# Unblind a RSA blinded signature

I have blinded $m = 16$ with $r= 13$, $e = 7$ and $n = 209$. This resulted in $m' = 464$, which led to a $s' = 8$.

Now, I want to unblind this $s'$ by using $s = s' \times r^{-1}$ but im stuck at filling in the $r^{-1}$?

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What's your value for $d$ and how have you computed it? You need to find $x$ such that $x \cdot 13\equiv 1 \pmod{209}$. This $x$ is the value $r^{-1}$ you are seeking. Hint: extended euclid –  DrLecter Nov 25 '13 at 20:43
@DrLecter If i calculate it via the extended euclid r$^{-1}$ = 16, is that correct? –  ziggy34 Nov 25 '13 at 23:59
Nope. You just have to check if $13\cdot 16 =1 \mod p$, which is not the case (it's $-1$ or equivalently $208$). –  DrLecter Nov 26 '13 at 7:51