Tangentially related, but since we're talking about Signal and have Moxie's attention ...

Does anybody know how the "privacy preserving contact discovery" works?

One of my unpleasant experiences with Signal was receiving a greeting from an unknown number when I installed it. Fortunately, it was a friend with a new number I hadn't known ... but the potential privacy leak - without any apparent warning, consent or opportunity to opt-out - bothered me.

Best info I could find is a blog basically saying "privacy preserving contact discovery is an unsolved problem" ... which is hardly reassuring:

https://whispersystems.org/blog/contact-discovery/

What's the story?