Comment by zajio1am | Hacker News Reader

Comment by zajio1am | Hacker News Reader

I use XMPP (with OMEMO) extensively and my main problem with it is absence of some basic key trust chains and bad handling of group chats

1) With OMEMO, each client/device of a user has its own key, but there is no trust relationship between them or some common master key. I can verify and trust counterparty key, but then counterparty adds another client/device and i have to do manual verification again.

2) Group chats do not have group key, but one encrypt messages for each group member (pairwise), which means that everybody has to do key exchange with everybody. This does not scale, especially with 1), which means that even if membership is stable, new keys may appear. Some clients (Conversations) do not allow to send message to a group when they do not know some keys of some group members/devices, which is fail situation that happen pretty often.

1) I think that's by design: you've got two ways to use OMEMO, the "friendly (default)-way" (with BTBV, Blind Trust Before Verification), and the "paranoid-way" (where you verify each and every device). In the latter, you don't want your messages to be encrypted for a new, unverified device (as it could potentially be that of an attacker): you can either verify the new device yourself, or your contact can verify theirs and you verify them again. Like I said, that's not for everyone. And if you are not that paranoid, then verification is an afterthought anyway and you don't lose or gain much with verification anyway.

2) That's where MLS¹ kicks in, essentially a new encryption scheme that's suitable for large group chats. There are projects² for its adoption in XMPP. I have no strong opinion about MLS, but I tend to think that it's more for the gimmick and having XMPP score well in random protocol benchmarks: if you've got such a large groupchat that OMEMO doesn't scale anymore, you have a groupchat with so many participants that it is only private in name and more or less encryption doesn't matter.

¹: https://en.wikipedia.org/wiki/Messaging_Layer_Security

²: https://nlnet.nl/project/XMPP-MLS/

I've tried designing an E2EE group chat system before and ran into all of these. They're not impossible to solve but are very hard. Telegram's official docs shrug off multi-device E2EE too, calling it a "mess."

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