[KvanteKat]

It doesn't involve cryptography, but mastodon has for at least a couple of years supported link-verification in profiles (it basically checks if a link back to your mastodon profile exists on a page linked on your profile), so a linking to a page that only you credibly control (say, a personal website) is the de-facto system of decentralized user-verification on mastodon.

Edit: supported since 2018 https://github.com/mastodon/mastodon/pull/8703

[jefftk]

It does sort of involve crypto: if the page you control weren't served over HTTPS it wouldn't be too hard (DNS poisoning) for someone else to trick a server into verifying the wrong user.

[account42]

If DNS poisoning is so easy why don't you perform the same attach on Let's Encrypt since that also uses plain HTTP.

[jefftk]

Certificate Authorities are extremely careful about DNS (and BGP hijacking), more so than basically everyone else.