>>153858
Admittedly I haven't read through your post thoroughly, but isn't the same thing used for PMs and Inboxes here?
https://img.heyuri.net/b/koko.php?mode=module&load=privateMessage&action=write
https://img.heyuri.net/b/koko.php?mode=module&load=privateMessage

I haven't used it myself, but it seems like it's similar to what you write about public and private keys.

You have your tripcode passwords (private) and the set of hash letters that comes after your username (public).