Ben Pate

2025-12-31 16:57:29

MLS in ActivityPub

In December 2025, the Social Web Foundation launched a project to build End-to-End-Encryption (E2EE) into ActivityPub and the Fediverse, with funding from The Soveriegn Tech Fund. Here’s the the official project announcement.

Emissary is proud to be one of the servers chosen for this work. We are aiming to deliver the first version of an encrypted messenger in mid 2026.

In broad terms, we’re using Messaging Layer Security (MLS) Protocol to manage keys and encrypt data locally on users’ devices. MLS is an open, flexible industry standard has is being used “at scale” by a number of large organizations. We’re now implementing this in ActivityPub, using our existing servers to provide two important services to the MLS protocol:

1. MLS “directory service” to locate users and their encryption keys (WebFinger, Actor profiles)
2. MLS “delivery service” to send encrypted messages between clients (ActivityPub inboxes and outboxes)

Here is my list of the online resources that are helping me to understand the MLS protocol and how it is used within ActivityPub.

Official Specifications

• Messaging Layer Security in ActivityPub describes how to use MLS in ActivityPub
• RFC 9420 is the primary MLS specification from the IETF
• SWICG Github Project where we’re organizing our work and discussing issues

Tutorials

• Phoenix R&D Blog has a good introduction to the MLS protocol
• Positive Intentions Blog has a deep dive on building MLS applications (with Typescript code!)
• Demystifying MLS by Wire, one of the original participants in creating the MLS spec

Audio/Video Tutorials

• Podcast with Raphael Robers that I still need to listen to
• Video Presentation by Konrad Kohbrok and Raphael Robert that I still need to watch
• YouTube Video by Chalk Talk that I still need to wath

Libraries

• ts-mls is a Typescript library for the MLS protocol, and is the library I’m using for Emissary
• OpenMLS is a Rust library for the MLS protocol

Project Plan

If you’re following along from home, here’s how I’m tackling this problem and building now.

Building E2EE into Emissary is tricky. As a programmable ActivityPub server, Emissary uses server-rendered HTML templates for everything – even outbound JSON-LD documents pass through server-based filters. So, encrypting messages on the browser client will require a number of new technologies to be built into Emissary:

• ActivityPub API Inbox/Outbox - So far, Emissary hasn’t needed to support the client-facing ActivityPub API. So, I’m reworking the [hannibal library] to better support this, and building a true ActivityPub inbox/outbox into Emissary. This also paves the way for future work to support other client-to-server (C2S) interactions on Emissary.
• Support for new MLS message types - Hannibal has also been updated to recognize encrypted MLS messages when they reach the server. Their contents are opaque to the Emissary server, but we now know that they exist and can route them to users’ client apps correctly.
• Browser Client App - I’ve made a proof-of-concept using Mithril.js and ts-mls that places a fully client-based “conversations” app alongside Emissary’s server-side “inbox” app. I’ve used Mithril in the past, and appreciate its light weight and fast design as compared to heavier JS frameworks like React. The client-side app is built with esbuild so that we can also use JSX templates in the Mithril application. It’s looking really nice.

Project Status

2026-01-02 The “conversations” app is coming along. I can send plaintext messages to the server, and am working to populate Conversation objects and route messages to the correct recipients.

2026-01-03 I’m working with a demonstration MLS app featured on the positive intentions blog as a baseline for the MLS integration. It looks really promising.

2026-01-05 Modeling new apps from scratch is fun, but hard. There are so many ways it could go, but I have to pick the one it will go.. at least for now. Now that I have a “nearly working” outbox mechanism (with more still due, unfortunately) I’ve turned to face the actual MLS portion of this. I’m hoping to have some rudimentary encryption working in the next few days.

2026-01-06 Working to integrate features of the MLS demo app. And, I’ve made some progress on modeling the various services that this thing is going to need. It’s slow going, because there’s a lot to ingest. However, once I get past a few more thorny issues, I’m expecting to repurpose large sections of the app quickly :)

Building Scalable Group Messaging with MLS (Message Layer Security)

⚠️ WARNING: This document is not finished. The details in this document are subject to change.

^{xoron (positive-intentions)}