Again, in the theme of "features every group messaging system had already, but Signal didn't, because they hadn't figured out a way to implement it without turning Signal's central servers into a database of who's talking to who about what". Signal didn't even have
user profiles until recently, for the same reason. Here, they've slightly expanded the state of the art in MAC-based anonymous credentials to accomplish their goal.
One interesting aspect of this is that Signal gets to do this, because they have immense goodwill with the cryptographic research and engineering communities; though it's no guarantee of soundness, they have the advantage of having the feature designed, implemented, and ultimately reviewed by cryptography engineers that aren't generally/economically available to other messaging projects.
This is either a reason you love Signal (raises hand) or can't stand Signal. My take is, if you're in the latter group, that's fine; I use Slack, too.
One interesting aspect of this is that Signal gets to do this, because they have immense goodwill with the cryptographic research and engineering communities; though it's no guarantee of soundness, they have the advantage of having the feature designed, implemented, and ultimately reviewed by cryptography engineers that aren't generally/economically available to other messaging projects.
This is either a reason you love Signal (raises hand) or can't stand Signal. My take is, if you're in the latter group, that's fine; I use Slack, too.