Post #3276911

2026-06-02 00:24 UTC

@kbal@fedia.io @mullvadnet@mastodon.online @ansuz@gts.cryptography.dog OK, so we're back to the difficult part being making the proof that someone is at least 18 without revealing their age... Yeah. Like I said originally when you replied and said the hard part was tying that to an ID: "So I'm trying to learn zkp right now and it seems to me that it doesn't do much." So maybe just refer back to that post to complete the circle. I'd maybe add that I'm not sure why you can't just brute force the proof to estimate the birthdate.

Replies (1)

@ansuz@gts.cryptography.dog 2026-06-02 06:27

@crazyeddie@mastodon.social @kbal@fedia.io @mullvadnet@mastodon.online It should be possible to prove that you are over 18 without revealing your exact age. There are systems for proving arbitrarily complex statements, though I'm not sure whether those are the ones the EU system has adopted. In any case, a simple approach would be to use an interactive ZKP, so both participants (the prover and the verifier) would need to exchange information for the verifier to be convinced of anything. In such a situation the prover would need to assist with any sort of brute force attempt, which they would presumably avoid. My point above about non-anonymous systems being transferable is basically saying that there's nothing stopping a person who is over a certain age from providing proofs for other people who are below that age. It's comparable to asking your friend to share their Netflix password and relaying the OTP that gets sent to their phone number, email, or whatever. Pseudonymous systems as you describe can't prevent that unless they force you to provide enough information for it to not even really be pseudonymous anymore. Without tying a proof to a specific device (a problematic concept itself that effectively relies on DRM) or otherwise forcing them the proof to reveal uniquely identifying information there is nothing to stop an older sibling or friend from providing proofs to others under 18. They could try to make that illegal, but they'd need to somehow catch them in the act (as with the crime of providing alcohol to minors), and the technology involved would make that very difficult. tl;dr the proposed systems cannot do the things politicians and lobbyists have claimed.

Open ##3276912