ROT-13 Cipher

Rot-13 (short for Rotation 13) is the name given to a mono-alphabetical substitution cipher which has the property of being reversible and very simple.

Combining the French/Latin alphabet of 26 letters and an offset of 13, Rot-13 replaces a letter with another located thirteen places further down the alphabet.

Rot-13 coding is popular to hide content because it is easily reversible, indeed, if it is applied twice, then the original message reappears.

From an alphabet, usually the classic 26-letter alphabet ABCDEFGHIJKLMNOPQRSTUVWXYZ, each letter is shifted by 13 positions in the English alphabet. The correspondence table is:

Rot-13 decryption is identical to encryption, due to the reciprocal substituting alphabet used:

NB: Rot-13 has the same function for encryption and decryption, it is said to be involutive. In other words, applying the ROT-13 Cipher twice on a text will return it to its original state.

Frequency analysis shows a shift of 13 letters (The E is replaced by an R, which should be the most present letter).

A message encoded by Rot13 has an index of coincidence similar to the language of the plain text.

The ROT13 code has been widely popularized on Usenet groups and discussion forums, for example as an anti-spoil method.

Rot-13 is in fact a Caesar cipher with a shift of thirteen. As this code only works with letters, it is possible to add the Rot5 to it for the digits (in this case it is sometimes called ROT13.5) or even to use the Rot47 to manage all the ASCII characters.

An offset of 13 allows the encryption to be reversible. The encryption and decryption method are identical. Applying 2 consecutive encryptions (2 shifts of 13) heads to find the original text.

Applying 2 shifts of 13 represents a shift of 26, and for the Latin alphabet of 26 letters, this amounts to shifting all the letters to their starting point, so in the end, not undergoing any transformation.

ROT13(ROT13(letter)) = letter