A ZKP must fulfill three properties:
• Zero knowledge: A malicious5 verifier cannot extract any information except for the statement being true or not.
• Completeness: A prover can convince the verifier (who follows the protocol) of a true statement.
• Soundness: A malicious prover cannot convince the verifier of a false statement with a sufficiently high
probability.
Data Confidentiality
1. Berentsen A, Lenzi J, Nyffenegger R. An Introduction to Zero-Knowledge Proofs in Blockchains and Economics. Federal Reserve Bank of St Louis Review. 2023;forthcoming.
Being private about personal characteristics is one example where ZKPs could be used for data confidentiality. As an example, Carlos wants to prove at the entrance of a nightclub that he is 18 without revealing his exact age or any other personal characteristic. Judy wants to enter a fancy club she has a membership to but does not want the doorman to know who she is. Both could create a ZKP to prove that they are allowed to enter. For both examples, we would need some kind of underlying cryptographic set-up. In the former case, the government might have provided a digital identity, whereas in the latter case the club owners could have issued a digital membership card that they signed. The doorman could verify that the proof includes the signature of the club owner and would let Judy in.
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