Suppose I consider the affine cipher defined by $E(x) = ax + b \bmod n$ on the standard alphabet of 26 letters. Which choices of $a$ and $b$ lead to proper encryption functions?
Using the information provided from this website:
They say that $a=5$ and $b=8$ works, but $a=4$ and $b=5$ does not work.
My understanding is that in cases where it does not work then different letters prior to applying this cipher results in the same ciphertext having applied the cipher.
However, I consider this explanation to be quite woolly. How would I know which choices would work prior to applying the cipher?
