You haven't provided an alternative way though? If you're looking for more complicated things they can do see: https://news.ycombinator.com/item?id=41430157

> Yeah you can, you log them in then link them to that userData with an identifier - typically an email address or unique user ID. You can easily write the login API to know nothing but hashes, or you can write it to respond with - to use your example that user's age - if the password is correct (without ever actually knowing the password).

This doesn't make sense, how do you verify someones age without getting their birthday? Hashes are binary yes/no checks, not range checks.

> Anybody can verify any document with enough identifying information about the document and a registry to match it up to.

So your alternate solution is that every government in the world runs an API you can check a passport against? Instead of them just providing a known public key they signed the passport with? Sounds way over complicated compared to a ZK proof.