[MatteoFrigo]

The role of the secure element is only to "bind" the credential to the device, so that if you copy the credential somewhere else then the credential is useless. Concretely, the secure element produces a ECDSA signature that must be presented together with the credential. This is the normal protocol without ZKP. Concretely, the SE is in the phone, but could be a yubikey or something else.

The ZKP library does not run on the secure element. It runs on the normal CPU and produces a proof that the ECDSA signature from the SE is valid (and that the ECDSA signature from the issuer is valid, and that the credential has not expired, and ...) If you crack the ZKP library, all you are doing is producing an incorrect proof that will not verify.

[tzs]

Am I correctly understanding that I'd get the credential from say my state DMV once, and then later whenever I want to prove my age to a website the proof protocol is just between that website and my device? The DMV gets no information about what websites I use the DMV credential with and they get no information about when I use the credential even if the website and the DMV decide to cooperate? All they would be able to get was that at time T someone used a credential on the site that came from the DMV?

I tried to sketch out a design an age verification system, but it involved the DMV in each verification, which made timing attacks a problem. Briefly the website would issue a token, you'd get a blind signature of the token from the DMV's "this person is 18+" service, and return the token and unblinded signature to the website. I think that can be made to work but if the site and DMV cooperated they would likely be able to unmask many anonymous site users by comparing timing.

Getting the DMV out of the picture once your device is set up with the credential from them nicely eliminates that problem.

[MatteoFrigo]

You are correct. The property that the colluding website and DMV still cannot identify you is called "unlinkability" and as far as I can tell cannot be achieved without zero-knowledge proofs. See https://github.com/user-attachments/files/15904122/cryptogra... for a discussion on this issue.

However, the timing attack resurfaces once you allow the DMV to revoke credentials. Exactly how the revocation is done matters. We are actively pushing back against solutions that require the DMV to be contacted to verify that the credential has not been revoked at presentation time, but this is a very nuanced discussion with inevitable tradeoffs between privacy and security.

[coldpie]

>> The DMV gets no information about what websites I use the DMV credential with and they get no information about when I use the credential even if the website and the DMV decide to cooperate?

> You are correct. The property that the colluding website and DMV still cannot identify you is called "unlinkability" and as far as I can tell cannot be achieved without zero-knowledge proofs.

Well, no. This is true only if you trust the unverifiable wallet software on your phone, which was provided by a for-profit, American big tech advertising company. In this protocol, the wallet may secretly leak the transaction details back to the DMV or whoever else they wish[1].

[1] "Yes, a malicious wallet could leak your information." https://news.ycombinator.com/item?id=44458549

[tzs]

MatteoFrigo is suggesting that unlinkability requires ZKPs.

Your observation that a bad wallet could compromise unlinkability is not a refutation of that. To refute it you need to show that it is possible to achieve unlinkability without using a ZKP.

[hobofan]

One part that I don't understand yet: How does the system ensure "sybil resistance"? (not sure if that's the right term in that context)

By providing both attestation of individual attributes combined with "unlikability", how would even a single verifying party ensure that different attestations don't come from the same identity?

E.g. In the case of age attestation a single willing dissenting identity could set up a system to mint attestations for anyone without it being traceable back to them, right? Similar to how a single of-age person could purchase beer for all their under age friends (+ without any feat of repercussions.

[MatteoFrigo]

Great question. The current thinking, at least in high level-of-assurance situations, is this. The identity document is only usable in cooperation with a hardware security element. The relying party picks a random nonce and sends it to the device. The device signs the nonce using the SE, and either sends the signature back to the relying party (in the non-ZKP case), or produces a ZKP that the signature is correct. The SE requires some kind of biometric authentication to work, e.g. fingerprint. So you cannot set up a bot that mints attestations. (All this has nothing to do with ZKP and would work the same way without ZKP.)

In general there is a tradeoff between security and privacy, and different use cases will need to choose where they want to be on this spectrum. Our ZKP library at least makes the privacy end possible.

[hobofan]

Okay, yeah that's what I assumed.

That seems a bit like a game of whack-a-mole where as long as the forging side is willing to go further and further into out-of-hardware emulation (e.g. prosthetic finger on a robot hand to trick fingerprint scanners), they are bound to win. Biometrics don't feel like they hold up much if you can have collusion without fear of accountability.

> Our ZKP library at least makes the privacy end possible.

Yes, that's also one of the main things that make me excited about it. I've been following the space for quite some time now, and I'm happy that it becomes more tractable for standard cryptographic primitives and thus a lot more use-cases.

Thanks for your contributions to the space and being so responsive in this thread!