> But if you are actually concerned about the identity of the person, passport numbers, etc are better uids.
Why not just name + email? I don't think putting anything like a passport number is a wise idea. It's just putting one more semi-secret information on the internet.
I think the encryption and signing+trust gets mixed here. If I know someone only by email address, I don't trust their key very much. But I'm still going to use it to encrypt messages to them, because it's better than nothing. I'm going to trust it if it's got a track record of reasonable messages on mailing lists or git commits over some period of time.
But back to the main topic: email-only, and passport number are two extremes and the second one is even hard to verify. Most people actually just use name+email combination in uids. And that's what gpg invites you to do when generating keys. Why didn't you mention it?