Comment by miki123211
6 months ago
If you can achieve RCE on the chip and run arbitrary code without invalidating signatures, does the protocol still stay secure?
If so, what's the point of requiring your implementation to run on a verified secure element? If not, the protocol seems only as strong as the weakest chip, as obtaining just a single private key from a single chip would let you generate arbitrary proofs.
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.
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.
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.
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