The alphabet read backwards: A and Z swap, B and Y, and so on. Encrypting and decrypting are the same.
Procedure
How it works
Atbash comes from the Hebrew alphabet: the first letter (aleph) is swapped with the last (taw), the second (beth) with the second to last (shin), and the name spells out these pairs. A few words in the Book of Jeremiah are written this way.
In the Latin alphabet letter $x$ becomes $25 - x$, so A and Z trade places, B and Y, C and X. Applying it twice gives the letter back, so the same table encrypts and decrypts. It is the affine cipher $E(x) = 25x + 25 \bmod 26$.
There is no key at all, so it hides nothing from anyone who knows the trick. It survives in puzzles and in the study of old texts.
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