ROT13 ("rotate by 13 places", sometimes hyphenated ROT-13) is a simple substitution 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 is an example of the Caesar cipher, developed in ancient Rome.
In the Basic Latin alphabet, ROT13 is its own inverse; that is, to undo ROT13, the same algorithm is applied, so the same action can be used for encoding and decoding. The algorithm provides no cryptographic security, and is often cited as a canonical example of weak encryption. ROT13 has inspired a variety of letter and word games on-line, and is frequently mentioned in newsgroup conversations.
Applying ROT13 to a piece of text merely requires examining its alphabetic characters and replacing each one by the letter 13 places further along in the alphabet, wrapping back to the beginning if necessary. 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. Only those letters which occur in the English alphabet are affected; numbers, symbols, whitespace, and all other characters are left unchanged. Because there are 26 letters in the English 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:
For example, in the following joke, the punchline has been obscured by ROT13:
How can you tell an extrovert from an
introvert at NSA? Va gur ryringbef,
gur rkgebireg ybbxf ng gur BGURE thl'f fubrf.
Transforming the entire text via ROT13 form, the answer to the joke is revealed:
Ubj pna lbh gryy na rkgebireg sebz na
vagebireg ng AFN? In the elevators,
the extrovert looks at the OTHER guy's shoes.
A second application of ROT13 would restore the original.
ROT13 was in use in the net.jokes newsgroup by the early 1980s. It is used to hide potentially offensive jokes, or to obscure an answer to a puzzle or other spoiler. A shift of thirteen was chosen over other values, such as three as in the original Caesar cipher, because thirteen is the value for which encoding and decoding are equivalent, thereby allowing the convenience of a single command for both. ROT13 is typically supported as a built-in feature to newsreading software. Email addresses are also sometimes encoded with ROT13 to hide them from less sophisticated spam bots.
ROT13 is an example of the encryption algorithm known as a Caesar cipher, attributed to Julius Caesar in the 1st century BC.
ROT13 is not intended to be used where secrecy is of any concern—the use of a constant shift means that the encryption effectively has no key, and decryption requires no more knowledge than the fact that ROT13 is in use. Even without this knowledge, the algorithm is easily broken through frequency analysis. Because of its utter unsuitability for real secrecy, ROT13 has become a catchphrase to refer to any conspicuously weak encryption scheme; a critic might claim that "56-bit DES is little better than ROT13 these days." Also, in a play on real terms like "double DES", the terms "double ROT13", "ROT26" or "2ROT13" crop up with humorous intent, including a spoof academic paper "On the 2ROT13 Encryption Algorithm". As applying ROT13 to an already ROT13-encrypted text restores the original plaintext, ROT26 is equivalent to no encryption at all. By extension, triple-ROT13 (used in joking analogy with 3DES) is equivalent to regular ROT13.
In December 1999, it was found that Netscape Communicator used ROT-13 as part of an insecure scheme to store email passwords. In 2001, Russian programmer Dimitry Sklyarov demonstrated that an eBook vendor, New Paradigm Research Group (NPRG), used ROT13 to encrypt their documents; it has been speculated that NPRG may have mistaken the ROT13 toy example—provided with the Adobe eBook software development kit—for a serious encryption scheme. Windows XP uses ROT13 on some of its registry keys.
The ROT13 encryption is used to cipher cache hints on Geocaching.com.
 Letter games and net culture
|aha ↔ nun
||ant ↔ nag
|balk ↔ onyx
||bar ↔ one
|barf ↔ ones
||be ↔ or
|bin ↔ ova
||ebbs ↔ roof
|envy ↔ rail
||er ↔ re
|errs ↔ reef
||flap ↔ sync
|fur ↔ she
||gel ↔ try
|gnat ↔ tang
||irk ↔ vex
|clerk ↔ pyrex
||purely ↔ cheryl
|PNG ↔ cat
||SHA ↔ fun
|furby ↔ sheol
||terra ↔ green
|what ↔ Jung
||URL ↔ hey
|purpura ↔ Chechen
||shone ↔ FUBAR
ROT13 provides an opportunity for letter games. Some words will, when transformed with ROT13, produce another word. The longest example in the English language is the pair of 7-letter words abjurer and nowhere; there is also the 7-letter pair chechen and purpura. Other examples of words like these are shown in the table. Gnat is an interesting example of a word which is reversed when encoded with ROT13.
The 1989 International Obfuscated C Code Contest (IOCCC) included an entry by Brian Westley. Westley's computer program can be ROT13'd or reversed and still compiles correctly. Its operation, when executed, is either to perform ROT13 encoding on, or to reverse its input.
The newsgroup alt.folklore.urban coined a word—furrfu—that was the ROT13 encoding of the frequently encoded utterance "sheesh". "Furrfu" evolved in mid-1992 as a response to postings repeating urban myths on alt.folklore.urban, after some posters complained that "Sheesh!" as a response to newcomers was being overused.
ROT47 is a derivative of ROT13 which, in addition to scrambling the basic letters, also treats numbers and common symbols. Instead of using the sequence A–Z as the alphabet, ROT47 uses a larger set of characters from the common character encoding known as ASCII. Specifically, the 7-bit printable characters, excluding space, from decimal 33 '!' through 126 '~', 94 in total, taken in the order of the numerical values of their ASCII codes, are rotated by 47 positions, without special consideration of case. For example, the character A is mapped to p, while a is mapped to 2. The use of a larger alphabet produces a more thorough obfuscation than that of ROT13; for example, a telephone number such as +1-415-839-6885 is not obvious at first sight from the scrambled result Z`\c`d\gbh\eggd. On the other hand, because ROT47 introduces numbers and symbols into the mix without discrimination, it is more immediately obvious that the text has been enciphered.
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