You could go the keybase route and tie in with various platforms using anywhere that you can post a public message to post "proof". So if you wanted to sign a

You could go the keybase route and tie in with various platforms using anywhere that you can post a public message to post "proof". So if you wanted to sign a windows executable, you'd need to have a microsoft account, and that microsoft account would need a place that you could publicly post a string of text. Then it's up to the Lets Encrypt service to validate that and cut the certificate