[messaging] Capsule: A Protocol for Secure Collaborative Document Editing

Nadim Kobeissi nadim at nadim.computer
Wed Mar 7 11:59:59 PST 2018

Dear respected peers,

Today's global society strongly relies on collaborative document editing, which plays an increasingly large role in sensitive workflows. 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 collaboratively 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 standard 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.

A proposal of Capsule is available here:


In this paper, I provide a full protocol description of Capsule. I also provide formal verification results on the Capsule protocol in the symbolic model. Finally, I present a full software implementation of Capsule, which includes a novel formally verified signing primitive implementation.

As it stands, Capsule is by no means a finalized protocol, and all that is presented in the preprint linked above is preliminary and very open to suggestions.

Capsule is by no means a protocol as involved or innovative as some others, such as the Signal protocol. However, I believe that it is valuable to see such a protocol solidly proposed and formalized, given that its targeted use case is underserved despite its legitimacy.

I welcome your feedback on the current Capsule draft. I enjoy working on this project and hope to make of it a good software soon. Let's discuss it and share our opinions!


Sent from my computer
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