ROT13 - Wikipedia

ROT13 is a self-reversing Caesar cipher popularly used on Usenet and other online forums as a means of masking joke punchlines, movie and story spoilers, and offensive expressions from the casual glance.

The name "ROT13" stands for "rotate by 13 places," which is a concise description of the cipher algorithm. To apply ROT13 to a piece of text, shift every English letter in it by 13 places down the alphabet, wrapping back to the beginning if necessary: 'A' becomes 'N', 'B' becomes 'O', and so forth, down to 'Z' becoming 'M'. Numbers, symbols, and other characters are not changed. Because there are 26 letters in the English alphabet and 26 = 2 × 13, the ROT13 function is its own inverse: x = ROT13(ROT13(x)) for any text x.

ROT13 is not intended to be secure. It is intended merely to require the viewer of a message to consciously choose to decipher it, which generally involves no more than invoking the ROT13 command of one's newsreader software. Rather than protecting the encrypted message from unauthorized readers, ROT13 protects readers from material they may not wish to view, such as spoilers in book or movie reviews.

Because ROT13 is sometimes mistaken for real encryption by those unfamiliar with it, it has become a catchphrase among cryptography buffs to refer to any badly chosen, poor encryption: "56-bit DES is no better than ROT13 these days." Worse yet, naturally, is double ROT13: "Sure, go ahead, it's perfectly safe to send your credit card number in email. Just make sure to encrypt it with double ROT13."

ROT13 Table

 ABCDEFGHIJKLM
 NOPQRSTUVWXYZ