Tech firms must tame toxic algorithms to protect children online

As an overly simple thought experiment, you could generate a random ed25519 ephemeral public key, hash it, then send it (blinded, and thus unreadable) to an age verification service (with some long term age verification credential or similar).

The age verification provider does a blind signature on your (blinded) public key hash, and sends it back to you. You un-blind that signature (meaning that provider can't identify which identifiable request led to it, but now it bears the hash of your public key), and you can now authenticate to a service by signing a challenge with your ephemeral ed25519 private key.

The service only knows your ephemeral public key, and that it has been "vouched" (signed) by the age verification provider.

The age verification provider knows "you" asked for a token , but doesn't know what public key you used.

Clearly there are challenges with replaying (authorised user could share the private key every day with a group of others), and revocation of a credential whose private key gets shared among a group is hard (beyond providers blocking a public key).

The risk is that this becomes a race towards "DRM" and platform attestation/authentication to try and prevent private keys being exported.

By my understanding , in principle yes you could use ZK proofs - you can imagine it as a way to prove a certain assertion (age >= 18) in a way that isn't directly linked to other data. You sometimes see this in conceptual ID card specifications - using keys on a smart card to give a signed attestation about a single attribute without sharing other ones.

Ultimately though, when you need to actually implement it, you'll end up needing the same core concept as the thought experiment above - you'll need one or more "trusted central authorities" whose word is trusted on a given asserted attribute (age, etc).

They'll need a way to prove that they vouched for a user (as there's no digital way to validate their age as that's an unverifiable claim). You'll then need a way for the "bearer" of a ZK proof to tie themselves to that trusted central authority's attestation, and you'll need a way to prevent the information needed to generate that ZK proof being shared with others for replay.

A ZK proof will still need that external trusted authority for an attribute like age, because age isn't something you can root some kind of cryptographic trust from.

I'm not an expert in the ZK crypto either, but it doesn't deliver a magical ability to prove a (biological analog world claim) without chaining back to a trusted verifier of said claim, and effectively delivering that sort of "thought experiment" protocol.

Sometimes though, the complex solutions tend towards the simple - you could issue people "age verification" smart cards (if you have enough confidence in CC EAL6 or similar cards, and their side channel resistance) which are "group keyed" with common attestation signing keys for every million (or another suitable anonymity threshold) users, and share the public keys used (to allow verification you haven't been given a special unique public key), and then allow signed card-issued anonymous attestations. That would work for as long as you can keep the smartcard-backed key secure against side channel/ key extraction attacks.

The user adoption challenge in all this is getting users onboard and demonstrating it's a private solution rather than an excuse to oversee their online activities more, but I do believe you could do this in a manner that's going to make it easier to just identify them from IP address and adtech trackers or similar external means.

This isn't exactly true.... All societies always have footed the bill, modern or not. If you cast the person out in a middle age society then expect to get robbed by bandits on your next visit to the town next door.

Which emphasizes why you should attempt to minimize wrongdoing before it gets expensive, when you don't it turns into that story of "there was an old lady who swallowed a fly"

Many people (myself included) think this is the true nature and shape of the world. There are lots of layers, institutions, and policy built on top, but what matters is ultimately who holds the hammer and what do they want.

This isn't as cynical as it might sound. Most people ultimately hold some real-world power, and have organized into systems that help them get what they want. Anger a human enough and they will withhold work, anger them more and they will resort to violence. This is the basis of all society.