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This is odd because I know a few mainland Chinese people that use XMPP without problems (and afaik without a VPN).

Sounds like your server got blocked for another reason?

Well, instead of leaking metadata to Signal, AWS, Cloudflare, Google/Apple and your ISP, like Signal does, RCS only leaks it to your ISP /s

You can easily redirect xmpp to port 443 which is not blocked by most firewalls. If you have problems with firewalls or public wifis your xmpp server is misconfigured.

The actual military grade (xmpp based) messengers implement security lables, meaning messages are tagged with the required security clearance and if you invite random people to a chat they can’t see the messages.

The server can swap to a modified JS that exfiltrates the e2ee key and thus allows the server owner to decrypt the messages, or in more advanced encryption schemes add additional keys without you knowing and achieve the same thing.

Yes you can juryrig something like that with cookies, but it is highly fragile and browser based e2ee is basically a scam anyways as the server serving the website can always swap out the javascript that decrypts the messages.

e2ee is not really compatible with what you want due to necessary key management, and once you drop that there are so many possible options for what you want that I don’t want to list them all here.