Earlier quoted context omitted.
> I don't see why a messaging service like Signal cares about mutual friends. Signal doesn't care; users care about mutual friends. Here's an example: - Alice installs Signal². She has many contacts, and doesn't know which contacts also use Signal². Notably, she doesn't want to give the Signal² servers all of her contacts. - Alice asks Bob to install Signal². He does, and they trade key information (e.g. via SMS, ema…
Users also won't see contacts who use Signal and do have mutual friends in common, but who simply haven't used Signal to talk to those mutual friends. Your solution is certainly better than nothing, but it relies on people having actually used Signal to talk to each other in the past. It also appears to involve a lot of P2P coordination. And while it may not share contacts with the Signal servers, it does leak your a…
Well, it is trying to incentivise Signal² usage:-)
> It also appears to involve a lot of P2P coordination.
Not necessarily a huge amount, I think — folks could SMS/email stuff as well as trade contacts face to face, depending on their risk preferences.
> it does leak your address book to all of your friends
Only where there are overlaps: since it's private set intersection, Alice & Bob only see their mutual acquaintances, not other folks. But yes, the mutual-thereapist issue exists. Presumably an expert mode might enable folks to whitelist/blacklist particular contacts when they're added.
Certainly, there are ways in which this approach is worse & more complex than the current Signal approach, but it is better in another way: it preserves privacy from Signal (and its hosting provider, and Intel, and any government which can coërce Signal, its hosting provider and/or Intel into subverting security).