# Fiat-Shamir zero knowledge protocol

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?
• cryptowiki.net/index.php?title=Fiat_-_Shamir_protocol – PDHide Jan 3 '19 at 16:44
• yes, why can't attacker send the correct value y, if x and v are public, and impersonate as A – PDHide Jan 3 '19 at 17:27
• Why don't you write more explicitly into your question? Also, you can delete the comments after that – kelalaka Jan 3 '19 at 17:48
• The original Fiat-Shamir Paper – kelalaka Jan 3 '19 at 19:21