@LargeCardinal The group messaging mechanism is very different. In Signal (app) group messages are layered on top of the existing pairwise messaging system. To add a new user, you (an existing user) have to send encrypted control messages to each existing user. The server can’t do this.
@LargeCardinal In fact, the Signal server doesn’t even know what a group message is. As far as it’s concerned, when I send a message to a group of N people, it just looks like I’m sending a bunch of pairwise messages at the same time.
@LargeCardinal In WhatsApp (as of a few months ago) the server knows what group messages are. It actually sends special control (add/remove) requests to the members of the group, and it handles multiplexing group messages to all of the other users.
@LargeCardinal Oh heck I just remembered that I wrote about this all. There’s a great research paper linked in here. google.com/amp/s/blog.cry…
