The German Enigma Cipher Machine
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The story of the famous Enigma cipher machine combines ingenious technology, military history and the mysterious world of espionage, codebreakers and intelligence into a real thriller. Never before has the fate of so many lives been so influenced by one cryptographic machine, as in the Second World War. Enigma is the most famous and appealing example of the battle between codemakers and codebreakers. Enigma showed the importance of cryptography to military and civil intelligence.

The German Wehrmacht Enigma
© D. Rijmenants

Origins of the Enigma

The need for secure communications for both military as civilian use became obvious in the early 1900s, with the rise of wireless communication. The search to replace the impractical and time-consuming hand ciphers began. In 1917, the American Edward Hugh Hebern developed a cipher machine with rotating disks, each disk performing a substitution cipher. Hebern's idea was the base for many similar machines, developed in several other countries.

In 1918, Engineer Arthur Scherbius patented a cipher machine using rotors. The German Navy and Foreign Offices were approached, but were not interested. In 1923, the rights for the patents went to Chiffriermaschinen-AG, a firm with Scherbius on the board of directors, that commercialized the machine. In 1927, Scherbius bought the 1919 patent from of a similar machine from the Dutchman Koch, in order to secure his own patent, approved in 1925.

The first cipher machine, Enigma A, came onto the market in 1923. It was a large and heavy machine with an integrated typewriter and weighed about 50 Kg. Soon after the Enigma B was introduced, a very similar machine. The weight and size of these machines made them unattractive for military use. The development of the reflector, an idea of Scherbius' colleague Willi Korn, made it possible to design the compact and much lighter Enigma C. Also, the type writer part was replaced by a lamp panel. In 1927, the Enigma D was introduced and commercialized in several versions with different rotor wirings, and sold across Europe to military and diplomatic services. The Enigma D had three normal rotors and one reflector that could be set in one of the 26 positions.

Several intelligence services succeeded in breaking the civil and military Enigma versions which were all based on the commercial D. The Enigma D had no plugboard, a military feature that would increase security considerably from 1935 onwards. The Italian Navy bought the commercial Enigma D, as did Spain during the Spanish Civil War. The Swiss army used the Enigma K, a slightly modified version of the Enigma D. Japan used the Enigma T, also called Tirpiz Enigma, an adapted Enigma D with modified entry rotor connections. Japan also developed their own version of the T, with horizontally placed rotors. The messages of both models T and K were broken as well. The Railway Enigma, another D clone which was used by the German Reichsbahn in Eastern Europe, was partially broken from 1941 onwards.

Military versions Top

In 1926, the commercial Enigma was purchased by the German Navy and adapted for military use. They called it Funkschlüssel C. Meanwhile, Chiffriermaschinen-AG developed a special Enigma with rotors that have the same contact alignment as the D rotors, but with teeth, multiple notches and are advanced by cog wheels instead of pawls and ratchets (see patent drawing left). It also had a rotating reflector and a counter on its left. Today, only one such machine is known. This probably experimental model, presented in 1928 but exceptionally only patented in 1931, lead to the Enigma G.

The Enigma G had different rotors with a zigzag pin placement and the counter on its right. Its rotors, which also had multiple notches, were moved by a system of gears, similar to the 1928 special predecessor. Already in 1928, the German Abwehr (Secret Service) bought the 12 Kg light Enigma G, also called Zahlwerk (clock-work) Enigma due to it's counter on the front panel. The Enigma G was exclusively used by the Abwehr.

In 1932, the Wehrmacht revised the commercial Enigma D and added the plugboard at the front of the machine. This version, the Enigma I, became known as the Wehrmacht Enigma and was introduced on a large scale in the Heer (Army) and public authorities. The Luftwaffe (Air Force) followed the Heer's lead in 1935. The Wehrmacht Enigma came initially with three rotors. From 1939 on they were equipped with five rotors.

In 1934, the German Navy adopted the Wehrmacht model, with its securer plugboard, and extended the set of rotors to eight. The Navy machine was called Funkschlüssel M or M3. In 1941, although reassured by the Abwehr that the Enigma M3 was unbreakable, Admiral Karl Dönitz insisted on improvement of the Kriegsmarine Enigma. Early in 1942, the famous four rotor M4 model was introduced in the Kriegsmarine.

During the war, different types of reflectors were introduced. The B and C reflector were used on Heer and Luftwaffe models, and also on the Kriegsmarine M3. The Kriegsmarine M4 used a thin B and C version, to fit in the 4 rotor machine, with other wirings, but if 'zeroized' in combination with its fourth rotor compatible with the Heer and Luftwaffe version. By the End of the war German Command tried to introduce a new type D rewireable reflector.

Early use of this reflector posed a significant problem to Allied codebreakers, but problems in distribution of this reflector and their key sheets prevented a widespread use of the D reflector. Another military add-on, introduced in 1944 by the Luftwaffe, was an extra plugboard switch, called the Uhr (clock), a switch with 40 positions, each position resulting in a different combination of plug wiring. For more information about Enigma codebooks and the message procedures, please visit the enigma procedures page. The technical details page explains how the Enigma works and shows the inside of the machine.

An estimated total of 100,000 Enigma machines were produced. Although generally known as Enigma, there were only a few machines that actually carried the name Enigma and the logo. Most machines only had a serial number and fabrication code. The machines were produced in different factories on various locations such as Ertel-Werk für Feinmechanik in München, Olympia Büromaschinenwerke in Erfurt, Chiffriermaschinengesellschaft Heimsoeth & Rinke in Berlin, Atlas-Werke Maschinenfabrik in Bremen and Konski & Krüger in Berlin. The machines that survived the war were confiscated by the Allies and mostly sold to other countries. The rotors of these machines were often rewired. Of course, they forgot to mention that they were able to break them.

Rare predecessor of the Enigma G
© D. Rijmenants

Extracted Wehrmacht rotors on their shaft
© D. Rijmenants

The Enigma M4 with open cover
© D. Rijmenants

Breaking the Code

When the Wehrmacht introduced the plugboard on the military Enigma, this added an astronomical number of possible key settings. The general idea was that this military Enigma, unlike the commercial types, would be impossible to break. No one even tried to break it. However, in 1932, Poland's Biuro Szyfrow (Cipher Bureau) initiated attempts to analyze and break the Enigma messages. Although the chief of this Bureau received copies of key sheets, sold by the German spy Hans-Thilo Schmidt, he did not give them to his codebreakers. He thought that keeping this information from them might stimulate their efforts.

Marian Rejewski, Henryk Zygalski and Jerzy Rozicki were convinced that mathematics could solve the problem and succeeded in breaking the Enigma messages. They also developed an electro-mechanical machine, called the Bomba, to speed up cryptanalysis. Two major security flaws in the German Enigma procedures were the global ground setting and the twice encrypted message-key, a procedure to exclude errors.

These flaws opened the door to cryptanalysis. In 1939 the Bureau was no longer able to break the keys due to increased sophistication in the design, new procedures and lack of funds for the codebreakers. When Germany invaded Poland, the Polish Biuro Szyfrow passed its secret knowledge and several replica Enigma machines to the baffled French and British intelligence. The work of the Biuro Szyfrow was vital, not only because their pioneering work itself, but also because it convinced other cipher bureaus that it was possible to break Enigma.

Bletchley Park Top

The Government Code and Cipher School (GC&CS) at Bletchley Park initially broke Enigma by hand. In August 1940 they started using their own Bombes, designed by Alan Turing and Gordon Welchman. It was also a rotary electro-mechanical device but it worked on an entirely different principle as Jerewski's Bomba. The Turing Bombe searched for the enigma settings for a given piece of plain and cipher text.

When an Enigma message was intercepted, codebreakers had to search for so-called cribs. These cribs were presumed pieces of plain text within the encrypted message. This could be "An Der Oberbefehlshaber", "An Gruppe", "Es Lebe Den Fuhrer" or any other standardized code from a codebook or piece of text.

Once a crib was located (special techniques existed to do this) the associations between the letters of the ciphertext and their plain version were entered in the Bombe. The Bombe, which contains a large number of drums, each replicating the rotors of the Enigma, ran through all possible settings to find the key settings that belong to the given pieces of cipher and plain text. Once these settings were found all messages, encrypted with these setting, could be deciphered.

All information retrieved by cryptanalysis, the breaking of codes and ciphers, was codenamed ULTRA and played a very important and often decisive role during the war, mainly in the Battle of the Atlantic. The ULTRA information was used very carefully, so as to avoid suspicion among the German forces. Special liaison officers, trained to deal with this valuable but delicate knowledge, were placed in Headquarters and other strategic places. Moreover, ULTRA was never used unless it could be confirmed by a second source in order to avoid giving the German Command reason to suspect that their communications security might be broken.

The Kriegsmarine Top

The German Kriegsmarine was very successful in applying their Rudeltaktik or "Wolfpack Tactics" with U-boats. They hunted individually for convoys. If a convoy was spotted, they shadowed it and called other U-boats into battle. Once all U-boats were on the spot, they sank the convoy with a closely coordinated attack. This technique was so devastating to the allied supplies that it almost decided the outcome of the war. Communication was the keyword and the U-boats used Enigma to send messages to co-ordinate their attacks. After some initial hard times, Bletchley Park broke the naval ciphers almost continuously.

Decreasing effectiveness of his U-boats made Admiral Donitz suspicious and, although reassured by German intelligence that Enigma was secure, he insisted on improving the Enigma's security. Early in 1942 the famous 4-wheel machine was introduced in the Kriegmarine and the complicated TRITON key and its procedure (called SHARK by the codebreakers) caused a big crisis at Bletchley Park. The Kriegmarine referred to the spring of 1942 as the "Happy Times" because the Allied forces were unable to decipher the messages and the U-boats were able to continue sinking ships without much interference. More information is found on Enigma and the U-boat War.

Turning the tide

The codebreakers in Bletchley Park discovered by cryptanalysis that a fourth rotor had entered the battlefield. After ten nerve-wracking months of heavy losses, Bletchley Park succeeded in breaking the TRITON keys. The major reason for this success was the capture of Kurzsignal codebooks by British Navy on German weather ships and the attacks on U-boats like Kapitanleutenant Heidtmann’s U-559 by HMS Petard. These boarding were not to steal Enigma machines or key sheets, as often wrongly portrayed in movies and books (they already had replicas of the Enigma from the Biuro Szyfrow). Enigma key sheets only gave access to a particular radio net and area for a single month.

However, only two editions of the Kurzsignal codebook, issued to all U-boats, were ever printed during the war. These codebooks encoded weather and operational reports in four-letter codes, prior to encryption with Enigma. By seizing them, Bletchley Park could use these four-letter codes as new cribs to attack all future Enigma setting. Moreover, new Bombes were developed to deal with the four-rotor Enigma, and by the end of 1943, another fifty of these Bombes became operational in the US Navy. More on the codebooks can be found on the enigma procedures and Kurzsignalen pages.

The tide of the U-boat war had turned. Except for some brief periods, the entire communication system was intercepted by a large number of listening stations, and the message were broken in Bletchley Park, which employed over 7000 workers at its peak. With the positions of the U-boats unveiled, Allied ships could now evade the U-boats and the Allies actively hunted for U-boats. The elite weapon of the Kriegsmarine got decimated, with heavy losses among the U-boat crews. An estimated 700 U-boats and 30,000 crewmen were lost at sea. U-boat command never suspected cryptanalysis of the Enigma and related these losses to new Allied submarine detection techniques like ASDIC sonar, surface radar, HF direction finding and anti-submarine airplanes.

All improvements, introduced by the German Forces, were tackled successfully by the codebreakers. The introduction of the rewireable D reflector, with its key changes every ten days, proved to be a big problem to the codebreakers. A widespread use of the D reflector would require five to ten days to break a particular key, which would render tactical information useless. Without the D reflector, keys were broken mostly within 24 hours. Fortunately, logistical problems prevented general use of the D reflector in the German forces.

German operators were also reluctant to use the D reflector and found it too elaborate to program in tactical situations. Instead, the B reflector remained the default reflector and the D reflector was used only for important messages, on the same machines with the same basic machine settings for rotors and plugboard. However, with the key already broken for these machines with the B reflector, the codebreaker only had to retrieve the unknown wiring of the D reflector, used on the same machines. A work that was performed by hand.

The fatal mixed use of B and D reflectors enabled the codebreakers to continue reading the once feared D reflector messages. The Enigma Uhr (clock), used by the Luftwaffe, was another useless effort by the Germans to increase the security of the Enigma. The Uhr was a switch that replaced the plugs of the Enigma and provided 40 different plug wirings. However, the unique design of the Allied Bombes, used to retrieve the key settings of the Enigma, excluded the plugboard wiring. The Enigma Uhr therefore had little or no effect on the codebreaking results.

Kriegsmarine Enigma M4
© D. Rijmenants

Enimga M4 thin reflector (left) and special
Beta rotor with spring-loaded pins on both
© D. Rijmenants

Kriegsmarine M4 box to store the five unused normal rotors and one Beta or
Gamma rotor.
© D. Rijmenants

Box to store the two unused rotors of a set
of five rotors.
© D. Rijmenants

The ULTRA information was kept highly secret during the entire war and played a decisive role. Breaking the Heer and Luftwaffe messages also provided crucial tactical information. The codebreakers exposed the weakness of Field Marshal Rommel's notorious Afrika Korps. The speed and success of the Afrika Korps created long stretches of poorly defended supply lines. ULTRA information revealed their logistical problems and provided Field Marshal Montgomery with a vital tactical advantage.

In the days before the D-day invasion of Normandy, the Wehrmacht, without realizing it, provided the Allies with an enormous quantity of detailed information on the coastal defenses, location and strength of all German tank divisions and the movement of troops in France. Experts estimate that the breaking of Enigma shortened the war by about three years. The number of saved lives is innumerable. The large scale breaking of German communications was one of the best kept secret of the Second World War. German armed forces kept on using Enigma during the entire war without any suspicion

Enigma Uhr
© D. Rijmenants

Enigma plugboard
© D. Rijmenants

Left view of an Enigma rotor
© D. Rijmenants

Theoretical versus Practical Security

How secure was the Enigma machine actually and why ended it up being the Achilles heel of the superior German war machine? During a top secret Allied operation in the final days of the war, special TICOM teams round up German cryptologists and Signals Intelligence personnel. The answer to our question is found in their only recently declassified TICOM reports, vol 2, “Notes on German High Level Cryptography and Cryptanalysis” (see NSA's Axis SIGINT). More about the mathematical security of the Enigma is found in the Technical Details page.

Summarized, it comes to this: to create a secure crypto device you need both excellent codemakers and codebreaker. You cannot effectively assess the security of a crypto machine unless you test it by trying to break it. According to TICOM, Germany had very capable cryptologists and developed some excellent crypto machines. Unfortunately, their codebreaking skills, although excellent, were not on par with their brilliant Polish, British and American counterparts.

It was this little difference in codebreaking skills that convinced the Germans that Enigma was secure. Their studies only revealed theoretical weaknesses. It was the same little difference in skills that enabled the Allies to find a practical solution to the theoretical weaknesses of the Enigma machine. German cryptologists did continue to develop various improvements to Enigma and other crypto machines during the war, some of which, according to TICOM reports, would prove impossible to break by the Allies at that time. Fortunately, as the war progressed, logistical problems, shortage of raw materials and lack of time and money kept these new machines from entering service.

The heritage of Enigma

After the Second World War, Enigma was the basis for many more sophisticated rotor cipher machines like the Swiss NEMA, the American KL-7 ADONIS and the until recently top secret Russian M-125 FIALKA. Although Enigma was very well designed and offered, for those days, an unbreakable security, the negligent use in the German Armed Forces and the compromised codebook material enabled the codebreakers to turn the best kept secret of the war into a Trojan horse and give the kick-off for cryptographic intelligence. Today, Signal Intelligence is considered to be a most vital part of the modern battle.

All images on this page copyright D Rijmenants.

References and Links Top

Further Information Top

More related to Enigma (offsite - opens in new tab) Top

  • Paul Reuvers' and Marc Simons' Paul and Marc host the Crypto Museum website with an extensive Enigma section that contains detailed information and many beautiful images of many different Enigma machines.
  • Tom Perera's Enigma museum Tom is a collector of Enigmas and other cipher machines. On his site, you can find a huge number of very detailed photographs. He's also the first to publish all details of the famous Russian Fialka M-125, top secret until recently.
  • Tom's Enigma In Action video Tom created a very nice video that shows how to set the daily key on the Enigma, change the rotors, ring settings and plugboard connections, and how a message is enciphered. Great way to see the Enigma in action.
  • Frode Weierud's Crypto Pages As a member of the Crypto Simulation Group, Frode has composed a large list of sims. Several Enigma models, PURPLE, the SIGBA, Hagelin CD57 and others. The site contains also a lot of historical information and documents. CSG is decoding a large number of original WW2 German messages, some of which are already published on Frode's site.
  • Breaking German Navy Cyphers Michael Hoerenberg's website about breaking authentic Enigma M4 messages, retrieved from salvaged U-534
  • Chiffriermaschine Patrick Hayes' pages about the Enigma machine with many detailed photos of its parts
  • Erik Vestergaard's Enigma page A clear as beautiful illustrated page on the breaking of the Enigma machine
  • Tony Sale's Codes & Ciphers Tony is walking computer history. One of the people who helped saving Bletchley Park. He even rebuilt the famous Colossus computer in the Park's museum.
  • Lech Maziakowski's Enigma History The importance of the Polish effort on breaking the Enigma.
  • Bletchley Park Official site of the Bletchley Park Thrust. In the Second World War, this was the center of all British code breaking efforts.
  • Enigma on the German U-Boats A great site with a huge archive on the German U-boats. There is a very interesting page on the use of Enigma and the efforts on breaking the U-boat Enigma codes
  • Enigma and the Bombe The story of breaking enigma and the use of Bombes by Graham Ellsbury.
  • Codebreaking and secret weapons in WWII Very complete historical information on the breaking of the Enigma messages.
  • NSA's National Cryptologic Museum NSA museum pages with lots of information on intelligence, codebreaking and Enigma
  • Enigma patents A list of original patens of various Enigma machines at the Foundation for German communication and related technologies website
  • 2012 ALAN TURING YEAR A centenary celebration of the life and work of Alan Turing, the famous British codebreaker
  • The TICOM report on Enigma Volume 2, "Notes on German High Level Cryptography and Cryptanalysis".
  • European Axis Signal Intelligence in World War II.NSA's declassified TICOM reports.
  • Enigma World Code Group Bruce Culp's excellent website, designed to join Enigma enthusiasts and globally exchange Enigma encrypted messages. Provides clear and simple instructions on how to encrypt and decrypt messages. codebooks provided.
  • OPERATION TURING Mitchel Thomas and Indiana Popovich made a website where you can decrypt enigma messages (keys provided) to unveil the story of a fascinating 1938 intelligence operation in a shadowy pre-war atmosphere. More info about how and why on my weblog
  • The Enigma Machine An interactive exploration of the encryption powers of the Enigma Machine by engineering students at Olin College.

Enigma Replicas and Simulations (offsite - opens in new tab) Top

The Enigma Simulator

This software is an exact simulation of the 3-rotor Wehrmacht and Luftwaffe Enigma, the 3-rotor Kriegsmarine M3, also called Funkschlussel M, and the famous 4-rotor Kriegmarine M4 Enigma cipher machine, used during World War II from 1939 until 1945. You can select between the three models, actually choose different rotors or 'Walzen', preset the rotor wiring positions or 'Ringstellung' and switch letters by using plugs or 'Stecker'. The internal wiring of all rotors is identical to those used by the Wehrmacht, Luftwaffe and Kriegsmarine. This simulator is therefore fully compatible with the real Enigma-machine and you can decrypt original messages and make your own encrypted text. Runs under Windows.

Download Enigma Simulator

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© Dirk Rijmenants 2004. Last changes: 20 November 2022

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