There's a unique identifier, but it's your secret and can't be used for tracking. Sites needing verification don't learn anything except that you "have" a toke

There's a unique identifier, but it's your secret and can't be used for tracking. Sites needing verification don't learn anything except that you "have" a token matching the condition they are checking. This includes not learning your unique identifier, so they can't use it for tracking. The issuer also doesn't learn anything about your verification queries. You have an incentive to keep the secret token to yourself, and would probably use existing mechanisms for that: You might manage like your phone number, private email and other personal accounts today. Not perfect, but effective most of the time for most people. You might decide to share it with someone you trust, like your sibling. That's up to you, but you wouldn't share it widely or with people you don't trust, even under pressure, because: To prevent mass reuse of stolen tokens, it's possible to use more cryptography to detect when the same token is reused in too many places, either on the same site or across many sites, without revealing tokens that don't meet the mass-reuse condition, so they still can't be used for tracking. If mass-reused tokens auto-revoke, they can't be reused thousands of times by anyone, and that also provides an incentive to avoid sharing with people you don't trust. I won't pretend this is trivial. It's fairly advanced stuff. But the components exist these days. The last paragraph above requires combining zero-knowledge proofs (ZKP) with other branches of modern cryptography, called multi-party computation (MPC) and perhaps fully homomorphic encryption (FHE).