> How do they manage to make the same messages appear on multiple devices?
For a long time, they didn’t.
I don’t know for sure, but I expect it involves keys that multiple devices share. Any “linked” device would be able to download the encrypted copy and decrypt the message that way. Once any device has done that, it can send a copy to any other devices using the unique keys it knows for that device.
This link describes independent queues for devices: https://support.signal.org/hc/en-us/articles/5532268300186-Disappearing-Messages-with-a-Linked-Device
I take it you weren’t aware that Google Wave was based on XMPP.
https://www.networkworld.com/article/757722/opensource-subnet-nine-new-apps-that-show-off-the-power-of-google-wave.html
@onlinepersona