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I am defining multi-party computation using the real-ideal paradigm (see A Pragmatic Introduction to Secure Multi-Party Computation). That is, for any successful attack on an MPC protocol in the real world, there exists a simulator that carries out this attack successfully in the ideal world. It follows that security in the real world must be equivalent to security in the ideal world.

I am defining interactive zero-knowledge proof systems for a language $L$ using the original definition from The Knowledge Complexity of Interactive Proof Systems. That is, a pair $(A, B)$ of interactive Turing machines must fulfill

  1. Completeness: given $x \in L$, $B$ accepts with very high probability;
  2. Soundness: given any prover $A'$ and $x \not\in L$ passed into $(A', B)$, $B$ accepts with very low probability;
  3. Zero-Knowledge: there exists a probabilistic polynomial-time simulator that can simulate the entire exchange of messages between $A$ and $B$ for any input $x \in L$.

Now, the paper Zero-Knowledge from Secure Multiparty Computation mentions the following:

Zero-knowledge protocols can be viewed as a special case of secure two-party computation, where the function verifies the validity of a witness held by the prover.

That is, given $L \in \mathcal{NP}$, there exists an algorithm $A$ such that $x \in L \iff \exists w\colon A(x, w) = 1$ (definition of $\mathcal{NP}$). One party $P_1$ acts as the prover, another $P_2$ as the verifier. $P_1$ knows $x$ and $w$, $P_2$ knows only $x$. They execute $A(x, w)$ together to determine whether $x \in L$ or not.

Clearly, $w$ is not revealed to the verifier $P_2$ due to the MPC protocol. However, is the definition of zero-knowledge not more general? If the prover $P_1$ sent, for some reason, the solution to some instance of an $\mathcal{NP}$-complete problem1, no polynomial-time simulator could simulate this assuming $\mathcal{P} \neq \mathcal{NP}$. The created proof system would not be zero-knowledge.

So, given that an MPC protocol could exchange non-simulatable messages, an MPC protocol cannot actually be used to implement a zero-knowledge proof system for some language $L \in \mathcal{NP}$, can it?


1 The solution can be made dependent on $x$ such that it is not constant and thus easily simulatable.

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Zero-knowledge was initially defined with respect to arbitrary (possibly unbounded) provers. However, when we use or discuss zero-knowledge in cryptography, we almost always implicitly assume ZK for NP where the prover runs in polynomial time given a witness for the statement. This is the type of zero-knowledge proof the paper was referring to, and this is indeed a special case of maliciously-secure two-party computation.

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  • $\begingroup$ Yes, but this scheme is what I described right below the citation with parties $P_1$ and $P_2$, isn't it? My question specifically addresses that zero-knowledge means not only not leaking $w$, but also not leaking anything else, while secure MPC might leak other knowledge than $w$. Therefore, it appears unreasonable to me that MPC could be used to construct a zero-knowledge proof. $\endgroup$
    – cadaniluk
    Aug 25 at 15:39
  • $\begingroup$ How would the polynomial-time prover (who is only given the witness for the statement) leak this "other knowledge"? Either this is hardcoded in its description - then it can be harcoded in the simulator, or it is easy to compute (then the simulator can compute it). Otherwise, there is no way anything hard-to-simulate is leaked by the MPC protocol. If you inspect the definition, it holds directly that malicious 2-party MPC is a strict generalization of zero-knowledge. $\endgroup$ Aug 25 at 17:41

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