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{{Short description|Encoding for text messages}}
The '''Tap Code''' is a [[cipher]], commonly used by prisoners to communicate with one another. The method of communicating is usually by "tapping" either the metal bars or the walls inside the cell, hence its name. It is a very simple cipher, not meant to avoid interception, since the messages are sent in [[cleartext]].▼
{| class="wikitable" align=right style="width:100%; width:200px; text-align:center;"
|+ Latin alphabet tap code table
!
|-
! 1
| [[A]] || [[B]] || [[C]]/[[K]] || [[D]] || [[E]]▼
|-
! 2
| [[F]] || [[G]] || [[H]] || [[I]] || [[J]]▼
|-
! 3
| [[L]] || [[M]] || [[N]] || [[O]] || [[P]]▼
|-
! 4
| [[Q]] || [[R]] || [[S]] || [[T]] || [[U]]▼
|-
! 5
| [[V]] || [[W]] || [[X]] || [[Y]] || [[Z]]
|}
The ''tap code'', sometimes called the '''knock code''', is a way to encode text messages on a letter-by-letter basis in a very simple way. The message is transmitted using a series of tap sounds, hence its name.<ref>The Handbook Of The SAS And Elite Forces. How The Professionals Fight And Win. Edited by Jon E. Lewis. p.199-Tactics And Techniques, Evasion, Capture And Escape. Robinson Publishing Ltd 1997. ISBN 1-85487-675-9</ref>
▲The
== Design ==
The tap code is based on a [[Polybius square]] using a 5×5 grid of letters representing all the letters of the [[Latin alphabet]], except for K, which is represented by C.<ref name="pbs"/>
{| class="wikitable"
▲! !! 1 !! 2 !! 3 !! 4 !! 5
|-
! H !! E !! L !! L !! O
▲| A || B || C || D || E
|-
| {{center|2, 3}}|| {{center|1, 5}}|| {{center|3, 1}}|| {{center|3, 1}}|| {{center|3, 4}}
▲| F || G || H || I || J
|-
| colspan = 5 | •• ••• • ••••• ••• • ••• • ••• ••••
|}
▲| L || M || N || O || P
The letter "X" is used to break up sentences, and "K" for acknowledgements. <!--What does it mean by acknowledgments? Please elaborate-->
Because of the difficulty and length of time required for specifying a single letter, prisoners often devise
By comparison, despite its messages being shorter, [[Morse code]] is harder to send by tapping or banging. Its short and long signals can be improvised as taps and thumps, or short and long whistles or scraping sounds, but tap codes are simpler to learn and can be used in a wider variety of situations.<ref name="denton">{{cite book |last1=Denton |first1=Jeremiah A. |last2=Brandt |first2=Ed |title=[[When Hell Was in Session]] |date=1976 |publisher=Reader's Digest Press |___location=New York |isbn=0-88349-112-5}}</ref> The tap system simply requires one to know the alphabet and the short sequence "AFLQV" (the initial letter of each row), without memorising the entire grid. For example, if a person hears four knocks, they can think "A... F... L... Q". If after a pause there are three knocks, they think "Q... R... S" to arrive at the letter S.
== History ==
{| class="wikitable" align=right style="width:100%; width:200px; text-align:center;"
|+ Russian alphabet tap code<ref>{{Cite web|url=http://faroutliers.blogspot.com/2005/04/russian-prison-tapping-code.html|title=Far Outliers: Russian Prison Tapping Code|date=April 14, 2005|access-date=August 8, 2014|archive-date=August 15, 2014|archive-url=https://web.archive.org/web/20140815063344/http://faroutliers.blogspot.com/2005/04/russian-prison-tapping-code.html|url-status=live}}</ref>
|-
! !! 1 !! 2 !! 3 !! 4 !! 5 !! 6
▲| Q || R || S || T || U
|-
! 1
|
|-
! 2
| [[Ж]] || [[З]] || [[И]]/[[Й]] || [[К]] || [[Л]] || [[М]]
|-
! 3
| [[Н]] || [[О]] || [[П]] || [[Р]] || [[С]] || [[Т]]
|-
! 4
| [[У]] || [[Ф]] || [[Х]] || [[Ц]] || [[Ч]] || [[Ш]]
|-
! 5
| [[Щ]] ||[[Ъ]]/[[Soft sign|Ь]]||[[Ы]] || [[Э]] || [[Ю]] || [[Я]]
|}
The origins of this encoding go back to the [[Polybius square]] of [[Ancient Greece]].
▲Or to communicate the word "WATER" the cipher would be the following (the time between each pair of numbers is smaller than the one between two different letters):
In Vietnam, the tap code became more widely used than Morse; despite messages taking longer to send, the system was easier to learn and could be applied in a wider variety of situations.<ref name="denton"/> Tap codes proved to be a very successful<ref>{{Cite journal|last=Peterson|first=Gordon I|last2=Taylor|first2=David C|date=March 2016|title=Intelligence Support to Communications with US POW's in Vietnam|url=https://www.cia.gov/library/center-for-the-study-of-intelligence|journal=Studies in Intelligence|publisher=Center for the Study of Intelligence Publications|volume=60|pages=1–15|access-date=2019-05-23|archive-url=https://web.archive.org/web/20190508044337/https://www.cia.gov/library/center-for-the-study-of-intelligence|archive-date=2019-05-08|url-status=dead}}</ref> way for otherwise isolated prisoners to communicate.<ref name="au"/><ref>{{Cite book|title=Lessons from the Hanoi Hilton: Six Characteristics of High Performance Teams|last=Fretwell|first=Peter|last2=Kiland|first2=Taylor Baldwin|publisher=Naval Institute Press|year=2013|isbn=978-1-61251-217-4|___location=Annapolis, Maryland|pages=63–67}}</ref> POWs would use the tap code in order to communicate to each other between cells in a way which the guards would be unable to pick up on.<ref>{{Cite book|title=Two Souls Indivisible: The Friendship that saved two POW's in Vietnam|last=Hirsch|first=James S|publisher=Houghton-Mifflin|year=2004|isbn=0618273484|___location=New York|pages=[https://archive.org/details/twosoulsindivisi00hirs/page/161 161-162]|url-access=registration|url=https://archive.org/details/twosoulsindivisi00hirs/page/161}}</ref> They used it to communicate everything from what questions interrogators were asking (in order for everyone to stay consistent with a deceptive story), to who was hurt and needed others to donate meager food rations. It was easy to teach and newly arrived prisoners became fluent in it within a few days.<ref>{{cite book | last=McCain | first=John | author-link=John McCain |author2=Mark Salter |author2-link=Mark Salter | title=Faith of My Fathers | title-link=Faith of My Fathers | publisher=[[Random House]] | year=1999 | isbn=0-375-50191-6|pages=[https://archive.org/details/faithofmyfathersmcc00mcca/page/211 211]–12}}</ref><ref name="brace"/> It was even used when prisoners were sitting next to each other but not allowed to talk, by tapping on another's thigh.<ref name="brace">{{cite book | last=Brace |first=Ernest C. |author-link=Ernest C. Brace | title=A Code to Keep: The true story of America's longest held civilian prisoner of war in Vietnam | publisher=[[St. Martin's Press]] | year=1988 | isbn=0-7090-3560-8|pages= 171–72, 187–88}}</ref> U.S. Navy Rear Admiral [[Jeremiah Denton]] developed a vocal tap code of coughs, sniffs and sneezes.<ref name="denton"/> By overcoming isolation with the tap code, prisoners were said to be able to maintain a [[chain of command]] and keep up morale.<ref name="au"/><ref>{{Cite journal|last=Naughton|first=Robert J|date=1975|title=Motivational Factors of American Prisoners of War Held by the Democratic Republic of Vietnam|url=https://digital-commons.usnwc.edu/nwc-review/vol28/iss1/2|journal=Naval War College Review|volume=28|access-date=2019-05-04|archive-url=https://web.archive.org/web/20190504141451/https://digital-commons.usnwc.edu/nwc-review/vol28/iss1/2/|archive-date=2019-05-04|url-status=live}}</ref>
In 1980, a doctor sentenced to life in solitary confinement in [[Somalia]] used tap code to share the entirety of Tolstoy's ''[[Anna Karenina]]'', nearly 2 million letters, via tap code with fellow prisoners.<ref>{{cite web |title=Rough Translation: How 'Anna Karenina' Saved A Somali Inmate's Life |url=https://www.npr.org/2017/09/11/550058353/rough-translation-how-anna-karenina-saved-a-somali-inmates-life |website=[[NPR]] |date=2017-09-11 |archive-url=https://web.archive.org/web/20230626065411/https://www.npr.org/2017/09/11/550058353/rough-translation-how-anna-karenina-saved-a-somali-inmates-life |archive-date=2023-06-26 |url-status=live}}</ref>
▲Because of the difficulty and length of time required for specifying a single letter, prisoners often devise [[abbreviations]] and [[acronyms]] for common items or phrases, such as "GN" for ''Good Night'', or "GBU" for ''God Bless You''.<ref name="pbs"/>
==
In the 1990 video game ''[[Metal Gear 2: Solid Snake]]'', the tap code is used by Dr. Drago Pettrovich Madnar to communicate to ''[[Solid Snake]]'' through a cell wall.
In Season 2 Episode 2 of ''[[Person of Interest (TV series)|Person of Interest]]'' in 2012, the tap code is used by Harold Finch to discreetly leave breadcrumbs of his ___location to John Reese by encoding his ___location as tap code on a telephone.
In the 2015 video game ''[[Her Story (video game)|Her Story]]'', the main characters use the tap code to surreptitiously communicate.
In Season 2 Episode 14 of ''[[The Flash (2014 TV series)|The Flash]]'' in 2016, the masked prisoner in Zoom's lair uses the tap code to try to communicate with the others.
▲The origins of this encoding go back to the [[Polybius square]] of [[Ancient Greece]]. As the "knock code", a [[Cyrillic alphabet]] version is said to have been used by [[nihilist movement| nihilist]] prisoners of the [[Russia]]n [[Czar]]s.<ref>[[David Kahn]], ''The Codebreakers - The Story of Secret Writing''. 1967. ISBN 978-0-684-83130-5.</ref> The knock code is featured in [[Arthur Koestler]]'s classic 1941 work ''[[Darkness at Noon]]''.<ref>Koestler, Arthur, ''Darkness at Noon'' (1941). Translated by Daphne Hardy. See page 19 of the Bantam Publishing paperback, 1981 printing for more info.</ref>
In the 2021 film ''[[The Ice Road]]'', the tap code is used on a metal pipe conduit by trapped miners to communicate with executives of the mining company.
▲[[United States]] [[prisoners of war]] during the [[Vietnam War]] are most known for having used the Tap Code. It was introduced in June 1965 by four POWs held in the [[Hanoi Hilton|Hoa Lo "Hanoi Hilton"]] prison: Captain Carlyle "Smitty" Harris, Lieutenant Phillip Butler, Lieutenant Robert Peel, and Lieutenant Commander Robert Shumaker.<ref name="pbs">{{cite web | url=http://www.pbs.org/wgbh/amex/honor/sfeature/sf_tap.html | title=''[[American Experience]]'': ''Return with Honor'': The Tap Code | publisher=[[PBS]] | date=1999 | accessdate=2008-04-08}}</ref> Smitty Harris had heard of the tap code being used by prisoners in [[World War II]]<ref name="au">{{cite news | url=http://www.au.af.mil/au/awc/awcgate/awc-pow.htm | title=Vets, Flyers discuss ideology, time in POW camps | author=Staff Sgt. Jason Tudor | publisher=[[Air Force News Service]] | date=1998-03-18 | accessdate=2008-04-08}}</ref> and remembered a [[United States Air Force]] instructor who had discussed it as well.<ref name="pbs"/>
==See also==
*[[Morse code]]
==References==
{{
==External links==
* [http://www.braingle.com/brainteasers/codes/tapcode.php Online Tap Code Encoder/Decoder]
* [http://faroutliers.blogspot.com/2005/04/russian-prison-tapping-code.html Russian Prison Tap Codes]
*[http://www.premioceleste.it/opera/ido:260668/ L' Alfabeto Quadrato(Codice a Colpi) - An Degrida] Artist An Degrida's Tap Code artistic illustration.
{{Clear}}
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[[Category:Classical ciphers]]
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