A very simplistic cipher where you change A to Z, B to Y, and so on.

The Atbash cipher is a very common and simple cipher that simply encodes a message with the reverse of the alphabet. Initially it was used with Hebrew. Basically, when encoded, an "A" becomes a "Z", "B" turns into "Y", etc.

The Atbash cipher can be implemented as an Affine cipher by setting both a and b to 25 (the alphabet length minus 1).