Does someone know more about the architecture of Signal's group calls? From looking at the ringrtc source, it seems that the calls are not fully meshed, but use some kind of group key and a SFU for server-side media distribution. Is that correct?
I work at Signal and wrote much of that code, so I'm familiar with it :). Yes, it uses a server for media distribution and uses encryption keys that the server does not know.
Do you rotate the group key regularly and/or on group events (e.g. a person leaving the group call)?
What SFU do you use, and did it require custom modifications? Or is it even a fully custom server-side solution?