direct messages... chat...
the "deprecated" kind that has both sender and receiver pubkeys in it, as opposed to the receiver only 1059/1060 "giftwrap"
that pesky thing that can only be moderately protecting of user privacy IF CLIENTS IMPLEMENT NIP-42 authentication!!!!!
which nobody seems to want to do, and i've got a colleague from the company i work for adding this feature to #nostrudel if nostr:npub1ye5ptcxfyyxl5vjvdjar2ua3f0hynkjzpx552mu5snj3qmx5pzjscpknpr won't do it (if you don't get it added it will be offered as a PR on a fork in a couple weeks time)