Comment by michaelt

The trick is to define "privacy-preserving age verification" in an extremely narrow way that ignores any other privacy concerns.

For example, imagine you put the same private key into the 'secure element' of every single iphone. You use code signing so that key is only unlocked when the phone is running unmodified iOS with all security updates. You use encryption and remote attestation for the front-facing camera and face id depth sensor. You use NFC to read government-authenticated age and appearance data from biometric passport chips (or digital ID cards) and you store it on-device.

Then, when you want to access pornhub, they send an age challenge to your device, your device makes sure your face matches the stored passport, and if so it signs the challenge with the private key.

Pornhub gets an Apple-signed attestation of age - but because every phone signs with challenges with the same private key, Pornhub can't link it to a particular phone or identity document.

So in a very narrow sense, privacy is preserved.

You can't use someone else's ID, as it checks your face every time. You can't fool it with a photo of the person because of the depth sensor. You can't MITM/replay the camera/depth data because the link is encrypted. You can't substitute software that skips the check with a rooted phone because of the code signing. Security holes can be closed by just pushing a mandatory OS update.

Sure, it doesn't work on PCs. Doesn't work on Linux, or on unlocked/rooted phones. It hands users' government ID documents over to Google and Apple. It requires people to carry foreign-made, battery powered, network connected GPS trackers (with cameras, microphones and speech recognition) with them. And there are non-negotiable terms of service everyone must agree to. But if you define "privacy-preserving" to ignore all that stuff and only consider whether Pornhub learns your identity, it's privacy-preserving.