The paper says ghost protocol has never been implemented, but wasn't it widely used on trump administration version of the signal-custom-client, which sent messages to a ghost account responsible for indexing and backup of plain text copies?
Good point. Most of this was written before the signal-custom-client thing happened, so it wasn't top of mind. We also considered "serious" systems, deployed en masse. Perhaps we should've included this as well.
Thanks for putting this together. Found via Schneier’s blog; I thought it was a good summary, and perhaps needed information in the face of attacks like Chat Control.
Oh, I didn't realize it'd made it to Schneier's blog, thanks for letting me know!
A goal of the work was to try to explain to a new audience (specifically nontechnical legal folks) the ongoing challenges with Going Dark. Hopefully it helps!