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I was looking at the exercises of Cryptography theory and practice and I could not find the answer to this question anyone has an idea?

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Well, here's one obvious attack:

  • Eve wants to impersonate Alice.

  • Bob wants Eve to prove that she's Alice. Bob gives a value $x$ to Eve

  • Eve turns around, and asks Alice to prove that she (Alice) is Alice. Eve gives $x$ to Alice

  • Alice gives the value $y$ to Eve

  • Eve then gives the value $y$ to Bob

Of course, that's not the attack that Stinson is looking for...

For the attack that Stinson is thinking of, here's a hint:

  • For a random quadratic residue $x$, how many values of $y$ are there s.t. $y^2 = x$?

  • Suppose Bob picks a random value $y$ and computes $x = y^2 \bmod n$ and gives it to Alice, what is the probability that Alice will compute a $y'$ that's neither $y$ nor $n-y$?

  • If Bob knows $y$ and $y'$, what could he do with those values? Hint: consider $y^2 - y'^2 = (y + y')(y - y')$

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