Joining the conversation: balancing privacy with usability for encrypted messages (matrix-conf-2025)

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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](https://github.com/matrix-org/matrix-spec-proposals/pull/4268), 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. Licensed to the public under https://creativecommons.org/licenses/by-sa/4.0/ about this event: https://cfp.2025.matrix.org/matrix-conf-2025/talk/VLF9QQ/