I rediscovered IndieAuth and wanted to try it finally.
As authentication method, I decided to use my public GPG key because it’s public anyway and something interesting and new to me (I actually never signed anything on the console before).
It took a little trial and error before I found the following solution for signing the key:
gpg --clearsign --default-key name@example.tld gpgmessage.txt
It’s not perfect; it requires putting the message that needs to be signed in a file and the result gets written in a new file with the extension .asc. It would be much nicer if it allowed doing both directly on the console.