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ROT13

2007 Schools Wikipedia Selection. Related subjects: Cryptography


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   ROT13 replaces each letter by its partner 13 characters further along
   the alphabet. For example, HELLO becomes URYYB (or, decrypting, URYYB
   becomes HELLO again).
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   ROT13 replaces each letter by its partner 13 characters further along
   the alphabet. For example, HELLO becomes URYYB (or, decrypting, URYYB
   becomes HELLO again).

   ROT13 ("rotate by 13 places", usually hyphenated ROT-13) is a simple
   Caesar cipher used for obscuring text by replacing each letter with the
   letter thirteen places down the alphabet. 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. The algorithm is used
   in online forums as a means of hiding joke punchlines, puzzle
   solutions, movie and story spoilers, 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" . The name "ROT13"
   originated on Usenet in the early 1980s, and the method has become a de
   facto standard. As a Caesar cipher (a method of encryption thousands of
   years old), 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.

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, preserving case: a becomes n, B becomes O, and so forth,
   down to Z, which becomes M. Only those letters which occur in the Latin
   alphabet are affected; numbers, symbols, whitespace, and all other
   characters are left unchanged. Because there are 26 letters in the
   Latin alphabet and 26 = 2 × 13, the ROT13 function is its own inverse:

          ROT[13](ROT[13](x)) = ROT[26](x) = x 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 answer (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.

History

   ROT13 originated in the net.jokes newsgroup in the early 1980s, in an
   effort to provide a voluntary means to hide jokes that some readers
   might have found offensive, or just to obscure the punchline of a joke
   to keep it from being read too soon. (Previous attempts to categorize
   offensive jokes by putting them in separate newsgroups had failed —
   site managers did not want to be seen as condoning such postings by
   creating a special place for them.) ROT13 was a convenient solution
   because of its simplicity.

   Since it replaces alphabetic letters with other letters, ROT13 was
   guaranteed not to cause problems for any newsgroup software which might
   have been incompatible with unusual kinds of characters. ROT-13 was
   chosen over ROT-N (for any other value of N, such as 3 as in the
   original Caesar cipher) because 13 is the value which arranges that
   encoding and decoding are equivalent, thereby allowing the convenience
   of a single command for both encryption and decryption. Strictly
   speaking, the value 13 works in this way only for languages like
   English which have 26 letters in their alphabets, although since the
   26-character Latin alphabet is embedded in the ASCII character set,
   ROT13 is pretty much universal, at least for those Western languages
   which can use ASCII or one of its extensions.

   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:

          tr A-Za-z N-ZA-Mn-za-m

   Automatic deciphering was soon added as a built-in feature to
   newsreading software. From the early 1990s, ROT13 was also used in
   Fidonet forums, to the extent that Fidonet mail reading software often
   included features for enciphering and deciphering automatically.

ROT13 as encryption

   ROT13 is one of a set of encryption algorithms known as Caesar ciphers,
   which are themselves a particularly simple type of substitution cipher.
   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, like any substitution cipher
   the algorithm is easily broken through frequency analysis or the
   exploitation of pattern words. (ROT13 is an example of what Bruce
   Schneier calls "kid sister encryption" -- at best it might keep a
   document secret from your kid sister.)

   The real effect of ROT13 is merely to make sure that the viewer of a
   message must consciously choose to decipher it, which typically means
   invoking the relevant ROT13 command in the message-reading software.
   Rather than protecting a confidential message from unauthorized
   readers, ROT13 safeguards authorized readers from material they may not
   wish to inadvertently read, such as spoilers in book or movie reviews.

   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" (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. The notional ROT26 has been used as a satirical jibe
   against the United States' Digital Millennium Copyright Act (DMCA);
   some online users append phrases such as "Encoded with ROT26 —
   circumvention will be prosecuted!" to their posts to online forums.
   (The DMCA introduced a broad ban on the circumvention of copy
   prevention systems, systems which are often found to internally employ
   woefully insecure methods of cryptography.) "Triple ROT13" is also
   sometimes seen - which is, of course, equivalent to regular ROT13. (The
   term is most probably a take on Triple DES.)

   In 2001, Russian cryptanalyst Dmitry Sklyarov was arrested after
   detailing weaknesses in ebook copy prevention systems. One ebook
   vendor, New Paradigm Research Group (NPRG), actually 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 (UserAssist). Didier
   Stevens wrote a tool to decrypt them. These keys track recently used
   programs.

ROT13 letter games and net culture

                                               abcdefghijklmnopqrstuvwxyz
                                               NOPQRSTUVWXYZABCDEFGHIJKLM
                                                 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

   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.

   The word ravine is transformed under ROT13 to enivar — its reversal,
   although not an English word. 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) 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
   to either 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 .

Variants

   There are other little-used obfuscation schemes with a similar purpose
   to ROT13. ROT13 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.

ROT47

   ROT47 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 special characters to numbers in
   the range 0–127. Viewed in ASCII, ROT13 covers the codes 65–90 and
   97–122 — the upper and lower case letters respectively. ROT47 uses 94
   characters instead, from ! (the exclamation 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.

   Applying ROT47 to the earlier example (entirely in plaintext) gives:
w@H 42? J@F E6== 2? 6IEC@G6CE 7C@> 2?
:?EC@G6CE 2E }$pn x? E96 6=6G2E@CD[
E96 6IEC@G6CED =@@< 2E E96 ~%wt# 8FJVD D9@6D]

   The Unix command to apply ROT47 is:

          tr '!-~' 'P-~!-O'

memfrob()

   The GNU C library (a set of standard routines available for use in
   computer programming) contains a function — memfrob() — which has a
   similar purpose to ROT13, although it is intended for use with
   arbitrary binary data. The function operates (humorously termed "
   frobnicates") by combining each byte with the binary pattern 00101010 (
   42 decimal) using the exclusive or (XOR) operation. This effects a
   simple XOR cipher. Like ROT13, memfrob() is self-reciprocal; it is also
   just about as insecure (that is, just as trivial to decrypt if the key
   is not known).

   Retrieved from " http://en.wikipedia.org/wiki/ROT13"
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   with only minor checks and changes (see www.wikipedia.org for details
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