[yuliyp]

(a) it means each code can only be redeemed once, which is a bit of a usability hit (b) it still doesn't stop a malicious server, which can try to MITM the group join and proxy messages to the client.

[baby]

If this is a usability problem then remove this line:

> and everyone deletes the code

I'm not sure I understand your attack in (b), the message is encrypted to the participants the server cannot relay or mitm it.