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Comment by pseudo0 | Hacker News Reader

You can decrypt all future messages in a noisy way that alerts all the users that something weird is going on.

> While the Matrix specification does not require a mitigation of this behaviour, when a user is added to a room, Element will display this as an event in the timeline. Thus, to users of Element this is detectable.

Virtually everyone uses Element, so it's blatantly tamper-evident. That makes it a really bad option if red-teaming.

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"Tamper-evident", meaning, Element will list the string name of the member the server injected into your group in the group list.

(Here it's worth noting that this is the behavior post-fix for this paper. Read the paper! It was so much worse.)

I have used Element, and new users joining a room are displayed as an event in the timeline. The paper confirms that in section III-A, and based on the context I'm 99% sure that they mean pre-fix, not post-fix.

With III-B, they can add an unverified device, but that causes a warning for every other room participant. Again, tamper evident.

We're clear that this is "an event in your timeline" that says "the server might be decrypting all your messages or maybe a new participant joined your group", right?

If some random user joins my E2EE groupchat, yes, my expectation is that some unknown actor is now reading my messages... It would be no different than some rando getting added to a Signal group. At least in the groups I'm in, that would cause a flurry of messages asking who they are and who added them.

What if someone already in the group adds a new device? That would hardly cause a flurry or surprise, it’s a common thing to happen. But using Matrix it could also mean whom ever controls the server could be reading all your messages. And the alert has to cover both scenarios. I’m not sure how you could message that in such a way to make it actionable. Especially to non-tech users. Even if they did understand it would require a mini investigation/“flurry” multiple times a month for even a modest sized group. My personal observation is that a group of about 50 have 1 new phone every two weeks, with seasonal increases around Sept/Oct and January 1st.

If verified that device will show up red. And you can enable sending keys only to verified contacts

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