ROT13 is a simple letter substitution cipher that replaces a letter with the 13th letter after it in the Latin alphabet.

ROT13 is a special case of the Caesar cipher which was developed in ancient Rome, used by Julius Caesar in the 1st century BC. An early entry on the Timeline of cryptography.

ROT13 can be referred by "Rotate13", "rotate by 13 places", hyphenated "ROT-13" or sometimes by its autonym "EBG13".

Applying ROT13 to a piece of text requires examining its alphabetic characters and replacing each one by the letter 13 places further along in the English alphabet, wrapping back to the beginning as necessary.

When encoding a message, A becomes N, B becomes O, and so on up to M, which becomes Z. Then the sequence continues at the beginning of the alphabet: N becomes A, O becomes B, and so on to Z, which becomes M. When decoding a message, the same substitution rules are applied, but this time on the ROT13 encrypted text. Other characters, such as numbers, symbols, punctuation or whitespace, are left unchanged.

Because there are 26 letters in the Latin alphabet and 26 = 2 × 13, the ROT13 function is its own inverse:

In other words, two successive applications of ROT13 restore the original text (in mathematics, this is sometimes called an involution; in cryptography, a reciprocal cipher).

The transformation can be done using a lookup table, such as the following: