Comment by tzs

You've almost got it right. You just need to modify this part:

> Just as the 3rd party site could use SSO login from your identity provider, perhaps the identity provider could provide signed evidence to the 3rd party site that asserts "I have verified that this person is age X" but not divulge their identity

The way you compared it so SSO login makes it sounds like there would be interaction between the 3rd party site and the identity provider. That's bad because if someone got a hold of the records from both the site and the identity provider they might be able to match access time logs and figure out who you are.

A fix is to make it so you get your signed document from the identity provider ahead of time, and that document is not tied to doing age verification with any particular site(s). You get it once and then use it with as many sites as you want.

When you use it with a site to demonstrate age we need to do that in such a way that neither of you have to communicate with the identity provider. If the site needs to verify a signature of the identity provider on something you present they use the provider's previously published public key.

We need to make it so that when you use the signed document from the identity provider to show your age to a site they don't see enough from the document to identify you, even if they have been compromised and are collaborating with the identity provider to try to identify you.

Finally, the signed document should be bound to you in some way so that you can't just make copies and give them to others or sell them on the black market to people who want to evade age checks.

BTW, since under this approach the identity provide isn't actively involved after their issue your signed document what probably makes the most sense is to have your government be the identity provider. In particular, the same agency that issues your driver's license or passport or nation ID (if your country has those).

Such a system can in fact be built. The EU is including one in their EU Digital Identity Wallet project, which has been in development for several years and is not undergoing large scale field testing in several countries. It is supposed to be deployed to the public this year or next.

The first version handles the binding of the document to you by tying it to your smart phone's hardware security element. They plan to later support other types of hardware security elements. 90+% of adults in the EU have smart phones (95-98% for adults under 54), and it is going up, so the first version will already cover most cases.

Google has published some libraries for implementing a similar system. Both the Google libraries and the EU system are open source.