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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.

Thanks! Since I've also written a lot of audio/video call code, it's always interesting to learn about design choices of other applications.

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?

We rotate the keys whenever someone joins or leaves, although the mechanism is more efficient when someone joins than one someone leaves.

I don't think we've made the SFU repository public yet, so I don't think I can comment on that yet. Likely I'll be able to in the future.