# ROT13

ROT13 ("rotate by 13 places", sometimes 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".[1] ROT13 is a variation of the Caesar cipher, developed in ancient Rome.

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 real cryptographic security and is not normally used for such. It 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.

## Description

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.[2] A becomes N, B becomes O, and so on up to M, which becomes Z, then the sequence reverses: 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:[2]

for any text x.

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:
 ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm

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.

## Usage

ROT13 was in use in the net.jokes newsgroup by the early 1980s.[3] It is used to hide potentially offensive jokes, or to obscure an answer to a puzzle or other spoiler.[2][4] A shift of thirteen was chosen over other values, such as three as in the original Caesar cipher, because thirteen is the value which arranges that encoding and decoding are equivalent, thereby allowing the convenience of a single command for both.[4] ROT13 is typically supported as a built-in feature to newsreading software.[4]

ROT13 is equivalent to an encryption algorithm known as a Caesar cipher, attributed to Julius Caesar in the 1st Century BC.[5] 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.[2] 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".[6] 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.[7] 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.[8] Windows XP uses ROT13 on some of its registry keys.[9]

## Letter games and net culture

 abcdefghijklmnopqrstuvwxyzNOPQRSTUVWXYZABCDEFGHIJKLM 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

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.[10]

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.[11]

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.[12]

## Variants

ROT47 is a generalisation of 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. 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.

The GNU C library, a set of standard routines available for use in computer programming, contains a functionmemfrob()[13]—which has a similar purpose to ROT13, although it is intended for use with arbitrary binary data. The function operates by combining each byte with the binary pattern 00101010 using the exclusive or (XOR) operation. This effects a simple XOR cipher. Like ROT13, memfrob() is self-reciprocal, and provides a similar level of security.

## Notes and references

1. ^ Horrocks, Bruce (June 28 2003). UCSM Cabal Circular #207-a. Usenet group uk.comp.sys.mac. Retrieved on 2007-09-17.
2. ^ Schneier, Bruce (1996). Applied Cryptography, Second, John Wiley & Sons, 11. ISBN 0-471-11709-9.
3. ^ Early uses of ROT13 found in the Google USENET archive date back to 8 October 1982, posted to the net.jokes newsgroup [1][2].
4. ^ Raymond, Eric S. (ed.) (2003-12-29). ROT13. The Jargon File, 4.4.7. Retrieved on 2007-09-19.
5. ^ Kahn, David. The Codebreakers: The Story of Secret Writing. New York: Macmillan. ISBN 0-684-83130-9.
6. ^ On the 2ROT13 Encryption Algorithm (PDF). Prüfziffernberechnung in der Praxis (2004-09-25). Retrieved on 2007-09-20.
7. ^ Hollebeek, Tim; Viega, John. Bad Cryptography in the Netscape Browser: A Case Study. Reliable Software Technologies. Retrieved on 2007-09-20.
8. ^ Perens, Bruce (2001-09-01). Dimitry Sklyarov: Enemy or friend?. ZDNet News. Retrieved on 2007-09-20.
9. ^ Ferri, Vic (2007-01-04). The Count Keys in the Windows Registry. ABC: All 'Bout Computers. Retrieved on 2007-09-20.
10. ^ De Mulder, Tom. ROT13 Words. Furrfu!. Retrieved on 2007-09-19.
11. ^ Westley, Brian (1989). westley.c. IOCCC. Retrieved on 2007-08-13.
12. ^ Furrfu. Foldoc (1995-10-25). Retrieved on 2007-08-13.
13. ^ 5.10 Trivial Encryption. The GNU C Library Reference Manual. Free Software Foundation (2006-12-03). Retrieved on 2007-09-20.

Caesar cipher, also known as a Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most widely known encryption techniques.
Internet forum is a web application for holding discussions and posting user generated content. Internet forums are also commonly referred to as web forums, message boards, discussion boards, (electronic) discussion groups, discussion forums,
spoiler is a summary or description of a narrative (or part of a narrative) that relates plot elements not revealed early in the narrative itself. Moreover, because enjoyment of a narrative sometimes depends upon the dramatic tension and suspense which undergird it, this early
punch line is the final part of a joke, usually the word, sentence or exchange of sentences which is intended to be funny and to provoke laughter from listeners.

For instance, in the following well-known joke:

A man walks into a bar with a duck under his arm.

Usenet (USEr NETwork) is a global, decentralized, distributed Internet discussion system that evolved from a general purpose UUCP architecture of the same name. It was conceived by Duke University graduate students Tom Truscott and Jim Ellis in 1979.
Caesar cipher, also known as a Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most widely known encryption techniques.
Cryptography (or cryptology; derived from Greek κρυπτός kryptós "hidden," and the verb γράφω gráfo "write" or λεγειν legein
A newsgroup is a repository usually within the Usenet system, for messages posted from many users at different locations. The term is somewhat confusing, because it is usually a discussion group.
ABCs redirects here, for the Alien Big Cats, see British big cats.

An alphabet is a standardized set of letters
The modern English alphabet consists of the 26 letters[1] of the Latin alphabet:

Majuscule Forms (also called uppercase or capital letters)
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Minuscule Forms (also called
inverse function for ƒ, denoted by ƒ−1, is a function in the opposite direction, from B to A, with the property that a round trip (a composition) returns each element to itself.
Mathematics (colloquially, maths or math) is the body of knowledge centered on such concepts as quantity, structure, space, and change, and also the academic discipline that studies them. Benjamin Peirce called it "the science that draws necessary conclusions".
involution, or an involutary function, is a function that is its own inverse, so that

f(f(x)) = x for all x in the domain of f.

## General properties

Any involution is a bijection.
A reciprocal cipher means, just as one enters the cleartext into the cryptography system to get the ciphertext, one could enter the ciphertext into the same place in the system to get the cleartext. Sometimes also referred as self-reciprocal cipher.
In computer science, a lookup table is a data structure, usually an array or associative array, used to replace a runtime computation with a simpler lookup operation. The speed gain can be significant, since retrieving a value from memory is often faster than undergoing an
National Security Agency/Central Security Service (NSA/CSS) is the United States government's cryptologic organization that was officially established on November 4, 1952. Responsible for the collection and analysis of foreign communications, it coordinates, directs, and performs
A newsgroup is a repository usually within the Usenet system, for messages posted from many users at different locations. The term is somewhat confusing, because it is usually a discussion group.
spoiler is a summary or description of a narrative (or part of a narrative) that relates plot elements not revealed early in the narrative itself. Moreover, because enjoyment of a narrative sometimes depends upon the dramatic tension and suspense which undergird it, this early
encryption is the process of transforming information (referred to as plaintext) to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key.
Gaius Julius Caesar
Dictator of the Roman Republic

Reign October, 49 BC–March 15, 44 BC
Full name Gaius Julius Caesar
Born 12 July 100 BC - 102 BC
Rome, Roman Republic
Died 15 March 44 BC (aged 57)
Confidentiality has been defined by the International Organization for Standardization (ISO) as "ensuring that information is accessible only to those authorized to have access" and is one of the cornerstones of Information security.
key is a piece of information (a parameter) that controls the operation of a cryptographic algorithm. In encryption, a key specifies the particular transformation of plaintext into ciphertext, or vice versa during decryption.
frequency analysis is the study of the frequency of letters or groups of letters in a ciphertext. The method is used as an aid to breaking classical ciphers.

Frequency analysis is based on the fact that, in any given stretch of written language, certain letters and
encryption is the process of transforming information (referred to as plaintext) to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key.
Data Encryption Standard
The Feistel function (F function) of DES

General
IBM
1975 (standardized on January 1977)

Lucifer
Triple DES, G-DES, DES-X, LOKI89, ICE

Cipher detail
Key size(s):| 56 bits

Block size(s):| 64 bits
plaintext is information used as input to an encryption algorithm; the output is termed ciphertext. The plaintext could be, for example, a diplomatic message, a bank transaction, an e-mail, a diary and so forth — any information that someone might want to prevent
Maintainer: Netscape Communications Corporation

OS: Cross-platform

Use: Internet suite

Website: wp.netscape.com/browsers/4

Netscape Communicator was a proprietary Internet suite produced by Netscape Communications Corporation.

Founded San Jose (1982)