The cipher is the simplest single-alphabet substitution cipher. It is really just a permutation of the Caesar cipher. All characters are rotated 13 characters through the alphabet.

ROT 13

Explanation:

The ROT 13 cipher is a substitution cipher with a specific key where the letters of the alphabet are offset 13 places. I.e. all 'A's are replaced with 'N's, all 'B's are replaced with 'O's, and so on. It can also be thought of as a Caesar cipher with a shift of 13.

The ROT 13 cipher offers almost no security, and can be broken very easily. Even if an adversary doesn't know a piece of cipher text has been enciphered with the ROT 13 cipher, they can still break it by assuming it is a substitution cipher and determining the key using hill-climbing. The ROT 13 cipher is also an Caesar cipher with a key of 13, so breaking it as a Caesar cipher also works.