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History of the Enigma
The rotor-based cipher machines

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 USA, Arvid Damm in Sweden, Hugo Koch in The Netherlands and Arthur Scherbius in Germany [1].

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

Invention of the Rotor Machine (1915)
There is one development however, that pre-dates the others, and that is the invention of Theo A van Hengel (1875-1939) and RPC Sprengler (1875-1955), two Dutch naval officers who produced working rotor-based cipher machines for the Dutch War Department (Ministerie van Oorlog) in 1915. This fact was discovered in 2003 and is described in a paper by Karl de Leeuw [2].

Officially though, the Enigma machine was invented by Arthur Scherbius in 1918, right at the end of World War I. After several years of improving his invention, the first machine saw the light of day in 1923. Just a year earlier he had secured the rights to patent NL10700 of Dutch inventor Hugo Koch [4].

It was a rather large typewriter-style machine that was developed by Scherbius' first company Scherbius & Ritter of Berlin-Wansee (Germany), but was built by Gewerkschaft Securitas (later: Chiffriermaschinen AG), also of Berlin. This machine would become known as the Enigma A.
  

As the machine prints its output directly on a sheet of paper (just like a typewriter), it was also known as the Schreibende Enigma (writing Enigma). It was first described in a technical article by Scherbius himself in 1923 [5]. As far as we know, there are no surviving Enigma A machines.

The glowlamp Enigma (1924)
There were a lot of problems with the Enigma A. It had reliability problems with the print wheel mechanism and was replaced a year later by the Enigma B (1924). More importantly though, the Enigma A and B were very expensive to build and would only be suitable for the high-end market.

For this reason, Scherbius developed a machine that produced it output on a lamp panel rather than on paper. The first model was the Enigma C that was introduced in 1924. It was also known as Gluhlampenmaschine (glow lamp machine).

The machine was available for about 1/8th of the price of the Enigma A and costed RM 1000 1 at the time. The machine is housed in a wooden case and looks pretty much like the later Enigma models, except that the keys are arranged in sequential order (ABCDE...) rather than the more common typewriter order (QWERTZ...).
  

The standard Enigma C 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 wheels that protrude the top lid. Each cipher wheel has 26 contacts at either side. Several variants of the Enigma C were produced, such as the so-called Funkschlüssel C (for the German Navy) and a Swedisch variant, both with 28 keys.

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

The commercial Enigma (1926)
Unlike the printing Enigma, the glowlamp machines had a reflector (UKW) that made the machine reciproke (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 wheels, which is why this machine is sometimes thought to be a 4-wheel Enigma.

The machine was internally known as model A26 and became known as the Enigma D. Like the Enigma C it was housed in a wooden transit case with a hinged lid. It had several improvements.
  
A lamp lights up when a key is pressed

The wheels 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 wheels. The Enigma D became the basis for most of the later machines.

The 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 wheel turnover mechanism that is comparable to the odometer of a car. The rightmost wheel makes a single step on each key press. After the rightmost wheel has completed a full revolution, the middle wheel 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).
  
The first Zählwerk Enigma (A28) introduced in 1928

Furthermore, the Zählwerk Enigma has a far more advanced wheel turnover mechanism that was driven by cogwheels rather than by pawls and levers. This allowed the mechanism to run in reverse as well, which was useful for correcting mistakes. The Zählwerk Enigma also introduces the concept of multiple turnover notches, which causes more frequent (irregular) wheel stepping.

The three cipher wheels 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 wheels.

The first Zählwerk Enigma was the model A28. It was introduced in 1928 and was built on the chassis of the Enigma D. A few years later, a design variant of the A28 was developed. It was slightly smaller and had smaller cipher wheels.
  
The Enigma G (model G31) that was introduced in 1931

Although the number and position of the turnover notches was identical (11, 15 and 17), the diameter of the wheels was smaller and the cogwheel driven mechanism was slightly simplified. This machine was introduced in 1931 and was known as model G31 (later: Abwehr Enigma).

Military Enigma (1932)
Apart from the Zählwerk Enigma, there is also a less well known version of the machine that is suitable for numbers only. It is introduced in 1930 and is known as Enigma Z or Z30. Two variants of this machine are known: one with a simple stepping mechanism and one with Zählwerk stepping. The machine was short lived and only a few units (±50) were ever built.

In 1926, the German Army (Reichswehr, later: Wehrmacht) started showing an interest in the machine. At their request a special variant of the commercial Enigma D is developed. It has three cipher wheels and a fixed reflector (UKW).

Furthermore, the new machine has a plugboard (Steckerbrett) at the front that adds an extra layer to the cipher. The Steckerbrett can be configured in the field and is used exclusively for the Reichswehr. The first prototype is ready in 1927 and features a single-ended plugboard. It is known as the Reichwehr Enigma D.
  
Enigma I with top lid and flap open

The final version is ready in 1932 and has an improved double-ended Steckerbrett. This version is known by the Reichswehr (now: Wehrmacht) as the Enigma-I and is only available to the Army. Until this time, all commercial Enigma models were freely available on the (international) market. This changes when in 1932 the German Army claims the exclusive rights to the machine. From then on, all commercial and international sales had to be approved by the German Army.

Apart from the Enigma-I, the manufacturer, now by the name of Heimsoeth & Rincke, also develops a new version of the printing Enigma. It is the successor to the Enigma B and is ready in 1929. The machine is called Enigma H (model H29) and is known by the Army as Enigma-II.

The Enigma H is also sold to the Hungarian Army, but was never very popular due to its high price. Apart from the Hungary Army, the Germans also kept selling Enigma machines to the Swiss and to the Dutch Army. The latter also bought Enigma G31 models as late as 1938.
  
View of the controls of the Enigma H29 with serial number H-221

In the mid-1930s the German Army is clearly preparing for war and starts ordering Enigma-I machines in large quantities for the Wehrmacht (Army) and the Luftwaffe (Air Force). The Enigma G is used by the German Abwehr (secret service). For the Kriegsmarine (German Navy) a model similar to and compatible with the Enigma-I is developed. It becomes known as the Enigma M1 (1934) which is later followed by the Enigma M2 (1938) and finally the Enigma M3 (1940).

Manufacturers
The first Enigma machines were developed in 1923 by Arthur Scherbius' first company Scherbius und Ritter but were built after the company had been renamed first to Gewerkschaft Securitas and a few years later to Chiffriermaschinen AG. 1 After Scherbius' untimely death in 1929, the company changed hands and in 1933, after the German Army had acquired the manufacturing rights to the Enigma machine, the name was changed once more to Heimsoeth und Rinke.

  • 1918
    Arthur Scherbius
  • 1918
    Gewerkschaft Securitas
  • 1920
    Scherbius und Ritter
  • 1920
    Chiffriermaschinen Aktiengesellschaft 2
  • 1933
    Heimsoeth und Rinke
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 manufactures 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

  1. The History of Scherbius' first companies is very clouded. The three companies: Gewerkschaft Securitas, Scherbius und Ritter and Chiffriermaschinen AG, seem to have existed more or less simultaneously, as patents were filed by all three companies during the first years, at least until 1923. It appears that from 1924 onwards, the name Chiffriermaschinen AG prevailed.
  2. In their article in Cryptologia of January 2002 [9], Kruh and Deavours state that Chiffriermaschinen AG was established on 9 July 1923. It is doubted whether this is correct however, as patent DE425147 was filed by Chiffriermaschinen AG on 26 September 1920, which means that the company already existed back then.

The Polish Breakthrough (1933)
Around 1930, the Polish Cipher Bureau, Biuro Szyfrów, is 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 start researching the commercial Enigma.

Marian Rejewski Jerzy Rózycki Henryk Zygalski

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

Rejewski is set to work on the problem in late 1932 and after a few weeks he achieves his first breakthrough, when he deduces the secret internal wiring of the Enigma. Together with his colleagues he starts developing various aids for the regular decryption of the German 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 is developed to exploit this weakness mechanically: the Bomba kryptologiczna (the cryptologic bomb).
  
The Polish Bomba kryptologiczna. Click for further information.

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

  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 gets access to the Enigma operating procedures that are used by the German Army. Hans Thilo Schmidt, a German playboy working at the German Cipher Office, is in need of money and sells infomation to the French secret service. The French, who gave Schmidt the codename Asché, pass it on to the Poles, who can now reconstruct the machine.

From 1933 onwards, the Poles intercept and decrypt a significant portion of the German radio traffic. In 1938 they see an increase in the number of messages sent by the Germans and it seems clear that Germany is preparing for war.

All this time, the Germans have been using a common Grundstellung (basic setting) for all Enigma traffic. On 15 September 1938 however, this procedure is abandonned. Around the same time, two new wheels (IV and V) are added to the existing three, which multiplies the maximum number of possible settings by a factor of 10.
  
An Enigma replica built by the Poles. Photograph courtesy David Hamer.

In the meantime, the Poles have built their own equivalent of the Wehrmacht Enigma with a plugboard added towards the rear. The wiring of the two additional wheels is soon recovered by Rejewski and suitably wired wheels are added to the Polish Replica. With the war imminent, the Poles start looking for ways to get their knowledge out of the country before it is 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 starts using the Steckered Enigma for communication between Germany and Spain in 1938, he mounts an attack on the military Enigma machine, but is not successful as he can't work out the wiring of the entry disc.

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

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.
  
The outstation of the Polish Cipher Bureay in Pyry

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 destroys all its secret documents and equipment, whilst the cryptanalysts escape to France. A few weeks later, on 14 August 1939, Bletchley Park is established by the British.

Only two weeks later, on 1 September, Germany invades Poland and two days after that, on 3 September, Great Britain and France declare war to Germany. World War II has started, just five weeks after the Poles had shared their secrets. In the early stage of the war, the Poles continue to work on Enigma from the French Cipher Bureau.
  
The Mansion at Bletchley Park. Click for more information.

Bletchley Park is an estate in the small town of Bletchley (Milton Keynes, UK), some 45 miles north of London, that would become the home of 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 scientist and army personnel to travel inconspicuously.

The first people to arrive at Bletchley Park (BP) are professional codebreakers, mathematicians, chess players and people with organising skills. Among them are 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).
  

Initially, Enigma messages are broken 'by hand', using simple pencil and paper methods, and with additional tools such as the so-called Jeffrey Sheets, the British equivalent of the Polish Zygalski Sheets. But as the volume of the traffic increases, Turing starts looking for automated solutions.

Based on the Polish Bomba and the information that was passed by the Polish codebreakers shortly before the start of WWII, Turing develops 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).

The Ultra Secret
The British Prime Minister, Winston Churchill, fully recognizes the impact and importance of the value of intelligence that is delivered by Bletchley Park and its codebreaking machinery. He introduces a new level of secrecy that superceedes all other levels: Top Secret Ultra, or ULTRA. He also commands that the source of this ULTRA intelligence has to be kept secret at all cost.

In the first stages of the war (1940), the British codebreakers are 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, impose a real problem as their operating procedures are much more complicated.

Furthermore, the German Navy (Kriegsmarine) uses three additional rotors (VI, VII and VII) of which the wiring was hitherto unknown. These extra wheels are used exclusively by the Navy and are not shared with other parts of the Army.
  
Prime Minister Winston Churchill (left) in the Cabinet War Room

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

Apparently, the Kriegsmarine uses a complex procedure that involves several codebooks, short message books and substitution tables. Long messages and status reports are 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 strikes when the German Navy, completely out of the blue, introduces a new Enigma machine. It causes an immediate black-out for the BP codebreakers.
  
The Cottage at Bletchley Park where Alan Turing worked in isolation on solving the Naval Enigma

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

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 USA 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 pray for the German U-boats that were organised in the so-called Wolfpacks.

The 4-wheel 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 costed, no doubt numerous, lives. Luckily however, the tide changes on 30 October 1942, when new codebooks are captured from a sinking U-boat. In the meantime, Turing has worked out the new Naval procedures and the wiring of the additional wheel. The codebooks complete the puzzle.

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

A better solution was the addition of external 4th-wheel attachments to the existing 3-wheel Bombes. Such add-ons were developed an built by the British Tabulating Company (BTM) as well as by the General Post Office (GPO) at. Some solutions even involved valve-based technology.
  
Lamp panel and wheels of Enigma M4

Finally, several variants of a true 4-wheel Bombe were built. Some of these featured an extra fast 4th wheel 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 a long discussion 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-wheel 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, has 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). Developement 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-wheel based Enigma messages (i.e. the U-boat traffic), leaving a modest part of it, plus the bulk of the 3-wheel traffic, to the codebreakers in the UK.

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 models and variants of the machine that also required their 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 cog wheel driven mechanism and multiple turnover notches on each wheel, causing irregular wheel 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 Commercial Enigma K, that was used by Spain, Switzerland and Italy. An improved version (Enigma T) was used by the Japanese Army.

The Abwehr networks yielded far less intercepts than the regular army networks, making it 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, who had worked in the Room 40 codebreaking unit during World War I.

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 amoung the first group of people to arrive at Bletchley Park 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.
  
Close-up of Enigma G

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' was top codebreaker Mavis Lever (later: Batey) who wrote an affectionate biography about Dilly Knox in 2009 [8].

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

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 to have 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 backwards compatible with the Enigma T.

Improvements
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 relibility of Enigma had been raised several times, for example by U-Boot 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 too...

Nevertheless, Donitz kept having doubts and took his own measures. Three additional cipher wheels (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 wheel 'Gamma') was introduced as an alternative to UKW-B, 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 wheel 'Beta'.

In January 1944, a field-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. 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 to use and could not be distributed effectively in 1944.

The German Air Force, the Luftwaffe took their own measures and developed a device to quickly alter the wiring of the Steckerbrett (plugboard). It became 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 errors it was broken within a few days after its introduction.

By far the most dangerous Enigma improvement however, was the so-called Lückenfüllerwalze (gap-filling wheel). It had been developed in 1943 by Regierungs-Oberinspektor 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 wheels were ordered. The Lückenfüllerwalze is a normal Enigma cipher wheel, with programmable turnover notches, 26 in total. It can easily be configured in the field and causes irregular wheel 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.

Relatives of Enigma
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. The Typex, which was virtually a plain copy of the German Enigma, had five cipher wheels, three of which were moving.

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. The discovery that the British used a machine similar to the Enigma, confirmed their believe that the machine was indeed unbreakable.

The Americans also used a rotor machine for some of their radio traffic. Whilst the Hagelin M-209 was used for tactical field messages, it was known to be broken by the Germans. For high-grade traffic however, the advanced 15 rotor SIGABA machine, shown here, was used instead.
  
Right angle view of the SIGABA, showing the paper path.

It was a combined development of US top cryptographers William Friedman, Frank Rowlett (US Army) and Laurence Safford (US Navy). Although the machine was clearly based on the design of the Enigma, it was improved in many areas and was able to print its output onto a 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).

After the War
When the war was over, the entire story of breaking the Enigma machine 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 CG&GS (now: GCHQ) and the NSA, or were given to other countries with the message 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. It is believed that the machine was also used in several African countries.

Funnily enough, there are no reports about the use of Enigma by the Russians, although it is pretty certain that they must have captured some machines. For a long time it was assumed that the Russians had no knowledge about the Allied achievements in WWII codebreaking, but it now seems likely that they were well informed.
  

In 1956, the Russians introduced the first version of a very advanced rotor-based cipher machine that was codenamed Fialka. The machine had 10 cipher wheels and featured irregular wheel stepping, with the wheels moving in both directions. More importantly, they had found solutions for all Enigma's weaknesses, such as the fact that a letter can never be encoded into itself.

Furthermore, the Steckerbrett was replaced by a card reader and the machine operated directly on teleprinter signals, allowing the use of letters and numbers. It had a built-in tape puncher and reader, and printed the output directly onto a paper strip. It was officially known as M-125 and was used by all countries of the Warsaw Pact.

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

The KL-7 was not only used by the USA, but also became the main cipher machine of the newly established NATO in the post-war era. Boris Hagelin, the developer of the US M-209 cipher machine, moved from Sweden to Switzerland and introduced a long range of pin-wheel based cipher machines during the 1950s. In the 1960s, rotor-based cipher machines were gradually replaced by all-electronic cipher machines, such as the KW-7, KG-84, Ecolex 4 and Aroflex. By the late 1980s, all machines had been replaced by a wealth of modern equipment like the KIV-7.

Other WWII German Cipher Machines
The 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 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, they even started using the Siemens T-43, a machine that was based on the unbreakable One-Time Pad cipher.

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.

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)
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 in the future.

 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 machine.

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

 More information
  

References
  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. Dutch 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.
  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.

Further information
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