Atbash

Atbash is a monoalphabetic substitution cipher that was originally used to encrypt the Hebrew alphabet. In a monoalphabetic substitution cipher every letter of the plaintext is replaced by one fixed letter, so the whole cipher is a single lookup table applied letter by letter. Atbash is a mirror substitution of this kind in which the alphabet is mapped onto itself in reverse order. Although it began with Hebrew, the scheme can be modified for use with any known writing system that has a standard collating order, since all it needs is an ordered alphabet to reverse. It is usually cited today as one of the oldest and simplest examples of a classical cipher, and it offers no real secrecy because it has no key.