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 <i>user profiles</i> 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.<p>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.<p>This is either a reason you <i>love</i> Signal (raises hand) or <i>can't stand</i> Signal. My take is, if you're in the latter group, that's fine; I use Slack, too.