Today's global society strongly relies on collaborative document editing, which plays an increasingly large role in sensitive work-flows. While other collaborative venues, such as secure messaging, have seen secure protocols being standardized and widely implemented, the same cannot be said for collaborative document editing. Popular tools such as Google Docs, Microsoft Office365 and Etherpad are used to col-laboratively write reports and other documents which are frequently sensitive and confidential, in spite of the server having the ability to read and modify text undetected. Capsule is the first formalized and formally verified protocol that addresses secure collaborative document editing. Capsule provides confidentiality and integrity on encrypted document data, while also guaranteeing the ephemeral identity of collaborators and preventing the server from adding new collaborators to the document. Capsule also, to an extent, prevents the server from serving different versions of the document being collaborated on. In this paper, we provide a full protocol description of Capsule. We also provide formal verification results on the Capsule protocol in the symbolic model. Finally, we present a full software implementation of Capsule, which includes a novel formally verified signing primitive implementation.