Enigma machine: Difference between revisions

{{short description|German cipher machine during World War II}}

{{Use British English|date=October 2025}}

{{Use dmy dates|date=October 2025}}

The '''Enigma machine''' is a [[cipher]] device developed and used in the early- to mid-20th century to protect [[Commerce|commercial]], diplomatic, and military communication. It was employed extensively by [[Nazi Germany]] during [[World War II]], in all branches of the [[Wehrmacht|German military]]. The Enigma machine was considered so secure that it was used to encipher the most top-secret messages.<ref>{{Cite web|title=EnigmaHistory|url=https://www.cryptomuseum.com/crypto/enigma/hist.htm|access-date=16 December 2020|website=Crypto Museum}}</ref>

The Enigma has an electromechanical [[Rotor machine|rotor mechanism]] that scrambles the 26 letters of the Latin alphabet. In typical use, one person enters text on the Enigma's keyboard and another person writes down which of the 26 lights above the keyboard illuminated at each key press. If [[plaintext]] is entered, the illuminated letters are the [[ciphertext]]. Entering ciphertext transforms it back into readable plaintext. The rotor mechanism changes the electrical connections between the keys and the lights with each keypress. In essence, the rotor's motion means every letter is encrypted with a different [[Key (cryptography)|cryptographic key]], making it highly resistant to conventional cryptographic attacks based on patterns the keys leave in the resulting [[cyphertext]].

For the system to be bidirectional, the receiving station would have to know and use the exact settings employed by the transmitting station to decrypt a message. This consisted of a series of initial settings that were generally changed daily, based on secret key lists distributed in advance. Due to the large number of messages transmitted every day, this could allow the system to be attacked if enough messages were intercepted. To complicate this, operators would choose some other (ideally) random settings of the rotors, say "GTZ", and then use the day settings to encode that key and send it. They would then change the rotors to those chosen settings and send the rest of the message. That meant that only those three letters were set to the day code, although normally typed twice for a total of six characters. This made it seemingly impossible to gather enough cyphertext to attack it.

Despite the seeming difficulty in decrypting its messages, Enigma contained a number of design issues that left patterns in the cyphertext. [[Second Polish Republic|Poland]] first cracked the machine as early as December 1932 and was able to read messages prior to and into the war. Poland's sharing of their achievements enabled the [[Allies of World War II|Allies]] to exploit Enigma-enciphered messages as a major source of intelligence.{{sfn|Comer|2021}} Although Nazi Germany introduced a series of improvements to the Enigma over the years that hampered decryption efforts, [[cryptanalysis of the Enigma]] continued throughout the war. Many commentators say the flow of [[Ultra (cryptography)|Ultra]] [[communications intelligence]] from the decrypting of Enigma, [[Lorenz cipher|Lorenz]], and other ciphers shortened the war substantially and may even have altered its outcome.{{sfn|Keegan|2003|p=283}}

The Enigma machine was invented by German engineer [[Arthur Scherbius]] at the end of [[World War I]].<ref name="Singh2011">{{cite book|first=Simon |last=Singh|title=The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography|url={{google books | plainurl=y | id=fbp9V9dkaNkC}}|date=26 January 2011|publisher=Knopf Doubleday Publishing Group|isbn=978-0-307-78784-2|page=139}}</ref> The German firm Scherbius & Ritter, co-founded by Scherbius, patented ideas for a cipher machine in 1918 and began marketing the finished product under the brand name ''Enigma'' in 1923, initially targeted at commercial markets.<ref>{{cite web | url=http://www.cryptomuseum.com/crypto/enigma/hist.htm | title=History of the Enigma | publisher=Crypto Museum | access-date=1 December 2017}}</ref> Early models were used commercially from the early 1920s, and adopted by military and government services of several countries, most notably [[Nazi Germany]] before and during [[World War II]].<ref>{{cite web|last=Lord|first= Bob|title=Enigma Manual|year=1998–2010|url=http://www.ilord.com/enigma-manuals|access-date=31 May 2011}}</ref>

Several Enigma models were produced,{{sfn|Hamer|Sullivan|Weierud|2010|pp=211-229}} but the [[Wehrmacht|German military]] models, having a [[plugboard]], were the most complex. Japanese and Italian models were also in use.<ref>{{Cite web |title=Four Rotor Enigma Machine |url=https://www.spymuseum.org/exhibition-experiences/about-the-collection/collection-highlights/four-rotor-enigma-machine/ |access-date=21 February 2023 |website=International Spy Museum |language=en}}</ref> With its adoption (in slightly modified form) by the German Navy in 1926 and the German Army and Air Force soon after, the name ''Enigma'' became widely known in military circles. Pre-war German military planning emphasised fast, mobile forces and tactics, later known as [[blitzkrieg]], which depended on radio communication for command and coordination. Since adversaries would likely intercept radio signals, messages had to be protected with secure encipherment. Compact and easily portable, the Enigma machine filled that need.

[[Hans-Thilo Schmidt]] was a [[Nazi Germany|German]] who spied for the [[French Third Republic|French]], obtaining access to German cipher materials that included the daily keys used in September and October 1932. Those keys included the plugboard settings. The French passed the material to [[Second Polish Republic|Poland]]. Around December 1932, [[Marian Rejewski]], a Polish mathematician and [[Cryptanalysis|cryptologist]] at the [[Biuro Szyfrów|Polish Cipher Bureau]], used the theory of permutations,{{sfn|Rejewski|1980}} and flaws in the German military-message encipherment procedures, to break message keys of the plugboard Enigma machine.{{sfn|Vázquez|Jiménez–Seral|2018}} Rejewski used the French supplied material and the message traffic that took place in September and October to solve for the unknown rotor wiring. Consequently, the Polish mathematicians were able to build their own Enigma machines, dubbed "[[Polish Enigma double|Enigma doubles]]". Rejewski was aided by fellow mathematician-cryptologists [[Jerzy Różycki]] and [[Henryk Zygalski]], both of whom had been recruited with Rejewski from [[Adam Mickiewicz University in Poznań|Poznań University]], which had been selected for its students' knowledge of the German language, since that area was held by [[German Empire|Germany]] prior to World War I. The Polish Cipher Bureau developed techniques to defeat the plugboard and find all components of the daily key, which enabled the Cipher Bureau to read German Enigma messages starting from January 1933.{{sfn|Kozaczuk|1984|p=21}}

In September 1939, British Military Mission 4, which included [[Colin Gubbins]] and [[Vera Atkins]], went to Poland, intending to evacuate cipher-breakers [[Marian Rejewski]], [[Jerzy Różycki]], and [[Henryk Zygalski]] from the country. The cryptologists, however, had been evacuated by their own superiors into Romania, at the time a Polish-allied country. On the way, for security reasons, the Polish Cipher Bureau personnel had deliberately destroyed their records and equipment. From Romania they travelled on to France, where they resumed their cryptological work, collaborating with the [[UK|British]], who began work on decrypting German Enigma messages, using the Polish equipment and techniques.{{sfn|Kozaczuk|1984|pp=69–94}}

Among those who joined the cryptanalytic effort in France was a team of seven Spanish cryptographers, known as "Equipo D" (Team D), led by [[Antonio Camazón]], former head of the cipher service ([[Servicio de Información Militar]]) of the [[Spanish Republican Army]] during the [[Spanish Civil War]]. After the fall of the Republic in 1939, Camazón and his colleagues sought refuge in France and were recruited by French intelligence officer [[Gustave Bertrand]]. They were assigned to the [[PC Bruno]] centre near Paris, where they worked alongside Polish cryptanalysts analysing Enigma-encrypted traffic and contributing to the adaptation of Polish decryption methods.

 

Following the [[Battle of France|German invasion of France in June 1940]], the Spanish team relocated first to the [[Cadix]] centre in the [[Vichy France|Vichy-controlled zone]] and later to [[Algiers]], continuing their work with the Western Allies. Their tasks included manual decryption, rotor setting reconstruction, and message traffic analysis. Though their contribution remained largely unknown for decades, recent historical research and documentaries have highlighted their role in the broader Allied effort to break Enigma.<ref>Quirantes, Arturo (2021). "Faustino Camazón: El español que descifró la máquina Enigma". The Conversation España. Retrieved 1 June 2024.</ref><ref>RTVE (2020). ''Equipo D: los códigos olvidados''. Directed by Jorge Laplace. RTVE Play. Retrieved 1 June 2024.</ref><ref>García Abadillo, Esteban (2019). "El olvidado matemático vallisoletano cuyo trabajo fue decisivo para derrotar a Hitler". ''El País''. Retrieved 1 June 2024.</ref>

During the war, British cryptologists decrypted a vast number of messages enciphered on Enigma. The intelligence gleaned from this source, codenamed "[[Ultra (cryptography)|Ultra]]" by the British, was a substantial aid to the [[Allies of World War II|Allied]] war effort.{{efn|Much of the German cipher traffic was encrypted on the Enigma machine, and the term "Ultra" has often been used almost synonymously with "[[Cryptanalysis of the Enigma|Enigma decrypts]]". Ultra also encompassed decrypts of the German [[Lorenz cipher|Lorenz SZ 40 and 42 machines]] that were used by the [[Oberkommando der Wehrmacht|German High Command]], and decrypts of [[C-36 (cipher machine)|Hagelin ciphers]] and other Italian ciphers and codes, as well as of Japanese ciphers and codes such as [[Type B Cipher Machine|Purple]] and [[Japanese naval codes#JN-25|JN-25]].}}

The Abwehr code had been broken on 8 December 1941 by [[Dilly Knox]]. Agents sent messages to the Abwehr in a simple code which was then sent on using an Enigma machine. The simple codes were broken and helped break the daily Enigma cipher. This breaking of the code enabled the [[Double-Cross System]] to operate.<ref name="Flem11"/>  

From October 1944, the German Abwehr used the [[Schlüsselgerät 41]] in limited quantities.<ref name="NSA-German">{{cite web |url=https://www.nsa.gov/Portals/70/documents/about/cryptologic-heritage/historical-figures-publications/publications/wwii/german_cipher.pdf |title=German Cipher Machines of World War II |pages=22–25 |work=Center for Cryptologic History |publisher=[[National Security Agency]]|archive-url=https://web.archive.org/web/20230514055454/https://www.nsa.gov/portals/75/documents/about/cryptologic-heritage/historical-figures-publications/publications/wwii/german_cipher.pdf|archive-date=14 May 2023  |access-date=21 January 2024 |year=2014 }}</ref>

Like other rotor machines, the Enigma machine is a combination of mechanical and electrical subsystems. The mechanical subsystem consists of a [[alphanumeric keyboard|keyboard]]; a set of rotating disks called ''rotors'' arranged adjacently along a [[axle|spindle]]; one of various stepping components to turn at least one rotor with each key press, and a series of lamps, one for each letter. These design features are the reason that the Enigma machine was originally referred to as the rotor-based cipher machine during its intellectual inception in 1915.<ref name="Enigma History">{{Cite web|title=Enigma History|url=https://www.cryptomuseum.com/crypto/enigma/hist.htm|access-date=16 December 2020|website=cryptomuseum.com}}</ref>

Each rotor can be set to one of 26 starting positions when placed in an Enigma machine. After insertion, a rotor can be turned to the correct position by hand, using the grooved finger-wheel which protrudes from the internal Enigma cover when closed. In order for the operator to know the rotor's position, each has an ''alphabet tyre'' (or letter ring) attached to the outside of the rotor disc, with 26 characters (typically letters); one of these is visible through the window for that slot in the cover, thus indicating the rotational position of the rotor. In early models, the alphabet ring was fixed to the rotor disc. A later improvement was the ability to adjust the alphabet ring relative to the rotor disc. The position of the ring was known as the ''Ringstellung'' ("ring setting"), and that setting was a part of the initial setup needed prior to an operating session. In modern terms it was a part of the [[initialisation vector]].

The current entry wheel (''Eintrittswalze'' in German), or entry [[Stator (electric machines)|stator]], connects the [[plugboard]] to the rotor assembly. If the plugboard is not present, the entry wheel instead connects the keyboard and lampboard to the rotor assembly. While the exact wiring used is of comparatively little importance to security, it proved an obstacle to Rejewski's progress during his study of the rotor wirings. The commercial Enigma connects the keys in the order of their sequence on a [[QWERTZ]] keyboard: ''Q''&rarr;''A'', ''W''&rarr;''B'', ''E''&rarr;''C'' and so on. The military Enigma connects them in straight alphabetical order: ''A''&rarr;''A'', ''B''&rarr;''B'', ''C''&rarr;''C'', and so on. It took inspired guesswork for Rejewski to penetrate the modification.

The plugboard (''Steckerbrett'' in German) permitted variable wiring that could be reconfigured by the operator. It was introduced on German Army versions in 1928,<ref>Craig P. Bauer: ''Secret History&nbsp;– The Story of Cryptology''. CRC Press, Boca Raton 2013, p.&nbsp;248. ISBN 978-1-4665-6186-1.</ref> and was soon adopted by the ''[[Reichsmarine]]'' (German Navy). The plugboard contributed more cryptographic strength than an extra rotor, as it had 150 trillion possible settings (see below).<ref name="158,962,555,217,826,360,000">{{Cite news|last1=Van Manen|first1=Dirk-Jan|last2=Johan O. A.|first2=Robertsson|date=2016|title=Codes and Ciphers|work=Geo ExPro|url=https://geoexpro.com/codes-and-ciphers-part-i/|access-date=13 October 2023}}</ref> Enigma without a plugboard (known as ''unsteckered Enigma'') could be solved relatively straightforwardly using hand methods; these techniques were generally defeated by the plugboard, driving Allied cryptanalysts to develop special machines to solve it.

Enigma was designed to be secure even if the rotor wiring was known to an opponent, although in practice considerable effort protected the wiring configuration. If the wiring is secret, the total number of possible configurations has been calculated to be around {{val|3e114}} (approximately 380 bits); with known wiring and other operational constraints, this is reduced to around {{val|e=23}} (76 bits).{{sfn|Miller|1995|pp=65–80}} Because of the large number of possibilities, users of Enigma were confident of its security; it was not then feasible for an adversary to even begin to try a [[brute-force attack]].

Most of the key was kept constant for a set time period, typically a day. A different initial rotor position was used for each message, a concept similar to an [[Initialization vector|initialisation vector]] in modern cryptography. The reason is that encrypting many messages with identical or near-identical settings (termed in cryptanalysis as being ''in [[Cryptanalysis#Depth|depth]]''), would enable an attack using a statistical procedure such as [[William F. Friedman|Friedman's]] [[Index of coincidence]].<ref>{{cite book|last=Friedman| first=W.F.|author-link=William F. Friedman|title=The index of coincidence and its applications in cryptology|series=Department of Ciphers. Publ 22|publisher=Riverbank Laboratories|location=Geneva, Illinois, USA|isbn=0-89412-137-5|oclc=55786052|year=1987|orig-year=1st pub. 1922|url=https://www.cryptomuseum.com/people/friedman/files/TIOC_Aegean_1987.pdf}}</ref> The starting position for the rotors was transmitted just before the ciphertext, usually after having been enciphered. The exact method used was termed the ''indicator procedure''. Design weakness and operator sloppiness in these indicator procedures were two of the main weaknesses that made cracking Enigma possible.

| [[File:Enigmas.jpg|upright=2.0|thumb|A selection of seven Enigma machines and paraphernalia exhibited at the US [[National Cryptologic Museum]]. From left to right, the models are: 1) Commercial Enigma; 2) Enigma T; 3) Enigma G; 4) Unidentified; 5) ''Luftwaffe'' (Air Force) Enigma; 6) ''Heer'' (Army) Enigma; 7) ''Kriegsmarine'' (Naval) Enigma&nbsp;— M4.]]

An estimated 40,000 Enigma machines were constructed.{{sfn|Bauer|2000|p=123}}<ref>[https://cryptocellar.org/enigma/e-history/enigma-reichswehr-wehrmacht-orders.pdf ''Reichswehr and Wehrmacht Enigma Orders''] {{Webarchive|url=https://web.archive.org/web/20210629133408/https://cryptocellar.org/enigma/e-history/enigma-reichswehr-wehrmacht-orders.pdf |date=29 June 2021 }} in Frode Weierud's CryptoCellar, accessed 29&nbsp;June 2021.</ref> After the end of World War II, the Allies sold captured Enigma machines, still widely considered secure, to developing countries.{{sfn|Bauer|2000|p=112}}

[[File:Enigma and Decoder (from above) at Discovery Park of America.jpg|alt=|left|thumb|Surviving three-rotor Enigma on display at [[Discovery Park of America]] in [[Union City, Tennessee|Union City, Tennessee, US]]]]

The ''[[Deutsches Museum]]'' in [[Munich]] has both the three- and four-rotor German military variants, as well as several civilian versions. The ''[[Deutsches Spionagemuseum]]'' in [[Berlin]] also showcases two military variants.<ref>{{Cite web |title=Enigma-Maschine: Die Entschlüsselung der Chiffriermaschine |url=https://www.deutsches-spionagemuseum.de/sammlung/enigma |access-date=3 February 2024 |website=Deutsches Spionagemuseum |language=de-DE}}</ref> Enigma machines are also exhibited at the [[National Codes Centre]] in [[Bletchley Park Museum|Bletchley Park]], the [[Government Communications Headquarters]], the [[Science Museum, London|Science Museum]] in [[London]], [[Discovery Park of America]] in Tennessee, the [[Polish Army Museum]] in Warsaw, the [[Swedish Army Museum]] (''Armémuseum'') in Stockholm, the Military Museum of [[A Coruña]] in Spain, the Nordland Red Cross War Memorial Museum in [[Narvik]],<ref>{{Cite web|url=http://www.warmuseum.no/no/English/|title=War Museum |work=Narviksenteret }}</ref> Norway, [[The Artillery, Engineers and Signals Museum]] in [[Hämeenlinna]], Finland<ref>{{Cite web|url=https://www.viestikiltojenliitto.fi/viestimuseo/_eng/index.html|title=The National Signals Museum|website=www.viestikiltojenliitto.fi}}</ref> the [[Technical University of Denmark]] in Lyngby, Denmark, in [[Skanderborg Bunkerne]] at Skanderborg, Denmark, and at the [[Australian War Memorial]] and in the foyer of the [[Australian Signals Directorate]], both in [[Canberra]], Australia. The Jozef Pilsudski Institute in London exhibited a rare [[Polish Enigma double]] assembled in France in 1940.<ref>{{Cite web|url=http://www.thenews.pl/1/10/Artykul/244703,Enigma-exhibition-in-London-pays-tribute-to-Poles |title=Enigma exhibition in London pays tribute to Poles|website=Polskie Radio dla Zagranicy|access-date=5 April 2016|archive-url= https://web.archive.org/web/20160423092753/http://thenews.pl/1/10/Artykul/244703,Enigma-exhibition-in-London-pays-tribute-to-Poles |archive-date=23 April 2016|url-status=dead}}</ref><ref>{{Cite web|url= http://pilsudski.org.uk/en/aktualnosci.php?news=205&wid=13&wai=&year=&back=%252Fen%252F |archive-url= https://web.archive.org/web/20160422230532/http://pilsudski.org.uk/en/aktualnosci.php?news=205&wid=13&wai=&year=&back=%252Fen%252F |url-status=dead|archive-date=22 April 2016|title=13 March 2016, 'Enigma Relay'– how Poles passed the baton to Brits in the run for WWII victory|website=J. Piłsudski Institute in London|access-date=5 April 2016}}</ref> In 2020, thanks to the support of the Ministry of Culture and National Heritage, it became the property of the Polish History Museum.<ref>{{Cite web|url=http://muzhp.pl/pl/c/1887/enigma-w-kolekcji-mhp|title=Enigma w kolekcji MHP - Muzeum Historii Polski|website=|access-date=11 November 2021|archive-date=11 November 2021|archive-url=https://web.archive.org/web/20211111122950/http://muzhp.pl/pl/c/1887/enigma-w-kolekcji-mhp|url-status=dead}}</ref>

[[File:Kriegsmarine Enigma.png|thumb|upright|A four-rotor ''[[Kriegsmarine]]'' (German Navy, 1. February 1942 to 1945) Enigma machine on display at the US National Cryptologic Museum]]

In the United States, Enigma machines can be seen at the [[Computer History Museum]] in [[Mountain View, California]], and at the [[National Security Agency]]'s [[National Cryptologic Museum]] in [[Fort Meade]], Maryland, where visitors can try their hand at enciphering and deciphering messages. Two machines that were acquired after the capture of {{Ship|German submarine|U-505||2}} during World War II are on display alongside the submarine at the [[Museum of Science and Industry (Chicago)|Museum of Science and Industry]] in [[Chicago]], Illinois. A three-rotor Enigma is on display at [[Discovery Park of America]] in [[Union City, Tennessee]]. A four-rotor device is on display in the ANZUS Corridor of the [[The Pentagon|Pentagon]] on the second floor, A ring, between corridors 8 and 9. This machine is on loan from Australia. The United States Air Force Academy in Colorado Springs has a machine on display in the Computer Science Department. There is also a machine located at [[The National WWII Museum]] in New Orleans. [[The International Museum of World War II]] near Boston has seven Enigma machines on display, including a U-boat four-rotor model, one of three surviving examples of an Enigma machine with a printer, one of fewer than ten surviving ten-rotor code machines, an example blown up by a retreating German Army unit, and two three-rotor Enigmas that visitors can operate to encode and decode messages. [[Mimms Museum of Technology and Art]] in [[Roswell, Georgia]] has a three-rotor model with two additional rotors. The machine is fully restored and CMoA has the original paperwork for the purchase on 7 March 1936 by the German Army. The [[National Museum of Computing]] also contains surviving Enigma machines in Bletchley, England.<ref>{{Cite web|title=The National Museum of Computing|url=https://www.tnmoc.org/ |access-date=16 December 2020|website=The National Museum of Computing|language=en-GB}}</ref> [[Carnegie Mellon University]] Libraries also has two enigma machines, stored within its Special Collections.<ref>{{Cite web |title=Enigma Machines at CMU {{!}} CMU Libraries |url=https://www.library.cmu.edu/about/news/2021-09/enigma-machines-cmu |access-date=1 August 2025 |website=www.library.cmu.edu}}</ref>  These models are a three-rotor A5005 and a four-rotor M16681.<ref>{{Cite web |date= |title=Inside the Enigma Machine |website=Carnegie Mellon University |url=http://www.cmu.edu/news/stories/archives/2019/october/inside-the-engima-machine.html |access-date=1 August 2025 |language=en}}</ref>

In Canada, a Swiss Army issue Enigma-K, is in Calgary, Alberta. It is on permanent display at the Naval Museum of Alberta inside the Military Museums of Calgary. A four-rotor Enigma machine is on display at the [[Military Communications and Electronics Museum]] at [[Canadian Forces Base]] (CFB) [[CFB Kingston|Kingston]] in [[Kingston, Ontario]].

On 3 December 2020, German divers working on behalf of the [[World Wide Fund for Nature]] discovered a destroyed Enigma machine in [[Flensburg Firth]] (part of the [[Baltic Sea]]) which is believed to be from a scuttled U-boat.<ref>{{Cite news|date=3 December 2020|title=Divers discover Nazi WW2 enigma machine in Baltic Sea|work=[[Reuters]]|url=https://www.reuters.com/article/us-germany-war-enigma-idUSKBN28D25F |url-status=dead|access-date=20 December 2025|archive-url= https://web.archive.org/web/20201203171005/https://www.reuters.com/article/us-germany-war-enigma-idUSKBN28D25F |archive-date=3 December 2020}}</ref> This Enigma machine will be restored by and be the property of the Archaeology Museum of [[Schleswig Holstein]].<ref>{{cite web |last1=Welle (www.dw.com) |first1=Deutsche |title=German divers hand over Enigma encryption machine in Baltic {{!}} DW {{!}} 4 December 2020 |url=https://www.dw.com/en/german-divers-hand-over-enigma-encryption-machine-in-baltic/a-55829171 |website=DW.COM}}</ref>

* {{Cite web |last1=Huttenhain |first1=Orr |last2=Fricke |title=OKW/Chi Cryptanalytic Research on Enigma, Hagelin and Cipher Teleprinter Messages |date=1 August 1945 |url=https://archive.org/details/ticom/I-45OkwchiCryptanalyticResearchOnEnigmaHagelinAndCipherTeleprinterMachines/|website=Internet Archive|publisher=TICOM}}

* {{Cite book |first=Richard James |last=Aldrich |title=GCHQ: The Uncensored Story of Britain's Most Secret Intelligence Agency |url={{google books |plainurl=y |id=4I2PmCtrHOgC}} |year=2010 |publisher=HarperPress |isbn=978-0-00-727847-3}}

* {{Cite book |first=Gustave |last=Bertrand |title=Enigma: ou, La plus grande énigme de la guerre 1939–1945 |url={{google books |plainurl=y |id=o2UNAAAAIAAJ}} |year=1973 |publisher=Plon}}

* {{Cite book |first=Peter |last=Calvocoressi |author-link=Peter Calvocoressi |title=Top Secret Ultra |url={{google books |plainurl=y |id=qxiHPwAACAAJ |page=98}} |pages=98–103 |year=2001 |publisher=M & M Baldwin |isbn=978-0-947712-41-9}}

* {{Cite web |last=Grime |first=James |title=The Enigma Flaw |url=http://www.numberphile.com/videos/enigma_flaw.html |work=Numberphile |publisher=[[Brady Haran]] |access-date=7 April 2013 |archive-url=https://web.archive.org/web/20130330065120/http://www.numberphile.com/videos/enigma_flaw.html |archive-date=30 March 2013 |url-status=dead}}

* {{Cite book |first=John |last=Herivel |author-link=John Herivel |title=Herivelismus: And the German Military Enigma |url={{google books |plainurl=y |id=voM0QwAACAAJ}} |year=2008 |publisher=M & M Baldwin}}

* {{Cite book |first=John |last=Keen |title=Harold 'Doc' Keen and the Bletchley Park Bombe |url={{google books |plainurl=y |id=tfq7MQEACAAJ}} |date=1 August 2012 |publisher=M & M Baldwin |isbn=978-0-947712-48-8}}

* {{Cite book |first=Christine |last=Large |title=Hijacking Enigma: The Insider's Tale |url={{google books |plainurl=y |id=jAkiAQAAIAAJ}} |date=6 October 2003 |publisher=Wiley |isbn=978-0-470-86346-6}}

* {{Cite book |last=Turing |first=Dermot |author1-link=Dermot Turing |title=[[X, Y & Z: The Real Story of How Enigma Was Broken]] |publisher=History Press |location=Gloucestershire England |year=2018 |isbn=978-0-7509-8782-0 |oclc=1029570490}}

* [https://web.archive.org/web/20250104092215/https://www.cgisecurity.com/2008/04/getting-to-see-an-enigma-machine-at-rsa-2008-.html Enigma Pictures and Demonstration by NSA Employee at RSA] Archived 4 January 2025 at the [[Wayback Machine]]