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The original 1986 Fiat-Shamir paper can be found here. The subsequent Feige-Fiat-Shamir 1988 paper can be found here, and contains the answer (Section 3): The $S_j$ (which are witnesses to the quadratic residuosity character of the $I_j$) are effectively hidden by the difficulty of extracting square roots $\bmod n$, and thus A can establish his ...


The problem is one of notation for modular arithmetic, at the point of the question reading if y^2 equals (x * v^e) % n then.. Likely the textbook is about if $y^2\equiv x\cdot v^e\pmod n$ then.. By definition of $a\equiv b\pmod n$, that holds if and only if $n$ divides $a-b$ (or equivalently: $|b-a|$ is a multiple of $n$). In the question, 437 ...


Without a sign the verifier learns that the number he received is a QR modulo n. Whether a number is a QR is a hard problem as he does not know the factors of n.


A just has to calculate $y=(x*v^e)^{\frac{1}{2}}$ and send it to B. Yes, if that was easy, the protocol would be breakable. However, finding square roots modulo a composite number is as difficult as factoring that number. See: Quadratic residue problem on composite integers


No, it wouldn't. Very simply put: If you can compute square roots wrt. a composite modulus, then you can calculate other fixed roots, too. This is based on the fact, that being able to compute square roots allows you to factor the modulus: Choose random $x$. Compute $x^2$. apply your square root algorithm to $x^2$. The result $y$ will be a different ...


Your question is related to the well known RABIN Cryptosystem which is similar to RSA, except the public exponent is 2. As fgrieu mentioned, decipherment can be easily processed by the CRT algorithm, but some precautions must beforehand be observed during the key generation. In fact the solution of the equation gives 4 roots, which means that the solution ...


Because we are working modulo $n$. There's not much more to say. There's no reason why any honest user would ever want to use a larger value. You might want to review modular arithmetic and its use in cryptography.

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