ROT13 originated in the net.jokes newsgroup in the early 1980s[2], in an effort to provide a voluntary means to hide jokes that some readers might have found offensive, or to just obscure the punchline of a joke, so it wouldn't be accidentally read too early. Previous attempts to categorize offensive jokes by putting them in different newsgroups had failed — site managers did not want to be seen as condoning such postings by coblems with unusual kinds of characters. ROT-13 was chosen over ROT-N (for other values of N between 1 and 25) because 13 is the only shift value which ensures that encoding and decoding are equivalent. The value of 13 works only for languages like English, which have 26 letters in their alphabets. While users could plausibly encode and decode messages by hand, automatic decryption is much more convenient; UNIX systems have a standard utility called "tr" (transliterate) that can be used to perform ROT13 encoding:Automatic deciphering was soon added to newsreading software. From the early 1990s, ROT13 was also used in Fidonet forums, to the extent where Fidonet mail reading software often included features for enciphering and deciphering automatically.

ROT13 can be viewed as one of a group of historical encryption algorithms known as Caesar ciphers, a type of substitution cipher. It is not intended to be used where secrecy is a serious concern—the use of a constant shift of 13 letters 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, like any substitution cipheal effect of ROT13 is to make sure that the viewer of a message must consciously choose to decipher it, which typically means invoking the relevant ROT13 command in his or her software. Rather than protecting a confidential message from unauthorized readers, ROT13 safeguards readers from material they may not wish to inadvertently read, such as spoilers in book or movie reviews.

Because of its unsuitability for secrecy, ROT13 has become a catchphrase to refer to any weak encryptn, fr eT13 are also used humorously, including a spoof academic paper "On the 2ROT13 Encryption Algorithm" (PDF). As explained above, applying ROT13 to an already ROT13 encrypted text restores the original plaintext, so ROT26 is equivalent to no encryption at all. ROT26 has been used as a satirical jibe against t "Encod a broad ban on the circumvention of copy prevention systems, which often employ insecure methods of cryptography. Triple ROT13 is also sometimes seen - which is, of course, identical to regular ROT13. This is most probably a take on Triple DES. 2001, Russian cryptanalyst Dmitry Sklyarov was arrested after detailing weaknesses in ebook copy prevention systems. Ebooks (books available in digital or electronic format) sometimes include technical measures to enforce copyright. One vendor, New Paradigm Research Group (NPRG), used ROT13 to encrypt their reports. 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

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 showord. The only known pair of common English words which are each other's ROT13 and reversal are gnat and tang.vex is transformed to irk under ROT13; the words are synonyms. Also, terra can be transformed to green to give a satisfying pair.The 1989 International Obfuscated C Code Contest (IOCCC) inclu an entry by Brian Wealt.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.There are other little-used obfuscation schemes with a similar purpose to only handles letters, and leaves other characters, such as punctuation, numbers and whitespace, untouched. Because of this, ROT13 may be unsuitable for some purposes, for example, hiding the numeric answer to a riddle, or handling arbitrary binary data. is a variant on ROT13 which, in addition to scrambling the basic letters, also treats numbers and many other characters. Instead of using the sequence A-Z as the alphabet, ROT47 uses a larger alphabet, derived from a common character encoding known as ASCII. ASCII maps letters, digits, punctuation and other speciallamation mark, ASCII code 33) to ~ (the tilde, ASCII code 126), rotating them by an offset of 47. The use of a larger alphabet is intended to produce a more thorough obfuscation than that of ROT13, but ROT47 is far less widely supported.

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