# Math Help - Help finding the decryptor

1. ## Help finding the decryptor

If Z(w) is equivalent to 21w+12 and this is an encryptor, then how would I find the decryptor?

I know the decryption law is w=A^(-1) * (Z-B) (mod N) where A^(-1) is the multiplicative inverse.

The encryption law is E(w) is equivalent to Aw + B (mod N)

2. Under the hypothesis that $N=26$ , if $A=21$ then $21 \cdot 5= 105 = 1 \mod 26 \rightarrow A^{-1}=5$, so that the decription law will be...

$w= 5 \cdot (Z-12) \mod 26$ (1)

Kind regards

$\chi$ $\sigma$