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Proof of personhood and ZK: a thread. Decided to do a brain dump while touching grass. Lets start with the components declared. Proof of personhood aka unique problem or PoP below is explored quite in accessible way by the glorious @vitalik.eth https://vitalik.eth.limo/general/2023/07/24/biometric.html
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Proof of Personhood: (a) PoP proves somehow cryptographically that a private key or an account owner is a human, of flesh and blood. (b) No matter how many private keys or accounts he or she has, they all can be linked to a cryptographic signal acquired during proving (a).
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ZK: proves cryptographically that some statement is true while not disclosing components of the statement. One infamous example is proving you're an adult. A proof would tell your birthdate is just far enough in the past. The date itself remains private.
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These two things ZK and PoP are independent. Mix and match at your will.
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Vitalik selects 2 general approaches to PoP. Social-graph based uses graph of endorsements you are human. Biometric approach relies on unique biological traits like iris scan or fingerprint or maybe an ear scan.
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There ZK might be applied as a sort of intermediation mechanism that hides a link between an account and unique human id derived from biometrics or social graph. Improves privacy, but contributes a little to the overall scheme.
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There is one more approach to prove Personhood though. We could bridge existing meatworld PoP to Web3. Most of the world population now has passports or government identities. Some of the governments, like Estonia, even use cryptography natively for national ids.
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Most of modern passports used for international travel use cryptography as well. You could scan a government-issued document, and use say a hash of its properties as a unique human identifier. ZK here plays a critical role.
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It proves whatever got scanned is correctly turned into an identifier. And then you could prove you have such a correctly received identifier without disclosing it.
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Obviously, the approach of piggybacking the legal id is not perfect. There are few different documents that could belong on the same human. At least, a national ID and an international passport.
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If we narrow that down to an international passport, you could have few of them as well. Either by a virtue of having multiple citizenships, or by a fact that in most countries it is allowed to have 2 international passports simultaneously.
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Yet, one could argue that is still an improvement. Legal id based PoP seems to be less intrusive than biometrics or social graph ceremonies.
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We could think of “sybilness” - number of non-unique identifiers belonging to the same unique human. PoP bases on a legal id seems to decrease sybilness to about 2. Biometrics would give exactly 1. Social graph based approach looks unbounded by design.
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Oh, and one more thing. While thinking about ZK and PoP you might recall fancy ZKML. That could be one way to do scanning of passports and/or faces alike.
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In short, ZKML is like a ML model that outputs not only signal like “this was a footage of a real human”, but also a cryptographic proof that the computation it did while calculating the signal is not tampered. It could be huge.
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