Atbash is a simple substitution very similar in nature to the Caesar Substitution. Whereas the Caesar substitution was Roman in origin, atbash is Jewish in origin.
In atbash, the last letter represents the first, the second to last represents the second and so on.
Atbash is even simpler to solve than the Caesar Substitution as there is only one solution to try!
You may start to wonder how we could start to solve a monoalphabetic substitution if we do not know whether the cipher alphabet is atbash, caesar, some combination of the two or just random! This will be addressed later when I discuss monoalphabetic substitution. In the meantime, can you decode the following atbash? I've provided a key to save some legwork.
PLAINTEXT a b c d e f g h i j k l m CIPHERTEXT Z Y X W V U T S R Q P O N PLAINTEXT n o p q r s t u v w x y z CIPHERTEXT M L K J I H G F E D C B A
Blf ziv mlg z Qvwr bvg (Wzigs Ezwvi gl Ofpv Hpbdzopvi)
You shouldn't have had a problem with this, it's fairly easy!
Atbash can also be combined with a caesar shift, to produce a Reversed Caesar substitution.
An example is shown below.
PLAINTEXT a b c d e f g h i j k l m CIPHERTEXT W V U T S R Q P O N M L K PLAINTEXT n o p q r s t u v w x y z CIPHERTEXT J I H G F E D C B A Z Y X