# 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' * 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