Editing ROT13

{{Short description|Simple encryption method}}
{{Use dmy dates|date=November 2023}}
[[File:ROT13_table_with_example.svg|right|frameless]]

'''ROT13''' is a simple letter [[substitution cipher]] that replaces a letter with the 13th letter after it in the [[Latin alphabet]].  

ROT13 is a special case of the [[Caesar cipher]] which was developed in ancient Rome, used by [[Julius Caesar]] in the 1st century BC.<ref>{{Cite book |last=Kahn |first=David |author-link=David Kahn (writer) |title=The Codebreakers: The Story of Secret Writing |publisher=Macmillan |isbn=0-684-83130-9 |location=New York}}</ref> An early entry on the [[Timeline of cryptography]]. 

ROT13 can be referred by "Rotate13", "rotate by 13 places", hyphenated "ROT-13" or sometimes by its [[Autological word|autonym]] "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]], wrapping back to the beginning as necessary.<ref name="schneier">{{Cite book|last=Schneier |first=Bruce |author-link= Bruce Schneier |title=Applied Cryptography |url=https://archive.org/details/appliedcryptogra00schn_605 |url-access=limited |edition=Second|year=1996|publisher=John Wiley & Sons |isbn= 0-471-11709-9|pages=[https://archive.org/details/appliedcryptogra00schn_605/page/n198 11] }}</ref>

When encoding a message, <kbd>A</kbd> becomes <kbd>N</kbd>, <kbd>B</kbd> becomes <kbd>O</kbd>, and so on up to <kbd>M</kbd>, which becomes <kbd>Z</kbd>. Then the sequence continues at the beginning of the alphabet: <kbd>N</kbd> becomes <kbd>A</kbd>, <kbd>O</kbd> becomes <kbd>B</kbd>, and so on to <kbd>Z</kbd>, which becomes <kbd>M</kbd>. 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 character|whitespace]], are left unchanged.

Because there are 26 letters in the [[Latin alphabet]] and 26 = 2 &times; 13, the ROT13 function is its own [[inverse function|inverse]]:<ref name="schneier" />

:<math>\mbox{ROT}_{13}(\mbox{ROT}_{13}(x))=x</math> for any basic Latin-alphabet text <math>x</math>.

In other words, two successive applications of ROT13 restore the original text (in [[mathematics]], this is sometimes called an ''[[involution (mathematics)|involution]]''; in cryptography, a ''[[reciprocal cipher]]'').

The transformation can be done using a [[lookup table]], such as the following:
{| class="wikitable"
| Input
| <kbd><span style="color: light-dark(darkred, pink)">ABCDEFGHIJKLM</span><span style="color: light-dark(darkblue, lightblue)">NOPQRSTUVWXYZ</span><span style="color: light-dark(darkred, pink)">abcdefghijklm</span><span style="color: light-dark(darkblue, lightblue)">nopqrstuvwxyz</span></kbd>
|-
| Output
| <kbd><span style="color: light-dark(darkblue, lightblue)">NOPQRSTUVWXYZ</span><span style="color: light-dark(darkred, pink)">ABCDEFGHIJKLM</span><span style="color: light-dark(darkblue, lightblue)">nopqrstuvwxyz</span><span style="color: light-dark(darkred, pink)">abcdefghijklm</span></kbd>
|}

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]], the encryption scheme was not represented by a [[mathematical structure]]. The [[Key (cryptography)|key]] to decrypt a message requires no more knowledge than the fact that ROT13 is in use. Even if [[secrecy]] 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 (cryptanalysis)|frequency analysis]]<ref name="schneier" /> or finding other [[String-searching algorithm|patterns]].

In the early 1980s, people used ROT13 in their messages on [[Usenet newsgroup|Usenet newsgroup servers]]<ref name="jargon">{{Cite web |date=29 December 2003 |editor-last=Raymond |editor-first=Eric S. |editor-link=Eric S. Raymond |title=ROT13 |url=http://www.catb.org/~esr/jargon/html/R/rot13.html |url-status=live |archive-url=https://web.archive.org/web/20120113164436/http://www.catb.org/~esr/jargon/html/R/rot13.html |archive-date=13 January 2012 |access-date=19 September 2007 |work=The Jargon File, 4.4.7}}</ref>
They did this to hide potentially offensive jokes, or to obscure an answer to a puzzle or other [[Spoiler (media)|spoiler]],<ref name="netiquette">{{Cite web | url=https://datatracker.ietf.org/doc/html/rfc1855#section-3.1.3 | title=RFC 1855 - Netiquette Guidelines | last=Hambridge | first=Sally | publisher=Internet Engineering Task Force | date=1995-10-01 | access-date=2025-01-02 | archive-url=https://web.archive.org/web/20250102045838/https://datatracker.ietf.org/doc/html/rfc1855#section-3.1.3 | archive-date=2025-01-02 }}</ref> or to fool less sophisticated [[spam bots]]<sup>[''[[Wikipedia:Accuracy dispute#Disputed statement|dubious]] – [[Talk:ROT13#Dubious|discuss]]'']</sup>.
ROT13 has been the subject of many jokes.

The 1989 [[International Obfuscated C Code Contest]] (IOCCC) included an entry by Brian Westley. Westley's [[computer program]] can be encoded in ROT13 or reversed and still [[compiler|compiles]] correctly. Its operation, when executed, is either to perform ROT13 encoding on, or to reverse its input.<ref>{{Cite web |last=Westley |first=Brian |year=1989 |title=westley.c |url=http://www.ioccc.org/1989/westley.c |url-status=live |archive-url=https://web.archive.org/web/20120609004219/http://www.ioccc.org/1989/westley.c |archive-date=9 June 2012 |access-date=13 August 2007 |work=[[IOCCC]]}}</ref>

In December 1999, it was found that [[Netscape Communicator]] used ROT13 as part of an insecure scheme to store email passwords.<ref>{{Cite CiteSeerX |citeseerx=10.1.1.15.9271 |author=Hollebeek, Tim |author2=Viega, John |author-link2=John Viega |title=Bad Cryptography in the Netscape Browser: A Case Study}}</ref> 

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 Systems|Adobe]] eBook [[software development kit]]—for a serious encryption scheme.<ref>{{Cite web |last=Perens |first=Bruce |author-link=Bruce Perens |date=1 September 2001 |title=Dimitry Sklyarov: Enemy or friend? |url=https://www.zdnet.com/article/dimitry-sklyarov-enemy-or-friend/ |url-status=live |archive-url=https://web.archive.org/web/20141017174003/http://www.zdnet.com/news/dimitry-sklyarov-enemy-or-friend/116424 |archive-date=17 October 2014 |access-date=3 February 2011 |publisher=[[ZDNet News]]}}</ref> Windows XP uses ROT13 on some of its registry keys.<ref>{{Cite web |date=24 July 2006 |title=ROT13 is used in Windows |url=https://blog.didierstevens.com/2006/07/24/rot13-is-used-in-windows-you%E2%80%99re-joking/ |url-status=live |archive-url=https://web.archive.org/web/20161220182919/https://blog.didierstevens.com/2006/07/24/rot13-is-used-in-windows-you%E2%80%99re-joking/ |archive-date=20 December 2016 |access-date=15 December 2016}}</ref> ROT13 is also used in the [[fortune (Unix)|Unix fortune program]] to conceal potentially offensive [[dicta]].

[[Johann Ernst Elias Bessler]], an 18th-century clock maker and constructor of [[perpetual motion]] machines, pointed out that ROT13 encodes his surname as ''Orffyre''. He used its [[latinisation of names|latinised]] form, ''Orffyreus'', as his pseudonym.<ref>{{cite web|url=https://lockhaven.edu/~dsimanek/museum/people/people.htm|title=Perpetual Futility: A short history of the search for perpetual motion|last=Simanek|first=Donald E.|date=2012|website=The Museum of Unworkable Devices|access-date=28 October 2020|archive-date=10 October 2020|archive-url=https://web.archive.org/web/20201010110525/https://www.lockhaven.edu/~dsimanek/museum/people/people.htm|url-status=live}}</ref>

== Net culture ==
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 [[Data Encryption Standard|DES]] is little better than ROT13 these days". In a play on real terms like "double DES" several terms cropped up with humorous intent:

* Double ROT13: applying ROT13 to an already ROT13-encrypted text restores the original [[plaintext]].
* ROT26: equivalent to no encryption at all.
* 2ROT13 was included in a spoof academic paper entitled "On the 2ROT13 Encryption Algorithm".<ref>{{Cite web |date=25 September 2004 |title=On the 2ROT13 Encryption Algorithm |url=http://www.pruefziffernberechnung.de/Originaldokumente/2rot13.pdf |url-status=live |archive-url=https://web.archive.org/web/20120415092223/http://www.pruefziffernberechnung.de/Originaldokumente/2rot13.pdf |archive-date=15 April 2012 |access-date=20 September 2007 |publisher=Prüfziffernberechnung in der Praxis}}</ref> 
* triple-ROT13: used jokingly in analogy with 3DES, it is equivalent to regular ROT13.
ROT13 jokes were popular on many [[newsgroup]] servers, like net.jokes as early as the 1980s.<ref name="jargon" />

The newsgroup alt.folklore.urban coined a word—''furrfu''—that was the ROT13 encoding of the frequently encoded utterance "[[:wikt:sheesh|sheesh]]". "<kbd>Furrfu</kbd>" evolved in mid-1992 as a response to postings repeating [[urban myth]]s on alt.folklore.urban, after some posters complained that "Sheesh!" as a response to [[newbie|newcomer]]s was being overused.<ref>{{Cite web |date=25 October 1995 |title=Furrfu |url=http://foldoc.org/furrfu |url-status=live |archive-url=https://web.archive.org/web/20140714182645/http://foldoc.org/furrfu |archive-date=14 July 2014 |access-date=3 October 2016 |publisher=[[Foldoc]]}}</ref>

Using a [[search engine]] on public social networks, yields results for ROT13 in jokes to this day.{{cn|date=April 2025}}{{when|date=April 2025}}

==Letter games==
{| class="wikitable" align="right" style="margin: 0 0 1em 1em; text-align: center"
|-
| colspan="2" | <kbd>abcdefghijklmnopqrstuvwxyz</kbd><br /><kbd>NOPQRSTUVWXYZABCDEFGHIJKLM</kbd>
|-
| <kbd>aha</kbd> ↔ <kbd>nun</kbd> || <kbd>ant</kbd> ↔ <kbd>nag</kbd>
|-
| <kbd>balk</kbd> ↔ <kbd>onyx</kbd> || <kbd>bar</kbd> ↔ <kbd>one</kbd>
|-
| <kbd>barf</kbd> ↔ <kbd>ones</kbd> || <kbd>be</kbd> ↔ <kbd>or</kbd>
|-
| <kbd>bin</kbd> ↔ <kbd>ova</kbd> || <kbd>ebbs</kbd> ↔ <kbd>roof</kbd>
|-
| <kbd>envy</kbd> ↔ <kbd>rail</kbd> || <kbd>er</kbd> ↔ <kbd>re</kbd>
|-
| <kbd>errs</kbd> ↔ <kbd>reef</kbd> || <kbd>flap</kbd> ↔ <kbd>sync</kbd>
|-
| <kbd>fur</kbd> ↔ <kbd>she</kbd> || <kbd>gel</kbd> ↔ <kbd>try</kbd>
|-
| <kbd>gnat</kbd> ↔ <kbd>tang</kbd> || <kbd>irk</kbd> ↔ <kbd>vex</kbd>
<!-- Nice ones: -->
|-
| <kbd>clerk</kbd> ↔ <kbd>pyrex</kbd> || <kbd>purely</kbd> ↔ <kbd>cheryl</kbd>
|-
| <kbd>PNG</kbd> ↔ <kbd>cat</kbd> || <kbd>SHA</kbd> ↔ <kbd>fun</kbd>
|-
| <kbd>furby</kbd> ↔ <kbd>sheol</kbd> || <kbd>terra</kbd> ↔ <kbd>green</kbd>
|-
| <kbd>what</kbd> ↔ <kbd>Jung</kbd> || <kbd>URL</kbd> ↔ <kbd>hey</kbd>
|-
| <kbd>purpura</kbd> ↔ <kbd>Chechen</kbd> || <kbd>shone</kbd> ↔ <kbd>FUBAR</kbd>
|-
| <kbd>Ares</kbd> ↔ <kbd>Nerf</kbd> || <kbd>abjurer</kbd> ↔ <kbd>nowhere</kbd>
|}

ROT13 provides an opportunity for letter games. Some words will, when transformed with ROT13, produce another word. Examples of 7-letter pairs in the [[English language]] are ''[[wiktionary:abjurer|abjurer]]'' and ''nowhere'', and ''[[Chechen people|Chechen]]'' and ''[[wiktionary:purpura|purpura]]''. Other examples of words like these are shown in the table.<ref>{{Cite web|url=http://www.furrfu.org/rot13words.html|last=De Mulder|first=Tom|title=ROT13 Words|access-date=19 September 2007|work=Furrfu!|archive-date=2 April 2012|archive-url=https://web.archive.org/web/20120402095657/http://www.furrfu.org/rot13words.html|url-status=live}}</ref> The pair ''[[gnat]]'' and ''tang'' is an example of words that are both ROT13 reciprocals and reversals.

==Variants and combinations==
ROT5 is a practice similar to ROT13 that applies to numeric digits (0 to 9). ROT13 and ROT5 can be used together in the same message, sometimes called ROT18 (18 = 13 + 5) or ROT13.5.

ROT47 is a derivative of ROT13 which, in addition to scrambling the basic letters, treats numbers and common symbols. Instead of using the sequence <kbd>A–Z</kbd> 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 '<kbd>!</kbd>' through 126 '<kbd>~</kbd>', 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 <kbd>A</kbd> is mapped to <kbd>p</kbd>, while <kbd>a</kbd> is mapped to <kbd>2</kbd>.

The use of a larger alphabet produces a more thorough obfuscation than that of ROT13; for example, a telephone number such as <kbd>+1-415-839-6885</kbd> is not obvious at first sight from the scrambled result <kbd>Z'\c`d\gbh\eggd</kbd>. 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 encoded.

Example:
:<kbd>The Quick Brown Fox Jumps Over The Lazy Dog.</kbd>
enciphers to
:<kbd>%96 "F:4< qC@H? u@I yF>AD ~G6C %96 {2KJ s@8]</kbd>

The [[GNU C library]], a set of standard routines available for use in [[computer programming]], contains a [[function (programming)|function]]—'''<kbd>mem<abbr title=frobnicate>frob</abbr>()</kbd>'''<ref>{{Cite web|url=https://www.gnu.org/software/libc/manual/html_node/Obfuscating-Data.html|date=3 December 2006|title=5.13 Obfuscating Data|work=The GNU C Library Reference Manual|publisher=[[Free Software Foundation]]|access-date=2 August 2019|archive-date=2 August 2019|archive-url=https://web.archive.org/web/20190802141847/https://www.gnu.org/software/libc/manual/html_node/Obfuscating-Data.html|url-status=live}}</ref>—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 number|binary]] pattern 00101010 ([[42 (number)|42]]) using the [[exclusive or]] (XOR) operation. This effects a [[simple XOR cipher]]. Like ROT13, XOR (and therefore <kbd>memfrob()</kbd>) is self-reciprocal, and provides a similar, virtually absent, level of security.

==Implementation==
=== tr ===
The ROT13 and ROT47 are fairly easy to implement using the Unix terminal application <kbd>[[Tr (Unix)|tr]]</kbd>; to encrypt the string "Pack My Box With Five Dozen Liquor Jugs" in ROT13:
<syntaxhighlight lang="console">
$ # Map upper case A-Z to N-ZA-M and lower case a-z to n-za-m
$ tr 'A-Za-z' 'N-ZA-Mn-za-m' <<< "Pack My Box With Five Dozen Liquor Jugs"
Cnpx Zl Obk Jvgu Svir Qbmra Yvdhbe Whtf
</syntaxhighlight>

and the string "The Quick Brown Fox Jumps Over The Lazy Dog" for ROT47:

 {{codett|lang=console|$ echo "The Quick Brown Fox Jumps Over The Lazy Dog" {{!}} tr '\!-~' 'P-~\!-O'}}
 {{codett|lang=output|%96 "F:4< qC@H? u@I yF>AD ~G6C %96 {2KJ s@8}}

=== Emacs and Vim ===
In [[Emacs]], one can ROT13 the buffer or a selection with the commands:<ref>[https://www.gnu.org/software/emacs/manual/html_node/emacs/Rmail-Rot13.html "Rmail Rot13 – GNU Emacs Manual"] {{Webarchive|url=https://web.archive.org/web/20160324051316/https://www.gnu.org/software/emacs/manual/html_node/emacs/Rmail-Rot13.html |date=24 March 2016 }}. ''www.gnu.org''.</ref> {{kbd|style=white-space:nowrap|M-x toggle-rot13-mode}}, {{kbd|style=white-space:nowrap|M-x rot13-other-window}}, or {{kbd|style=white-space:nowrap|M-x rot13-region}}.

In the [[Vim (text editor)|Vim text editor]], one can ROT13 a buffer with the command:<ref>{{Cite web|archive-url=https://web.archive.org/web/20160303184604/http://rayninfo.co.uk/vimtips.html|url=http://rayninfo.co.uk/vimtips.html|title=Best of VIM Tips, gVIM's Key Features zzapper|archive-date=3 March 2016|date=3 March 2016|website=rayninfo.co.uk}}</ref> {{kbd|style=white-space:nowrap|ggg?G}}.

===Python===

The module {{mono|codecs}} provides {{mono|'rot13'}} text transform.<ref>{{Cite web |date=2 November 2023 |title=codecs – Codec registry and base classes – Python 3.9.6 documentation |url=https://docs.python.org/3/library/codecs.html#text-transforms |url-status=live |archive-url=https://web.archive.org/web/20201006065104/https://docs.python.org/3/library/codecs.html#text-transforms |archive-date=6 October 2020 |access-date=7 October 2020 |website=ww6.python.org}}</ref>

<syntaxhighlight lang="pycon">
>>> import codecs
>>> print(codecs.encode("The Quick Brown Fox Jumps Over The Lazy Dog", "rot13"))
Gur Dhvpx Oebja Sbk Whzcf Bire Gur Ynml Qbt
</syntaxhighlight>
Without importing any libraries, it can be done by creating a translation table manually:{{efn|This source code is a slight variation in [[Zen of Python]]<ref>{{GitHub|https://github.com/python/cpython/blob/main/Lib/this.py}}</ref>}}
<syntaxhighlight lang="pycon">
>>> def gen_rot13_table(func=lambda x: x):
...     for c in (ord("A"), ord("a")):
...         for i in range(26):
...             yield func(i + c), func((i + 13) % 26 + c)
>>> table = dict(gen_rot13_table(chr))
>>> table
{'A': 'N', 'B': 'O', 'C': 'P', 'D': 'Q', 'E': 'R', 'F': 'S', 'G': 'T', 'H': 'U', 'I': 'V', 'J': 'W', 'K': 'X', 'L': 'Y', 'M': 'Z', 'N': 'A', 'O': 'B', 'P': 'C', 'Q': 'D', 'R': 'E', 'S': 'F', 'T': 'G', 'U': 'H', 'V': 'I', 'W': 'J', 'X': 'K', 'Y': 'L', 'Z': 'M', 'a': 'n', 'b': 'o', 'c': 'p', 'd': 'q', 'e': 'r', 'f': 's', 'g': 't', 'h': 'u', 'i': 'v', 'j': 'w', 'k': 'x', 'l': 'y', 'm': 'z', 'n': 'a', 'o': 'b', 'p': 'c', 'q': 'd', 'r': 'e', 's': 'f', 't': 'g', 'u': 'h', 'v': 'i', 'w': 'j', 'x': 'k', 'y': 'l', 'z': 'm'}
>>>
>>> s = "Quartz glyph job vext cwm porshrop finks?!"
>>> print("".join(table.get(c, c) for c in s))
Dhnegm tylcu wbo irkg pjz cbefuebc svaxf?!

</syntaxhighlight>

For Python 3, you can use the method [https://docs.python.org/3/library/stdtypes.html#str.translate {{code|str.translate()}}] (with [https://docs.python.org/3/library/stdtypes.html#str.maketrans {{code|str.maketrans()}}]):
<syntaxhighlight lang="pycon">
>>> x, y = zip(*gen_rot13_table(chr))
>>> ''.join(x)
'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
>>> ''.join(y)
'NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm'
>>> table = str.maketrans(''.join(x), ''.join(y))
>>> table
{65: 78, 66: 79, 67: 80, 68: 81, 69: 82, 70: 83, 71: 84, 72: 85, 73: 86, 74: 87, 75: 88, 76: 89, 77: 90, 78: 65, 79: 66, 80: 67, 81: 68, 82: 69, 83: 70, 84: 71, 85: 72, 86: 73, 87: 74, 88: 75, 89: 76, 90: 77, 97: 110, 98: 111, 99: 112, 100: 113, 101: 114, 102: 115, 103: 116, 104: 117, 105: 118, 106: 119, 107: 120, 108: 121, 109: 122, 110: 97, 111: 98, 112: 99, 113: 100, 114: 101, 115: 102, 116: 103, 117: 104, 118: 105, 119: 106, 120: 107, 121: 108, 122: 109}
>>>
>>> print(s.translate(table))
Dhnegm tylcu wbo irkg pjz cbefuebc svaxf?!
</syntaxhighlight>

==See also==
* [[Cryptanalysis]]
* [[Atbash]]

==References==
{{notelist}}
{{reflist}}

== External links ==
{{Wikifunctions|Z10627|ROT13}}
* [http://multidec.web-lab.at/rot.php Online converter] for ROT13, ROT5, ROT18, ROT47, [[Atbash]] and [[Caesar cipher]].
*[https://puretables.com/rot13-to-text ROT13 to Text on PureTables.com]

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