Hey,

I have a question to decipher an affine cipher. The cipher text is RBYDKLQTSLQJBYN and I am told that E(A)=L and E(O) = B.

Cipher

L -> 12

B -> 2

Plain

A -> 1

O -> 15

So basically I set up two equations in Z26:

$\displaystyle

12 = a + b

$

$\displaystyle

2 = 15a + b

$

I take the top and subtact the bottom:

$\displaystyle

10 = -14a

$

$\displaystyle

10 = 12a

$

Now my problem is I cannot find the inverse of 12 in Z26 because they are not relatively prime. I am not sure how to proceed from this point? Can someone point me in right direction? I've tried subtracting the equations in the reverse order, but run into the same relatively prime issue.

Thanks