The identity here would be based on cryptographic signatures so it would look ultimately have to look like some sort of key fingerprint. For example, an OpenPG

The identity here would be based on cryptographic signatures so it would look ultimately have to look like some sort of key fingerprint. For example, an OpenPGP identity contains one or more email addresses. So the fingerprint is all you need to give anyone. In practice this can be provided in the form of a QR code. There is even a standard way to do this. A secure mail list running on a server has to reencrypt the message to each recipent (it has access to the messages). So you would give the mail list server something like a key fingerprint. The original signatures would ideally survive the trip through the server.