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In Fiat-Shamir protocol, the final calculation is

$$y^2=x\,v^c$$

Where $c$ is the random $\{1,0\}$, $x$ is the witness and $v$ is the public key $s^2 \bmod n$.

My questions are:

  1. What stops the attacker from reading $c$, which is never encrypted, $v$ which is public and then calculate $y^2$ correctly?
  2. How does this improve entity authentication?
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Man-in-the-Middle attack are possible for all almost all zero-knowledge-proofs. Victor, can copy everything sent by Alice, the prover, to the Bob, the verifier, and reversely to impersonate the Alice, . In short, Victor can relay every message.

To mitigate, one can use time limit to prevent the relay, however this may not be enough.

A better solution is first creating a secure channel that is free of MitM attack then use Fiat-Shamir Protocol.

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  • $\begingroup$ So is there any practical use of this protocol? considering a channel to be free of MitM is not a good security practice. How can this be used for entity authentication if there isn't any way to check the authenticity of messages $\endgroup$ – Praveen David Mathew Jan 3 at 20:01
  • $\begingroup$ Securing channel and zero-knowledge-proofs are different subject. As in your previous question, you can use SAS to detect MitM-attacker. But can you prove something without revealing? $\endgroup$ – kelalaka Jan 3 at 20:12

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