2/ - Knowledge soundness: If the statement is false, a fraudulent prover cannot convince an honest verifier that it is true except by a tiny probability. - Zero-Knowledge: Given that the statement is true, a verifier learns nothing more than the fact that the statement is true.

3/ Thus, we get: Zero-Knowledge Proof = I know the secret. I can't tell you the secret. But I can prove to you that I know the secret.