ROT13 is a simple letter

substitution cipher In cryptography, a substitution cipher is a method of encrypting in which units of plaintext are replaced with the ciphertext, in a defined manner, with the help of a key; the "units" may be single letters (the most common), pairs of letters, t ...

that replaces a letter with the 13th letter after it in the

Latin alphabet The Latin alphabet, also known as the Roman alphabet, is the collection of letters originally used by the Ancient Rome, ancient Romans to write the Latin language. Largely unaltered except several letters splitting—i.e. from , and from � ...

. ROT13 is a special case of the

Caesar cipher In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code, or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in t ...

which was developed in ancient Rome, used by

Julius Caesar Gaius Julius Caesar (12 or 13 July 100 BC – 15 March 44 BC) was a Roman general and statesman. A member of the First Triumvirate, Caesar led the Roman armies in the Gallic Wars before defeating his political rival Pompey in Caesar's civil wa ...

in the 1st century BC. An early entry on the

Timeline of cryptography Below is a timeline of notable events related to cryptography. B.C.E. * 36th century The Sumerians develop ''cuneiform'' writing and the Egyptians develop ''hieroglyphic'' writing. * 16th century The Phoenicians develop an alphabet * 600-500 He ...

. ROT13 can be referred by "Rotate13", "rotate by 13 places", hyphenated "ROT-13" or sometimes by its

autonym Autonym may refer to: * Autonym, the name used by a person to refer to themselves or their language; see Exonym and endonym * Autonym (botany), an automatically created infrageneric or infraspecific name See also * Nominotypical subspecies, in zo ...

"EBG13".

Description

Applying ROT13 to a piece of text requires examining its alphabetic characters and replacing each one by the letter 13 places further along in the

alphabet An alphabet is a standard set of letter (alphabet), letters written to represent particular sounds in a spoken language. Specifically, letters largely correspond to phonemes as the smallest sound segments that can distinguish one word from a ...

, wrapping back to the beginning as necessary. When encoding a message, 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. When decoding a message, the same substitution rules are applied, but this time on the ROT13 encrypted text. Other characters, such as numbers, symbols, punctuation or

whitespace White space or whitespace may refer to: Technology * Whitespace characters, characters in computing that represent horizontal or vertical space * White spaces (radio), allocated but locally unused radio frequencies * TV White Space Database, a m ...

, are left unchanged. Because there are 26 letters in the

and 26 = 2 × 13, the ROT13 function is its own

inverse Inverse or invert may refer to: Science and mathematics * Inverse (logic), a type of conditional sentence which is an immediate inference made from another conditional sentence * Additive inverse, the inverse of a number that, when added to the ...

: :

\mbox_(\mbox_(x))=x

for any basic Latin-alphabet text

x

. In other words, two successive applications of ROT13 restore the original text (in

mathematics Mathematics is a field of study that discovers and organizes methods, Mathematical theory, theories and theorems that are developed and Mathematical proof, proved for the needs of empirical sciences and mathematics itself. There are many ar ...

, this is sometimes called an ''

involution Involution may refer to: Mathematics * Involution (mathematics), a function that is its own inverse * Involution algebra, a *-algebra: a type of algebraic structure * Involute, a construction in the differential geometry of curves * Exponentiati ...

''; in cryptography, a ''

reciprocal cipher Symmetric-key algorithms are algorithms for cryptography that use the same cryptographic keys for both the encryption of plaintext and the decryption of ciphertext. The keys may be identical, or there may be a simple transformation to go between ...

''). The transformation can be done using a

lookup table In computer science, a lookup table (LUT) is an array data structure, array that replaces runtime (program lifecycle phase), runtime computation of a mathematical function (mathematics), function with a simpler array indexing operation, in a proc ...

, such as the following: For example, in the following joke, the punchline has been obscured by ROT13: : Why did the chicken cross the road? :Gb trg gb gur bgure fvqr! Transforming the entire text via ROT13 form, the answer to the joke is revealed: : Jul qvq gur puvpxra pebff gur ebnq? : To get to the other side! A second application of ROT13 would restore the original.

Usage

ROT13 is not intended to be used in modern times. At the time of conception in an era of

Ancient Roman technology Ancient Roman technology is the collection of techniques, skills, methods, processes, and engineering practices which supported Roman civilization and made possible the expansion of the economy and military of ancient Rome (753 BC – 476 AD). ...

, the encryption scheme was not represented by a

mathematical structure In mathematics, a structure on a set (or on some sets) refers to providing or endowing it (or them) with certain additional features (e.g. an operation, relation, metric, or topology). Τhe additional features are attached or related to the ...

. The key to decrypt a message requires no more knowledge than the fact that ROT13 is in use. Even if

secrecy Secrecy is the practice of hiding information from certain individuals or groups who do not have the "need to know", perhaps while sharing it with other individuals. That which is kept hidden is known as the secret. Secrecy is often controver ...

does not fail, any alien party or individual, capable of intercepting the message, could break the code by spending enough time on decoding the text through

frequency analysis In cryptanalysis, frequency analysis (also known as counting letters) 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 th ...

or finding other

patterns A pattern is a regularity in the world, in human-made design, or in abstract ideas. As such, the elements of a pattern repeat in a predictable manner. A geometric pattern is a kind of pattern formed of geometric shapes and typically repeated li ...

. In the early 1980s, people used ROT13 in their messages on Usenet newsgroup servers They did this to hide potentially offensive jokes, or to obscure an answer to a puzzle or other

spoiler Spoiler or Spoilers may refer to: Arts, entertainment and media * Spoiler (media), something that reveals significant plot elements * The Spoiler, DC Comics superheroine Stephanie Brown Film and television * ''Spoiler'' (film), 1998 American ...

, or to fool less sophisticated spam bots^{dubious – Talk:ROT13#Dubious">discuss

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Description

Usage

Net culture

Letter games

Variants and combinations

Implementation

tr

Emacs and Vim

Python

See also

References

External links