ROT13

ROT13 ("rotate by 13 places", usually hyphenated ROT-13) is a simple Caesar cipher used in online forums as a means of hiding spoilers, punchlines, puzzle solutions, and offensive materials from the casual glance. ROT13 has been described as the "Usenet equivalent of a magazine printing the answer to a quiz upside down".

ROT13 provides no real cryptographic security and is not used for such; in fact it is often used as the canonical example of weak encryption. An additional feature of the cipher is that it is symmetrical; that is, to undo ROT13, the same algorithm is applied, so the same code can be used for encoding and decoding. The name "ROT13" originated on Usenet in the early 1980s, and the method has become a de facto standard.