encrypted communication channel. We then elicit additional deployment requirements for E2EE-CD for investigative journalists and design SignalCD, an E2EE-CD system built on top of Signal's group messaging protocol tailored for this setting. We analyze the security guarantees of SignalCD, implement a prototype, and empirically show that our solution is efficient enough to permit real-time collaboration.