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History of the Enigma
The rotor-based cipher machines

Invention of the Rotor Machine (1915)
The history of the Enigma starts around 1915, with the invention of the rotor-based cipher machine. As usual in history, the rotor machine was invented more or less simultaneously in different parts of the world. In 1917 there were inventions from Edward Hebern in the US, Arvid Damm in Sweden, Hugo Koch in The Netherlands and Arthur Scherbius in Germany [1][2].

The inventors of the rotor machine, from left to right: Theo van Hengel, Edward Hebern and Arthur Scherbius.

There is one development however, that pre-dates the others, and that is the invention of Theo A van Hengel (1875-1939) and RPC Spengler (1875-1955), two Dutch naval officers who produced working rotor-based cipher machines for the Dutch War Ministry (Ministerie van Oorlog) in 1915. This novum has been described in detail by Karl de Leeuw in Cryptologia in January 2003 [2].

Early prototypes (1917)
The actual Enigma cipher machine 1 though, was invented and built towards the end of WWI, in 1917, by Arthur Scherbius and patented on 23 February 1918. This so-called Probemaschine, 2 was a prototype that eventually evolved into the well-known series of glow lamp cipher machines. Scherbius later acquired patent NL10700 of the Dutch inventor Hugo Koch for a similar 3 device.

 First Enigma prototypes

  1. The name 'Enigma' was not yet used at that point.
  2. Literally translated: test machine or prototype machine.
  3. Apart from electricity, this patent describes the use of air, oil, mechanics and light as the information transport medium. It was important for the development of the Štolba cipher machine in the mid-1930s.

Printing Enigma (1923)
In 1919, Scherbius started the development of a rather large typewriter-style cipher machine, which became known as Die Handelsmaschine. It was marketed from 1923 by Scherbius & Ritter of Berlin-Wannsee (Germany), and built by the company Gewerkschaft Securitas, also of Berlin.

Like a regular typewriter, the machine prints its output directly onto a sheet of paper. It was first described in a technical article by Scherbius him­self in 1923 [5]. As far as we know, none of the early Handelsmaschinen have survived.

 More about Die Handelsmaschine 

A year later, in 1924, developement of another printing Enigma machine was started, probaby at the request of the German Reichswehr. Known as Die schreibende Enigma, it was never very popu­lar as it faced many mechanical problems.

 Die schreibende Enigma


Glow lamp Enigma (1924)
There were a lot of problems with the printing Enigma machines. The first ones had reliability issues with the print wheel, and the later model with the type bars. As a result, the schreibende Enigma would not be a suitable and reliable solution for the German Army (Reichswehr).

For this reason, Scherbius took his early glow-lamp-based based Probemaschine and improved it. The first new model was Enigma A, and was introduced in 1924. It also became known as Glühlampenmaschine (glow lamp machine).

The machine was available for about 1/8th of the price of the printing Enigma and costed RM 1000 1 . It was housed in a wooden transit case and resembles later Enigma models, except for the fact that the keys and lamps are arranged in sequential order (ABCDE...) rather than the common typewriter order (QWERTZ...).

Enigma A was succeeded by Enigma B (1924-1925) – of which various variants were made – and eventually by Enigma C (1925). The latter has 26 keys (A-Z) for the input, and 26 lamps (A-Z) for the output. The text is scrambled by means of three cipher rotors that protrude the top lid. Each cipher rotor has 26 contacts at either side. Several variants of Enigma C were produced, such as the so-called Funkschlüssel C (for the German Navy) and a Swedisch variant, both with 28 keys.

 More about Enigma C

  1. In 1924, the currency in Germany was the Reichsmark (RM).

Commercial Enigma (1926)
Unlike the printing Enigma, the glowlamp machines had a reflector (UKW) that made the machine reciprocal (symmetric). As a result, the settings of the machine for encoding and decoding were identical, which greatly improved its usability. The UKW had two or four fixed positions. The idea for the reflector came from Scherbius' colleague Willy Korn, who would later lead the company.

In 1926, the design of the glow lamp Enigma was drastically improved. A new chassis was developed and the standard (German) keyboard layout (QWERTZ...) was introduced. Furthermore the reflector (UKW) could be set to 26 different positions. It was mounted to the left of the three cipher rotors, which is why this machine is sometimes thought to be a 4-rotor Enigma.

The machine was known as Enigma D or Enigma Model A26. Like its predecessor – Enigma C – it was housed in a wooden transit case with a hinged lid, but had several improvements.

The rotors could be accessed more easily (i.e. the top lid could be opened), there was an optional sunlight filter for the lamp panel, and it had a power selector that was mounted to the right of the cipher rotors. The Enigma D became the basis for most of the later Enigma designs. Enigma D evolved into a line of commercial machines known as Enigma K, with a range of variants.

 More about Enigma D
 More about Enigma K

Zählwerk Enigma (1928)
In 1927, a series of new developments were started, all based on the chassis of the Enigma D. First of all there was the Commercial Enigma, that later became known as the Enigma K. There were several variants of this machine, such as the Swiss K that was built for the Swiss Army.

All commercial Enigma machines had a simple rotor turnover mechanism that is comparable to the odometer of a car. The rightmost rotor makes a single step on each key press. After the rightmost rotor has completed a full revolution, the middle rotor makes a single step and so on.

At the same time (1927) the development of a more advanced range of machines was started. This range became known as Zählwerk Enigma (counter Enigma) or Zählwerksmaschine (counter machine), probably because it has a counter that shows the length of a message (key presses).

Furthermore, the Zählwerk Enigma has a far more advanced rotor turnover mechanism that is driven by cogwheels rather than by pawls and levers. This allows the mechanism to be wound back as well, which is useful for correcting mistakes. The Zählwerk Enigma also introduces the concept of multiple turnover notches, which causes more frequent (irregular) rotor stepping.

The three cipher rotors have 11, 15 and 17 turnover notches respectively, each of which are relative primes of 26, which increases the period of the machine (i.e. the number of steps before the sequence is repeated). Furthermore, the reflector (UKW) is now part of the stepping mechanism and is driven by the other rotors.

The first Zählwerk Enigma was Model A28. It was introduced in 1928 and was built on the chassis of Enigma D or Enigma K. A few years later, a design variant of the A28 was developed. It was slightly smaller and had smaller cipher rotors.

Although the number and position of the turnover notches is identical (11, 15 and 17), the rotor diameter is smaller and the cogwheel driven mechanism is slightly simplified. Introduced in 1931 and designated Model G31, this machine is sometimes referred to as the Abwehr Enigma.

 More about Zählwerk Enigma A28
 More about Enigma G31

Military Enigma (1930)
Probably from the beginning of Scherbius' developments in 1917, the German Army (Reichswehr, from 1935 known as Wehrmacht), had opted for a machine that could be used in the field and that was secure enough to keep messages secret for longer periods of time. Scherbius knew that the best possible security would be obtained by increasing the number of rotors, but came to the conclusion that this was not a viable solution due to mechanical and practical contraints.

In 1926, development was started of a machine for the Rechswehr known as Enigma I. Based on the chassis of Enigma D/K, it had 3 rotors and a fixed UKW. Furthermore, it had a unique feature, known as the Schaltbrett (switch board), that significantly increased the strength of the cipher.

The first version of the Enigma I was ready in 1927 and featured a single-ended Schaltbrett in a 2 x 13 circular arrangement. After several redesigns of the Schaltbrett – by now known as Steckerbrett (plug board) – they settled in 1928 for a much simplified double-ended plugboard.

It had been designed by the Reichwehr themselves, and is by far the weakest variant of all tested plugboards. The final version of Enigma I, with the double-ended plugboard, was put into service on 1 June 1930. The plugboard was used exclusively on military machines, and was not sold to commercial customers. In 1932 the German Army claimed the exclusive rights to the machine, after which all commercial and international sales had to be approved by the German Army.

Meanwhile, in 1929, manufacturer ChiMaAG had successfully produced a new and improved version of the ill-fated schreibende Enigma. It was known as Enigma H or Model H29 and was designated Enigma II by the Reichswehr. It was not compatible with the glow lamp Enigma I.

The Enigma H29 was also sold to the Hungarian Army, but was never in popular demand due to its high price. Apart from the Hungarian Army, the Germans also kept selling Enigma machines to the Swiss and to the Dutch Navy. It is known that the Dutch bought Enigma G as late as 1938.

In the mid-1930s the German Army started preparing for war and began ordering Enigma I in large quantities for the Reichswehr (Army) and the Luftwaffe (Air Force). Enigma G was used by the German intelligence service, the Abwehr. For the Kriegsmarine (German Navy) a model similar to and compatible with the Enigma I was developed. It become known as the Enigma M1 (1934), later followed by the Enigma M2 (1938), Enigma M3 (1940) and finally Enigma M4 (1942).

 More about Enigma I

Arthur Scherbius
The first Enigma machines were developed by Arthur Scherbius between 1917 and 1920, but were built after the company had been dissolved into Gewerkschaft Securitas and a few years later into Chiffriermaschinen AG. After Scherbius' untimely death in 1929, the company changed hands, and in 1935, after the German Army had acquired the manufacturing rights to the Enigma machine, the company's assets were taken over by the newly established Heimsoeth und Rinke.

In the history of Scherbius and Enigma, the following events are of importance [11]:

  • 1911
    Dipl.-Ing. E. Richard Ritter & Co (household appliances)
  • 1915
    Scherbius member of the telegraph troops
  • 1917
    War Ministry order for development of a cipher machine
  • 1918
    Arthur Scherbius, 1st patent (prototype)
  • 1920
    Scherbius & Ritter
  • 1921
    Gewerkschaft Securitas
  • 1922
    Securitas GmbH (workshop, Berlin, Germany)
  • 1922
    N.V. Ingenieursbureau Securitas (Amsterdam, Netherlands)
  • 1923
    Chiffriermaschinen Aktiengesellschaft
  • 1929
    Scherbius dies in accident
  • 1935
    Heimsoeth und Rinke
From 21 November 1921 to 1925, Scherbius was not in the management of the above listed companies and did not own the majority of shares, except for his part in Scherbius & Ritter [11].

 Enigma-related patents

Other Enigma manufacturers
As many Enigma machines were needed for the German war effort, other companies were contracted to build the machines under licence. This also reduced the risk of supply problems should any of the manufacturers be bombed by the Allies. With Heimsoeth & Rinke in Berlin being the engineering company, the machines were manufactured by Konski & Krüger in Berlin. Later, the military machines were also manufactured by Olympia in Erfurt, Ertel-Werk in München, and Atlas-Werke in Bremen, all under licence of Heimsoeth und Rinke.

 List of manufacturers

The Polish Breakthrough (1933)
Around 1930, the Polish Cipher Bureau, Biuro Szyfrów, was the first to make an attempt to break the Enigma cipher. As one of the closest neightbours of Germany, they are very much aware of the clear and present danger of another war. They started researching the commercial Enigma.

Marian Rejewski
Jerzy Rózycki
Henryk Zygalski

From the University of Poznan, three young brilliant mathematicians were recruited: Marian Rejwski, Jerzy Rózycki and Henryk Zygalski. They started working on the Enigma cipher with nothing more that a handfull of intercepted messages and a description of a commercial Enigma.

Rejewski was set to work on the problem in late 1932 and after a few weeks he achieved his first breakthrough, when he deduced the secret internal wiring of the Enigma. Together with his colleagues he started developing various aids for the regular decryption of German Enigma traffic.

Zygalski developed the so-called Zygalski sheets that were used to exploit the double-enciphered message indicator 1 , a weakness in the German procedures. Later a machine was developed to exploit this weakness mechanically: the Bomba kryptologiczna (the cryptologic bomb).

With only three rotors available for the Enigma I, there are 6 possible rotor orders. On the Bomba, six sets of Enigma rotors are driven simultaneously by a cog wheel at the center. Around 100 intercepted messages were needed to recover the rotor order and initial settings.

 More about the Bomba

  1. The double-enciphered message indicators — that had enabled the Poles to break a significant part of the Enigma traffic — dates back to the late 1920s. It was part of a proposal on how to use the commercial Enigma. In early 1940, newly hired mathematicians at OKW/ln7 discovered the weakness that it introduced. The procedure would eventually be abandonned by the Germans on 1 May 1940, but by that time WWII had already started and the British had beome involved in codebreaking (see below) [10].

World War II
In 1933, the Polish Cipher Bureau even got access to the Enigma operating procedures that were used by the German Army. Hans Thilo Schmidt, a German playboy working at the German Cipher Office, needed money and sold infomation to the French secret service. The French, who gave Schmidt the codename Asché, passed it on to the Poles, who could then reconstruct the machine.

From 1933 onwards, the Poles intercepted and decrypted a significant portion of the German radio traffic. In 1938 they noticed an increase in the number of messages, which confirmed that the Germans were probably preparing for war.

Until that point, the Germans had been using a common Grundstellung (basic setting) for all traffic. On 15 September 1938 however, this procedure was abandonned. Around the same time, two new rotors (IV, V) were added to the existing three, which multiplied the maximum number of possible settings by a factor of 10.

In the meantime, the Poles had built their own equivalent of the Wehrmacht Enigma with a plug­board added towards the rear. The wiring of the two additional rotors was soon recovered by Rejewski and suitably wired rotors were added to the Polish Replica. With the war imminent, the Poles started looking for ways to get their knowledge out of the country before it was too late.

The Polish Gift
Dillwyn (Dilly) Knox was one of the Room 40 codebreakers during World War I. Since 1925 he had been trying to break the Enigma machine and had his first success on 4 April 1937 when he broke Franco's Enigma K during the Spanish Civil War. When Germany started using the Steckered Enigma for communication between Germany and Spain in 1938, he mounted an attack on the military Enigma, but did not succeed, as he unable to work out the wiring of the entry disc (ETW).

In 1938, the British GC&CS started discussing the Enigma machine with the French cipher bureau – Deuxième Bureau – from whom they acquired the details that the French had obtained from the German spy Asché. The French also dislosed their contacts with the Poles. In January 1939, at the first Polish-French-British meeting in Paris (France), GC&CS was represented by Dilly Knox, Hugh Foss and Alastair Denniston. Dilly described the system of rodding that he had developed, but the Poles had been instructed by their superiors not to disclose any vital information at that meeting.

Dilly had clearly impressed the Poles and on 25-26 July 1939, with the war imminent, a second meeting was arranged, this time in Poland at a facility of the Polish Cipher Bureau in a forest near Pyry, south of Warsaw (Poland). At this meeting, the Poles revealed their achievements.

Also at this meeting, the Poles gave a replica machine to both the French and the British.

Rejewski had used a different approach to Knox, as he used (mathematical) permutation theory to solve the problem, whilst Knox applied linguistics. Nevertheless, the two quickly established a good relationship during the conference. Knox also learned that the Enigma entry disc was simply wired in alphabetical order. Something that neither he nor Alan Turing had ever considered.

Some of the key players of Bletchley Park: Alastair Denniston, Dilly Knox, Alan Turing and Gordon Welchman

Bletchley Park
The meeting in Pyry was attended by Dilly Knox as codebreaker, Alastair Denniston as head of GC&CS (and codebreaker), and Humphrey Sandwith as head of the Admiralty's intercept and direction-finding service. On behalf of the French Deuxième Bureau, Captain Gustave Bertrand was present. The Polish contribution would soon prove to be of vital importance to the war effort.

Immediately after the meeting, the Polish Cipher Bureau destroyed all their secret documents and equipment, whilst the cryptanalysts escaped to France. A few weeks later, on 14 August 1939, Bletchley Park was established by the British.

Only two weeks later, on 1 September, Germany invaded Poland and two days after that, on 3 September, the UK and France declared war on Germany. World War II had started, just five weeks after the Poles had shared their secrets. In the early stage of the war, the Poles continued to work on Enigma from the French Cipher Bureau.

Bletchley Park is an estate in the small town of Bletchley (Milton Keynes, UK), some 45 miles north of London, that became home to the Government Code and Cypher School (GC&CS) — the British Cipher Bureau. The location was choosen because it had direct railway connections to London, Cambridge and Oxford, allowing scientists and army personnel to travel unobtrusively.

The first people to arrive at Bletchley Park (BP) were professional codebreakers, chess players, mathematicians and people with organising skills. Among them: Dillwyn (Dilly) Knox, Gordon Welchman, Alan Turing and Stuart Milner-Barry.

Knox had already worked for the codebreaking unit Room 40 during World War I, and helped with the decryption of the famous Zimmermann Telegram which brought the US into the war. Stuart Milner-Barry was a chess player and chess writer. Gordon Welchman and Alan Turing both were mathematicians from Cambridge (UK).

Enigma messages were initially broken by hand, using simple pencil and paper methods, and with additional tools like the so-called Jeffrey Sheets — the British equivalent of the Polish Zygalski Sheets. But as the volume of the traffic increased, Turing had to look for automated solutions.

Based on the Polish Bomba and the information that had been passed by the Polish codebreakers shortly before the start of WWII, Turing developed the Bombe. Although the Polish method of exploiting the German weakness of the double-encypered message indicator could no longer be used, Turing developed a more universal method based on Cribs (pieces of guessed plain text).

 More about the Bombe

The Ultra Secret
The British Prime Minister, Winston Churchill, recognized the value, impact and importance of the intelligence delivered by Bletchley Park (BP) and its codebreaking apparatus. He introduced a new level of secrecy that superceedes all others: Top Secret Ultra, commonly abbreviated ULTRA, and demanded that the source of this ULTRA intelligence be kept secret at all times and at all cost.

At the first stages of the war (1940), the British codebreakers were able to read the majority of radio messages from the German Air Force (Luftwaffe) and a modest part of the Army traffic (Wehrmacht). The Naval messages on the other hand, imposed a real problem as their operating procedures were much more complicated.

Furthermore, the German Navy (Kriegsmarine) used three additional rotors (VI, VII and VII) of which the wiring was initially unknown. These extra rotors were used exclusively by the Navy and were not shared with the other forces.

In 1941, Turing achieved a breakthrough when he was working in isolation in The Cottage at BP. He discovered the wiring of the additonal rotors and the naval message indicator procedure. Aided by the catch of a large amount of codebooks from U-Boot U-110, captured on 9 May 1941, Turing managed to find a way into the Naval Enigma M3 and to decrypt part of the naval traffic.

Apparently, the Kriegsmarine used a complex procedure that involved several codebooks, short message books and substitution tables. Messages and status reports were shortened by translating them into a short letter combination. The British even develop a unique system for direction finding, known as HFDF (or Huf-Duf), in order to obtain useful cribs for the Bombe.

Then, on 2 February 1942, disaster struck when the German Navy, completely out of the blue, introduced a new Enigma machine. It caused an immediate black-out for the BP codebreakers.

The new machine had an extra cipher rotor, inserted between the leftmost rotor and the reflector. At the same time, the indicator system was changed and new codebooks were introduced. The new machine was known as the Enigma M4 and was used exclusively by the U-boat section of the Kriegsmarine. The Bombes, that were made for the 3-rotor Enigma, were not suitable for this.

 More about Enigma M4

The Battle of the Atlantic
During WWII, there was en enormous shortage of nearly everything in the UK. Large convoys of supply ships, the so-called Liberty Ships, travelled from the US to the UK, bringing people, food, ammunition and anything else that was needed, to wartime Britain and to the Soviet Union [6].

Althoug the Liberty Ships were designed in the UK, they were adapted by the US and were quick and cheap to build. The convoys were often protected by military vessels. Nevertheless they were an easy prey for the German U-boats that were organised in the so-called Wolfpacks.

The 4-rotor Enigma M4 had a serious impact on The Battle of the Atlantic. As it was no longer possible to read the messages to and from the U-boats, it was impossible to determine the location of the Wolfpacks, resulting enormous losses of ships, people, supplies and war cargo.

The black-out that started on 2 February 1942, lasted for nearly nine months and cost no doubt numerous lives. Luckily however, the tide changed on 30 October 1942, when new codebooks were captured from a sinking U-boat. In the meantime, Turing had worked out the new Naval procedures and the wiring of the additional rotor. The codebooks completed the puzzle.

As the Bombe was only suitable for attacking 3-rotor Enigma machines, several solutions were developed. A 3-rotor Bombe, that contained the equivalent of 36 Enigma machines, was modified into a 24-Enigma 4-rotor Bombe. Although the resulting machine was rather slow, it worked.

A better solution was to add external 4th-rotor attachments to the existing 3-rotor Bombes. Such add-ons were developed and built by the British Tabulating Company (BTM) as well as by the General Post Office (GPO) at Dollis Hill. Some solutions even involved valve-based technology.

Finally, several variants of a true 4-rotor Bombe were built. Some of these featured an extra fast 4th rotor and an electronic valve-based sensing circuit that was developed by Tommy Flowers at the GPO in Dollis Hill. By this time, the US had already entered the war and after long discussions it was decided to share the knowledge about the Bombe technology with the American Allies.

This decision — that allowed the Americans to develop their own 4-rotor Bombe — came just at the right time. As the UK suffered shortages of nearly all kinds of material, it became more and more difficult to build reliable machines.

The Americans on the other hand, had sufficient supplies and resources and were able to allocate funding and production capacity to it. The US Bombe was developed by Joe Desch, an engineer at the National Cash Registers (NCR) in Dayton (Ohio). Development started at the end of 1942 and by mid-1943 the first US Bombe was ready.

It appeared to be much faster the UK Bombes and involved valve-based electronic circuits. By the end of 1943, no less than 120 machines were installed and for the remainder of the war, the US took care of breaking the bulk of the 4-rotor based Enigma messages (i.e. the U-boat traffic), leaving a modest part of it, plus the bulk of the 3-rotor traffic, to the codebreakers in the UK.

 More about the US Bombe

The Abwehr Enigma
The most common version of the Enigma machine that was broken by the codebreakers at Bletchley Park (BP) was the Enigma I, the machine that was used by the German Army and Air Force. The Naval machines, the M3 and M4, were also broken on a regular basis. Nevertheless, there were other Enigma models and variants that required the BP codebreakers' attention.

Some less important networks, sometimes used Commercial Enigma machines. Such machines, generally the Enigma K, were also used by other countries, such as Italy, Spain and Switzerland.

One of the most difficult machines to be broken, appeared to be the Enigma G. It was a variant of the commercial Enigma that had a cogwheel driven mechanism and multiple turnover notches on each rotor, causing irregular rotor stepping. Such machines were used by the German Secret Service, the Abwehr (hence the nickname Abwehr Enigma) and could not be broken by the Bombe.

The Abwehr networks yielded far less intercepts than the regular army networks, making it very difficult to find any messages in depth. Furthermore, the Abwehr used different keys on each link, requiring each radio link to be broken individually. The Enigma G was attacked by a team led by Dillwyn (Dilly) Knox, the WWI Room 40 codebreaker who had already broken the Spanish Enigma.

Knox had helped to decrypt the Zimmermann Telegram which was responsible for bringing the USA into WWI. After WWI he joined GC&CS and as a cryptographer he was among the first group of people to arrive at Bletchley Park (BP) in August 1939, where he started working in The Cottage.

After breaking the Italian Naval Enigma in 1941, something that was decisive in winning the Battle of Matapan (Greece), he and his group of female codebreakers – known as Dilly's Girls – started working on the Abwehr Enigma, and by the end of 1941 they had their first success.

After the first breakthrough in October 1941, a special unit was established to work on the Abwehr decrypts. It became known as Intelligence Services Knox (ISK) and by the end of the war, ISK had processed about 140,800 Abwehr messages [7]. Knox himself didn't live to see the results of his work. Already diagnosed with lymph cancer at the start of the war, he died in February 1943. One of 'his girls' — top codebreaker Mavis Lever (later: Batey) who had broken the Italian Naval Enigma in 1941 — wrote an affectionate biography of Dilly Knox in 2009 [8].

 More about Abwehr Enigma

The Japanese Enigma
Before and during World War II, Japan was arguably Germany's most important ally. Germany mainly fought its war in Europe, North Africa and Russia, whilst Japan took care of the southern hemisphere. During the war, Japan had observers in all parts of the European war theatre.

For communication between the observers and their headquarters, the Japanese used two manual cipher systems, known as Sumatra and TOGO (later: Sumatra 2 and TOGO 2), but they preferred a mechanical system like the Enigma.

Although they preferred the Military Enigma I (with Steckerbrett), the Germans didn't want to give away their most secure Enigma machine. Instead it was agreed in 1942 to build a special version of the commercial Enigma K with a differently wired entry disc (ETW) and five turnover notches on each of the eight rotors.

The Japanese ordered 800 of these machines and the first units were delivered to them in August 1943. Due to material shortages however, the full order was never delivered. Furthermore, the Japanese had their doubts about the security of the commercial machine and insisted on having the military variant instead. In the end it was agreed that the remainder of the order would consist of military Enigma I machines that were made backwards compatible with the Enigma T.

 More about Enigma T

In retrospect it may seem strange that the Germans kept using Enigma for so long and that its security was never questioned. In reality however, questions about the cryptographic strength of Enigma had been raised several times, for example by U-boat commander Admiral Karl Dönitz.

On each occasion, the Army Intelligence Service, (Abwehr) was asked to investigate any incidents. But the Abwehr, who had been responsible for choosing the Enigma in the first place, always concluded that it was imposible to break the machine. After all, the British used it as well...

Nevertheless, Donitz kept having doubts and took his own measures. Three additional cipher rotors (VI, VII and VIII) were introduced in 1939 for exclusive use by 'his' Navy and in 1942, out of the blue, he introduced the M4 Enigma. But these were not the only security measures.

In 1943, a new reflector Umkehrwalze C or UKW-C (with 4th rotor 'Gamma') was introduced as an alternative to UKW-B/Beta, but it was only available to a limited number of users. Nevertheless it was used until the end of the war, sometimes even mixed with UKW-B and 4th rotor 'Beta'.

In January 1944, a rewirable reflector, UKW-D or Dora, was introduced. It could be fitted in place of the existing UKW-B and there even was a special Naval version of it. Code­books were updated to include the UKW-D wiring, which was changed every 10 days. Nevertheless UKW-D saw limited use as it was difficult in operation and could not be distributed effectively in 1944.

The German Air Force, the Luftwaffe, took their own measures and developed a device to alter the wiring of the Steckerbrett (plugboard) for each message. It was known as the Enigma Uhr.

The Uhr was a small wooden device that could be attached to the right side of an Enigma machine and had 20 wires that were connected to the Steckerbrett instead of the normal patch cables. A large wooden knob on top of the device could be set to any of 40 positions, marked 00 - 39.

The Uhr was even combined with UKW-D on the so-called Red key, making it a real challenge for the codebreakers at Bletchley Park. Had the Uhr been used correctly, it might even have defeated them, but due to operator mistakes it was broken within a few days after its introduction.

The most dangerous Enigma improvement how­ever, was the so-called Lückenfüllerwalze (gap-filling rotor). It had been developed in 1943 by Regierungs-Oberinspektor Fritz Menzer, but was put off several times as the Enigma was still considered to be a secure machine at the time.

Towards the end of the war, 12,000 of these rotors were ordered. The Lückenfüllerwalze is a normal Enigma cipher rotor, with programmable turnover notches, 26 in total. It can easily be configured in the field and causes irregular rotor stepping, something that could have defeated the codebreakers. But the war ended before it was ready for release. After the war, the American TICOM immediately confiscated the Lückenfüllerwalze and kept it under wraps for many years.

 More about UKW-D   Enigma Uhr   Luckenfüllerwalze

After the War
Once the war was over, the fact that the the Enigma had been broken was kept secret for many years. Apart from a few exceptions, people went on with their lives and most of the Bombes were dismantled. The captured Enigma machines ended up in the vaults of GC&CS (later: GCHQ) and the SIS (now: NSA), or were used by other countries in the believe that they could not be broken.

In countries like Norway, Germany and Austria, the Enigma-I was used for many years after the war, until they were replaced by newer and better equipment. Rumour has it that Enigma was also used in some African countries.

There are no reports about the use of Enigma by the Russians, although it is certain that several machines were captured. For a long time, histo­rians assumed that the Russians had no know­lege about the Allied codebreaking suc­cesses during WWII, but it has since become clear that they were well informed through espionage.

In 1956, the Russians introduced the first version of a sophisticated rotor-based cipher machine: the M-125 — codenamed FIALKA. The machine has 10 rotors and features irregular stepping, whilst alternate rotors move in opposite directions. More importantly, it has fixes for virtually all known design flaws of the Enigma, including the fact that a letter cannot be encoded as itself.

Furthermore, the Steckerbrett is replaced by a card reader and the machine can operate directly on teleprinter signals, allowing the use of letters and numbers. It has a built-in tape puncher and reader, and prints its output directly onto a paper strip. Available in several variants, it was used by the USSR and all Warsaw Pact countries.

Shortly after the end of WWII, the Americans started the development of a new rotor-based cipher machine that would replace SIGABA. The machine became known as KL-7, and also by its key-procedure names ADONIS and POLLUX.

KL-7 was used by all US Forces (Navy, Army and Air Force), as well as by government agencies like the FBI, CIA and the White House itself. It also became the main cipher machine of the newly established NATO in post-war Europe. From the 1960s onwards, rotor-based cipher machines were gradually succeeded by electronic alternatives, such as KW-7, KL-51 (RACE) and Aroflex.

Fialka M-125 cipher machines
KL-7 rotor-based cipher machine (USA)
 Other rotor-based cipher machines

Related subjects
Enigma Family Tree
Based on many years of research by Frode Weierud, we've been able to put together the most accurate family tree of Enigma machines to date. It shows the relationship between the various models and variants, and provides a lot of additional information.

Please note that the tree is based on ongoing research and is therefore subject to changes.

 More information


Enigma Timeline
The history of the Enigma machine is extremely complex. There were many different models and variations, and they were used by many different customers. During the war, a mixture of military and commercial Enigma machines were used by different branches of the war apparatus.

Based on the above research, we've created a timeline of events, patents, enigma models, accessories and peripherals.

 More information


Other WWII German Cipher Machines
Enigma was by no means the only cipher machine used by the Germans during WWII. In fact the Enigma, of which over 20,000 units were produced, was mainly used at a tactical level, whilst the German High Command (OKW) used other machines, such as the Siemens T-52 Geheimschreiber and the Lorenz SZ-40/42 teleprinter add-on. Towards the end of the war, Germany even started using the Siemens T-43, a machine that was based on the unbreakable One-Time Pad cipher.

Siemens T-52 Geheimschreiber
The Lorenz teleprinter cipher attachment SZ-40 and SZ-42
Siemens T-43 One-Time Tape cipher machine
Schlüsselgerät 41 (SG-41), also known as the Hitlermühle (Hitler Mill)

Furthermore, it was decided that the Abwehr Enigma would be replaced by the SG-41, also known as the Hitlermühle (Hitler Mill) – an improved version of the Hagelin C-38/M-209 – developed by Fritz Menzer. It was avalable in an alphabetical and a numerical variant, but came too late to have a significant effect on the course of the war. Only a handful of the above machines have survived.

Contemporary relatives
The Enigma wasn't the only rotor-based cipher machine that was used during World War II. The UK used the so-called Typex cipher machine for all high-grade traffic. Typex — basically a copy of the German Enigma — had five cipher rotors, three of which moved during encipherment. In fact, the British codebreaking centre Bletchley Park had huts full of Typex machines that were converted into Enigma analogues, to decrypt German messages once the key had been broken.

As far as we know, Typex was never broken by the Germans during the war, despite the fact that the Germans had captured some machines. On the contrary: the fact that the British used a similar machine, confirmed the German believe that the Enigma was indeed unbreakable.

The Americans also used rotor machines for some of their radio traffic. For low-grade tactical messages they used the Hagelin M-209, which they knew could be broken by the Germans. For high-grade traffic however, they used the 15-rotor SIGABA, shown in the image on the right.

A combined development of US top cryptographers William Friedman, Frank Rowlett (US Army) and Laurence Safford (US Navy), SIGABA is based on the same rotor principle as Enigma, but is improved in many ways. In addition, it prints its output directly onto a gummed paper strip.

As far as we know, SIGABA was never broken by the Germans during the war. In the latter part of the war, around November 1943, the need arose for the Americans and the British to securely exchange cipher messages. As they couldn't agree on which machine was the better one, Typex or SIGABA, it was decided to define a common standard and modify both machines to comply with that standard. The common machine became known as Combined Cipher Machine (CCM).

The Hagelin-designed M-209 (C-38)
The Hagelin BC-38 (compatible with M-209)
British rotor-based cipher machine used extensively during WWII
ECM Mark II (SIGABA, CSP-888/889, CSP-2900, CSP-1600, CSP-1700)
Combined Cipher Machine based on SIGABA
  1. Wikipedia, Rotor machine
    Retrieved January 2014.

  2. Karl de Leeuw, The Dutch invention of the Rotor Machine, 1915-1923
    Cryptologia, January 2003, Volume XXVII, Number 1, pp. 73-94.

  3. Wikipedia, Enigma machine
    Retrieved January 2014.

  4. Hugo Alexander Koch, Patent NL10700
    7 October 1919. Transferred to Securitas on 5 May 1922. 1

  5. Dr.-Ing. Arthur Scherbius. Enigma Chiffriermaschine
    Elektrotechnische Zeitschrift. 1923. Heft 47/48. p. 1035-1036.

  6. Wikipedia, Battle of the Atlantic
    Retrieved January 2014.

  7. Wikipedia, Dilly Knox
    Retrieved January 2014.

  8. Mavis Batey, Dilly, The Man Who Broke Enigmas
    2009. Hard cover, ISBN 978-1-906447-01-4.

  9. Kruh and Deavours, The Commercial Enigma: Beginnings of Machine Cryptography
    Cryptologia, Volume XXVI, Number 1, January 2002.

  10. Frode Weierud, Personal correspondence
    August 2017.

  11. Claus Taaks, Scherbius and Enigma History
    Personal correspondence, April 2023.
  1. The rights to patent NL10700 were transferred to Naamloze Vennootschap Securitas in Amsterdam (Netherlands) on 5 May 1922 and then to Chiffriermaschinen AG in Germany on 28 January 1927.

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