Under the standard affine convention, an alphabet of m letters is mapped to the numbers 0, 1, ... , ''m'' - 1. (The Hebrew alphabet has 1=''m'' = 22, and the standard Latin alphabet has 1=''m'' = 26). The Atbash cipher may then be enciphered and deciphered using the encryption function for an affine cipher by setting 1=''a'' = ''b'' = (''m'' - 1):

This may be simplified to
:

If, instead, the m letters of the alphabet are mapped to 1, 2, ..., ''m'', then the encryption and decryption function for the Atbash cipher

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