Who has age authority? A state agency or service. Like the state issues an ID with age.
Preferable, we want the user to interact with a website, that website request age authentication, but not the website to talk to the government, but through the user.
Thus, something/somewhat like
- State agency issues a certificate to the user
- User assigns a password to encrypt the user certificate
- User connects to random website A
- Random website A creates an age verification request signed to only be resolveable by state agency but sends it to the user
- User sends the request to a state service with their user certificate for authentication
- State agency confirms-signs the response
- User passes the responds along to the random website A
There may be alternative, simpler, or less verbose/complicated alternatives. But I'm sure it would be possible, and I think it lays out how "double-blind"(?) could work.
The random website A does not know the identity or age of the user - only to the degree they requested to verify - and the state agency knows only of a request, not its origin or application - to the degree the request and user pass-along includes.