Atbash cipher
Atbash replaces each letter with its mirror image in the alphabet: A becomes Z, B becomes Y, and so on. There is no key, and applying it twice restores the original text, so one button both encrypts and decrypts.
About the Atbash cipher
Atbash is one of the oldest ciphers on record, invented for the Hebrew alphabet: aleph maps to tav, bet to shin, which gives the cipher its name (A-T-B-Sh). The Hebrew Bible uses it — the book of Jeremiah writes "Sheshach" for Babel — and applied to the Latin alphabet it becomes the mirror substitution this page computes.
As cryptography it is weaker than a Caesar shift: there is exactly one possible key, so recognizing the cipher is the same as breaking it. It is a fixed substitution alphabet, which makes it a special case of thesimple substitution cipher, and any monoalphabetic solver breaks it instantly.