[dodgerdan]

> The point is that if your threat model cares about interception, you have to verify the identity of users out of band - same as Signal or WhatsApp or whatever.

I’ve read your blog posts and this comment, congratulations it seems to have satisfied most people. Howerver it is NOT the same as Signal. Signal servers cannot just add a device to a group chat.

I would challenge you to get one reputable cryptographer to back what you’re claiming about these vulnerabilities and your proposed fixes.

To me Matrix isn’t secure, the organizations responses to these disclosures has been poor and the “fixes” weak.

[Arathorn]

You are completely misinterpreting my quote, which makes me question whether you are acting in good faith.

Totally agreed that Signal servers cannot just add a device to a group chat.

What I saying was: in any system, you have to verify users for security in general. Having verified users in Matrix, you then get a massive red warning if an unverified device is added to their accounts. Given we have cross-signing (i.e. users are heavily encouraged to verify their own devices when they log in), you can be sure that such unverified devices are malicious and take appropriate action.

The obvious thing we could do is to go one step further (as we used to, until we backed it out in https://github.com/matrix-org/matrix-react-sdk/pull/3837) and stop messages from flowing until the unverified device has been dealt with. Even better would be to make group membership controlled by the clients, so the server can't add devices at all. And we're working on this, as part of shifting the implementations over to the audited matrix-rust-sdk-crypto implementation to avoid having to solve the problem in quadruplicate.

> I would challenge you to get one reputable cryptographer to back what you’re claiming about these vulnerabilities and your proposed fixes.

Hopefully someone will pop up here and confirm that I'm not talking shit :) Failing that, you'll have to wait for the next Least Authority audit - we have another independent public audit queued once this wave of work finishes to address the "To me Matrix isn't secure" polemicists. You can see the last one (on the crypto layer, rather than the group membership layer) at https://matrix.org/blog/2022/05/16/independent-public-audit-... fwiw.

[cvwright]

It’s encouraging to hear that you’re getting an audit of the new approach.

Reading over the new design doc for authenticating membership events, and the Github comments, it feels like this is a case for some sort of model checking or automated theorem proving. Manually verifying that the new system does everything we want seems… tough.