2025-10-17 –, Alan Turing
Being able to read the conversation in a room before you joined is a critical feature for some usecases, but end-to-end encryption makes it tricky to implement.
In this technically-focussed talk, we'll cover the challenges behind implementing "history sharing", why previous attempts in this area failed, and how we solved those problems with minimal impact on the security or efficiency of the protocol.
The Element Crypto team have been working on MSC4268, aka "Sharing room keys for past messages". We'll talk about why this is difficult from the cryptographic perspective, and discuss the architecture behind the implementation.
Rich is a Staff Engineer at Element, and has been part of the Matrix project since 2015. In that time he's touched all sorts of things, including Element-Web and Synapse. He implemented the early steps towards encryption in Element Web, and more recently is back in the encryption sphere as part of Element's Crypto team.