# Enigma machine

Military Enigma machine

An Enigma machine was any of a family of related electro-mechanical rotor cipher machines used in the twentieth century for enciphering and deciphering secret messages. Enigma was invented by the German engineer Arthur Scherbius at the end of World War I.[1] Early models were used commercially from the early 1920s, and adopted by military and government services of several countries—most notably by Nazi Germany before and during World War II.[2] Several different Enigma models were produced, but the German military models are the most commonly discussed.

German military texts enciphered on the Enigma machine were first broken by the Polish Cipher Bureau, beginning in December 1932. This success was a result of efforts by three Polish cryptologists, Marian Rejewski, Jerzy Różycki and Henryk Zygalski, working for Polish military intelligence. Rejewski "reverse-engineered" the device, using theoretical mathematics and material supplied by French military intelligence. Subsequently the three mathematicians designed mechanical devices for breaking Enigma ciphers, including the cryptologic bomb. From 1938 onwards, additional complexity was repeatedly added to the Enigma machines, making decryption more difficult and necessitating larger numbers of equipment and personnel—more than the Poles could readily produce.

On 25 July 1939, in Warsaw, the Poles initiated French and British military intelligence representatives into their Enigma-decryption techniques and equipment, including Zygalski sheets and the cryptologic bomb, and promised each delegation a Polish-reconstructed Enigma. The demonstration represented a vital basis for the later British continuation and effort.[3] During the war, British cryptologists decrypted a vast number of messages enciphered on Enigma. The intelligence gleaned from this source, codenamed "Ultra" by the British, was a substantial aid to the Allied war effort.[4]

Though Enigma had some cryptographic weaknesses, in practice it was German procedural flaws, operator mistakes, laziness, failure to systematically introduce changes in encipherment procedures, and Allied capture of key tables and hardware that, during the war, enabled Allied cryptologists to succeed.[5][6]

The exact influence of Ultra on the course of the war is debated; an oft-repeated assessment is that decryption of German ciphers advanced the end of the European war by two years.[5][7][8] Winston Churchill told the United Kingdom's King George VI after World War II: "It was thanks to Ultra that we won the war."[9]

## Design

Enigma in use, 1943

Like other rotor machines, the Enigma machine is a combination of mechanical and electrical subsystems. The mechanical subsystem consists of a keyboard; a set of rotating disks called rotors arranged adjacently along a spindle; and one of various stepping components to turn one or more rotor with each key press.

### Electrical pathway

Enigma wiring diagram with arrows and the numbers 1 to 9 showing how current flows from key depression to a lamp being lit. The A key is encoded to the D lamp. D yields A, but A never yields A; this property was due to a patented feature unique to the Enigmas, and could be exploited by cryptanalysts in some situations.

The mechanical parts act in such a way as to form a varying electrical circuit. When a key is pressed, a circuit is completed. Current flows through various components in their current configuration, ultimately lighting one display lamp, revealing an output letter. For example, when encrypting a message starting ANX..., the operator would first press the A key, and the Z lamp might light, so Z would be the first letter of the ciphertext. The operator would next press N, and then X in the same fashion, and so on.

The detailed operation of Enigma is shown in the wiring diagram to the left. To simplify the example, only four components of a complete Enigma machine are shown. In reality, there are 26 lamps and keys, rotor wirings inside the rotors (of which there are either three or four) and between six and ten plug leads.

The scrambling action of Enigma's rotors is shown for two consecutive letters with the right-hand rotor moving one position between them.

Current flowed from the battery (1) through a depressed bi-directional keyboard switch (2) to the plugboard (3). Next, it passed through the (unused in this instance, so shown closed) plug "A" (3) via the entry wheel (4), through the wiring of the three (Wehrmacht Enigma) or four (Kriegsmarine M4 and Abwehr variants) installed rotors (5), and entered the reflector (6). The reflector returned the current, via an entirely different path, back through the rotors (5) and entry wheel (4), proceeding through plug "S" (7) connected with a cable (8) to plug "D", and another bi-directional switch (9) to light the appropriate lamp.[10]

The repeated changes of electrical path through an Enigma scrambler implemented a polyalphabetic substitution cipher that provided Enigma's security. The diagram on the right shows how the electrical pathway changed with each key depression, which caused rotation of at least the right-hand rotor. Current passed into the set of rotors, into and back out of the reflector, and out through the rotors again. The greyed-out lines are other possible paths within each rotor; these are hard-wired from one side of each rotor to the other. The letter A encrypts differently with consecutive key presses, first to G, and then to C. This is because the right-hand rotor has stepped, sending the signal on a completely different route. Eventually other rotors step with a key press.

### Rotors

Enigma rotor assembly. In the Wehrmacht Enigma, the three installed movable rotors are sandwiched between two fixed wheels: the entry wheel, on the right, and the reflector on the left.
Main article: Enigma rotor details

The rotors (alternatively wheels or drums, Walzen in German) formed the heart of an Enigma machine. Each rotor was a disc approximately 10 cm (3.9 in) in diameter made from hard rubber or bakelite with brass spring-loaded pins on one face arranged in a circle; on the other side are a corresponding number of circular electrical contacts. The pins and contacts represent the alphabet—typically the 26 letters A–Z (this will be assumed for the rest of this description). When the rotors were mounted side-by-side on the spindle, the pins of one rotor rested against the contacts of the neighbouring rotor, forming an electrical connection. Inside the body of the rotor, 26 wires connected each pin on one side to a contact on the other in a complex pattern. Most of the rotors were identified by Roman numerals, and each issued copy of rotor I was wired identically to all others. The same was true for the special thin beta and gamma rotors used in the M4 naval variant.

Three Enigma rotors and the shaft, on which they are placed when in use.

By itself, a rotor performs only a very simple type of encryption—a simple substitution cipher. For example, the pin corresponding to the letter E might be wired to the contact for letter T on the opposite face, and so on. Enigma's security came from using several rotors in series (usually three or four) and the regular stepping movement of the rotors, thus implementing a polyalphabetic substitution cipher.

When placed in an Enigma, each rotor can be set to one of 26 possible positions. When inserted, it can be turned by hand using the grooved finger-wheel, which protrudes from the internal Enigma cover when closed. So that the operator can know the rotor's position, each had an alphabet tyre (or letter ring) attached to the outside of the rotor disk, with 26 characters (typically letters); one of these could be seen through the window, thus indicating the rotational position of the rotor. In early models, the alphabet ring was fixed to the rotor disk. A later improvement was the ability to adjust the alphabet ring relative to the rotor disk. The position of the ring was known as the Ringstellung ("ring setting"), and was a part of the initial setting prior to an operating session. In modern terms it was a part of the initialization vector.

Two Enigma rotors showing electrical contacts, stepping ratchet (on the left) and notch (on the right-hand rotor opposite D).

Each rotor contained a notch (or more than one) that controlled rotor stepping. In the military variants, the notches are located on the alphabet ring.

The Army and Air Force Enigmas were used with several rotors, initially three. On 15 December 1938, this changed to five, from which three were chosen for a given session. Rotors were marked with Roman numerals to distinguish them: I, II, III, IV and V, all with single notches located at different points on the alphabet ring. This variation was probably intended as a security measure, but ultimately allowed the Polish Clock Method and British Banburismus attacks.

The Naval version of the Wehrmacht Enigma had always been issued with more rotors than the other services: at first six, then seven, and finally eight. The additional rotors were marked VI, VII and VIII, all with different wiring, and had two notches, resulting in more frequent turnover. The four-rotor Naval Enigma (M4) machine accommodated an extra rotor in the same space as the three-rotor version. This was accomplished by replacing the original reflector with a thinner one and by adding a thin fourth rotor. That fourth rotor was one of two types, Beta or Gamma, and never stepped, but could be manually set to any of 26 positions. One of the 26 made the machine perform identically to the three-rotor machine.

### Stepping

To avoid merely implementing a simple (and easily breakable) substitution cipher, every key press caused one or more rotors to step by one twenty-sixth of a full rotation, before the electrical connections were made. This changed the substitution alphabet used for encryption, ensuring that the cryptographic substitution was different at each new rotor position, producing a more formidable polyalphabetic substitution cipher. The stepping mechanism varied slightly from model to model. The right-hand rotor stepped once with each keystroke, and other rotors stepped less frequently.

### Turnover

The Enigma stepping motion seen from the side away from the operator. All three ratchet pawls (green) push in unison as a key is depressed. For the first rotor (1), which to the operator is the right-hand rotor, the ratchet (red) is always engaged, and steps with each keypress. Here, the middle rotor (2) is engaged because the notch in the first rotor is aligned with the pawl; it will step (turn over) with the first rotor. The third rotor (3) is not engaged, because the notch in the second rotor is not aligned to the pawl, so it will not engage with the rachet.

The advancement of a rotor other than the left-hand one was called a turnover by the British. This was achieved by a ratchet and pawl mechanism. Each rotor had a ratchet with 26 teeth and every time a key was pressed, the set of spring-loaded pawls moved forward in unison, trying to engage with a ratchet. The alphabet ring of the rotor to the right normally prevented this. As this ring rotated with its rotor, a notch machined into it would eventually align itself with the pawl, allowing it to engage with the ratchet, and advance the rotor on its left. The right-hand pawl, having no rotor and ring to its right, stepped its rotor with every key depression.[11] For a single-notch rotor in the right-hand position, the middle rotor stepped once for every 26 steps of the right-hand rotor. Similarly for rotors two and three. For a two-notch rotor, the rotor to its left would turn over twice for each rotation.

The first five rotors to be introduced (I–V) contained one notch each, while the additional naval rotors VI, VII and VIII each had two notches. The position of the notch on each rotor was determined by the letter ring which could be adjusted in relation to the core containing the interconnections. The points on the rings at which they caused the next wheel to move were as follows.[12]

Position of turnover notches
Rotor Turnover position(s) BP mnemonic
I R Royal
II F Flags
III W Wave
IV K Kings
V A Above
VI, VII and VIII A and N

The design also included a feature known as double-stepping. This occurred when each pawl aligned with both the ratchet of its rotor and the rotating notched ring of the neighbouring rotor. If a pawl engaged with a ratchet through alignment with a notch, as it moved forward it pushed against both the ratchet and the notch, advancing both rotors. In a three-rotor machine, double-stepping affected rotor two only. If in moving forward the ratchet of rotor three was engaged, rotor two would move again on the subsequent keystroke, resulting in two consecutive steps. Rotor two also pushes rotor one forward after 26 steps, but since rotor one moves forward with every keystroke anyway, there is no double-stepping.[11] This double-stepping caused the rotors to deviate from odometer-style regular motion.

With three wheels and only single notches in the first and second wheels, the machine had a period of 26 × 25 × 26 = 16,900 (not 26 × 26 × 26, because of double-stepping).[11] Historically, messages were limited to a few hundred letters, and so there was no chance of repeating any combined rotor position during a single session, denying cryptanalysts valuable clues.

To make room for the Naval fourth rotors, the reflector was made much thinner. The fourth rotor fitted into the space made available. No other changes were made, which eased the changeover. Since there were only three pawls, the fourth rotor never stepped, but could be manually set into one of 26 possible positions.

A device that was designed, but not implemented before the war's end, was the Lückenfüllerwalze (gap-fill wheel) that implemented irregular stepping. It allowed field configuration of notches in all 26 positions. If the number of notches was a relative prime of 26 and the number of notches were different for each wheel, the stepping would be more unpredictable. Like the Umkehrwalze-D it also allowed the internal wiring to be reconfigured.[13]

### Entry wheel

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

### Reflector

With the exception of models A and B, the last rotor came before a 'reflector' (German: Umkehrwalze, meaning 'reversal rotor'), a patented feature unique to Enigma among the period's various rotor machines. The reflector connected outputs of the last rotor in pairs, redirecting current back through the rotors by a different route. The reflector ensured that Enigma is self-reciprocal: conveniently, encryption was the same as decryption. However, the reflector also gave Enigma the property that no letter ever encrypted to itself. This was a severe conceptual flaw and a cryptological mistake subsequently exploited by codebreakers.

In Model 'C', the reflector could be inserted in one of two different positions. In Model 'D', the reflector could be set in 26 possible positions, although it did not move during encryption. In the Abwehr Enigma, the reflector stepped during encryption in a manner like the other wheels.

In the German Army and Air Force Enigma, the reflector was fixed and did not rotate; there were four versions. The original version was marked 'A', and was replaced by Umkehrwalze B on 1 November 1937. A third version, Umkehrwalze C was used briefly in 1940, possibly by mistake, and was solved by Hut 6.[14] The fourth version, first observed on 2 January 1944, had a rewireable reflector, called Umkehrwalze D, allowing the Enigma operator to alter the connections as part of the key settings.

### Plugboard

The plugboard (Steckerbrett) was positioned at the front of the machine, below the keys. When in use during World War II, there were ten connections. In this photograph, just two pairs of letters have been swapped (A↔J and S↔O).

The plugboard (Steckerbrett in German) permitted variable wiring that could be reconfigured by the operator (visible on the front panel of Figure 1; some of the patch cords can be seen in the lid). It was introduced on German Army versions in 1930, and was soon adopted by the Navy. The plugboard contributed more cryptographic strength than an extra rotor. Enigma without a plugboard (known as unsteckered Enigma) can be solved relatively straightforwardly using hand methods; these techniques are generally defeated by the plugboard, driving Allied cryptanalysts to special machines to solve it.

A cable placed onto the plugboard connected letters in pairs; for example, E and Q might be a steckered pair. The effect was to swap those letters before and after the main rotor scrambling unit. For example, when an operator presses E, the signal was diverted to Q before entering the rotors. Up to 13 steckered pairs might be used at one time, although only 10 were normally used.

Current flowed from the keyboard through the plugboard, and proceeded to the entry-rotor or Eintrittswalze. Each letter on the plugboard had two jacks. Inserting a plug disconnected the upper jack (from the keyboard) and the lower jack (to the entry-rotor) of that letter. The plug at the other end of the crosswired cable was inserted into another letter's jacks, thus switching the connections of the two letters.

### Accessories

The Schreibmax was a printing unit which could be attached to the Enigma, removing the need for laboriously writing down the letters indicated on the light panel.
The Enigma Uhr attachment

Other features made various Enigma machines more secure or more convenient.[15]

Some M4 Enigmas used the Schreibmax, a small printer that could print the 26 letters on a narrow paper ribbon. This eliminated the need for a second operator to read the lamps and transcribe the letters. The Schreibmax was placed on top of the Enigma machine and was connected to the lamp panel. To install the printer, the lamp cover and light bulbs had to be removed. It improved both convenience and operational security; the printer could be installed remotely such that the signal officer operating the machine no longer had to see the decrypted plaintext.

Another accessory was the remote lamp panel Fernlesegerät. For machines equipped with the extra panel, the wooden case of the Enigma was wider and could store the extra panel. A lamp panel version could be connected afterwards, but that required, as with the Schreibmax, that the lamp panel and lightbulbs be removed.[10] The remote panel made it possible for a person to read the decrypted plaintext without the operator seeing it.

In 1944, the Luftwaffe introduced a plugboard switch, called the Uhr (clock), a small box containing a switch with 40 positions. It replaced the standard plugs. After connecting the plugs, as determined in the daily key sheet, the operator turned the switch into one of the 40 positions, each producing a different combination of plug wiring. Most of these plug connections were, unlike the default plugs, not pair-wise.[10] In one switch position, the Uhr did not swap letters, but simply emulated the 13 stecker wires with plugs.

### Mathematical analysis

The Enigma transformation for each letter can be specified mathematically as a product of permutations.[16] Assuming a three-rotor German Army/Air Force Enigma, let $P$ denote the plugboard transformation, $U$ denote that of the reflector, and $L, M, R$ denote those of the left, middle and right rotors respectively. Then the encryption $E$ can be expressed as

$E = PRMLUL^{-1}M^{-1}R^{-1}P^{-1}$.

After each key press, the rotors turn, changing the transformation. For example, if the right-hand rotor $R$ is rotated $i$ positions, the transformation becomes $\rho^iR\rho^{-i}$, where $\rho$ is the cyclic permutation mapping A to B, B to C, and so forth. Similarly, the middle and left-hand rotors can be represented as $j$ and $k$ rotations of $M$ and $L$. The encryption transformation can then be described as

$E = P(\rho^iR\rho^{-i})(\rho^{j}M\rho^{-j})(\rho^{k}L\rho^{-k})U(\rho^kL^{-1}\rho^{-k})(\rho^{j}M^{-1}\rho^{-j})(\rho^{i}R^{-1}\rho^{-i})P^{-1}$.

Combining three rotors from a set of five, the rotor settings with 26 positions, and the plugboard with ten pairs of letters connected, the military Enigma has 158,962,555,217,826,360,000 (158 quintillion) different settings.[17]

## Operation

German Kenngruppenheft (a U-boat codebook with grouped key codes)

In use, the Enigma required a list of daily key settings and auxiliary documents. The procedures for German Naval Enigma were more elaborate and more secure than those in other services. Navy codebooks were printed in red, water-soluble ink on pink paper so that they could easily be destroyed if they were endangered.

In German military practice, communications were divided into separate networks, each using different settings. These communication nets were termed keys at Bletchley Park, and were assigned code names, such as Red, Chaffinch, and Shark. Each unit operating in a network was assigned a settings list for its Enigma for a period of time. For a message to be correctly encrypted and decrypted, both sender and receiver had to configure their Enigma in the same way; rotor selection and order, starting position and plugboard connections must be identical. All these settings (together the key in modern terms) were established beforehand, distributed in codebooks.

An Enigma machine's initial state, the cryptographic key, has several aspects:

• Wheel order (Walzenlage) – the choice of rotors and the order in which they are fitted.
• Initial position of the rotors – chosen by the operator, different for each message.
• Ring settings (Ringstellung) – the position of the alphabet ring relative to the rotor wiring.
• Plug connections (Steckerverbindungen) – the connections of the plugs in the plugboard.
• In very late versions, the wiring of the reconfigurable reflector.

Note that although the ringstellung was a required part of the setup, they did not affect encryption because the rotors were positioned independently of the rings. The ring settings were only necessary to determine the initial rotor position based on the message setting that was transmitted at the beginning of a message, as described in the "Indicators" section, below. Once the receiver's rotors were set to the indicated positions, the ring settings no longer played any role.

In modern cryptographic language, the ring settings did not actually contribute entropy to the key used for encrypting the message. Rather, the ring settings were part of a separate key (along with the rest of the setup such as wheel order and plug settings) used to encrypt an initialization vector for the message. The session key consisted of the complete setup except for the ring settings, plus the initial rotor positions chosen arbitrarily by the sender (the message setting). The important part of this session key was the rotor positions, not the ring positions. However, by encoding the rotor position into the ring position using the ring settings, additional variability was added to the encryption of the initialization vector.

Enigma was designed to be secure even if the rotor wiring was known to an opponent, although in practice considerable effort protected the wiring configuration. If the wiring is secret, the total number of possible configurations has been calculated to be around 10114 (approximately 380 bits); with known wiring and other operational constraints, this is reduced to around 1023 (76 bits).[7] Users of Enigma were confident of its security because of the large number of possibilities; it was not then feasible for an adversary to even begin to try a brute force attack.

### Indicator

Most of the key was kept constant for a set time period, typically a day. However, a different initial rotor position was used for each message, a concept similar to an initialisation vector in modern cryptography. The reason is that encrypting many messages with identical or near-identical settings (termed in cryptanalysis as being in depth), would enable an attack using a statistical procedure such as Friedman's Index of coincidence.[18] The starting position for the rotors was transmitted just before the ciphertext, usually after having been enciphered. The exact method used was termed the indicator procedure. Design weakness and operator sloppiness in these indicator procedures were two of the main weakness that made cracking Enigma possible.

Figure 2. With the inner lid down, the Enigma was ready for use. The finger wheels of the rotors protruded through the lid, allowing the operator to set the rotors, and their current position, here RDKP, was visible to the operator through a set of windows.

One of the earliest indicator procedures was used by Polish cryptanalysts to make the initial breaks into the Enigma. The procedure was for the operator to set up his machine in accordance with his settings list, which included a global initial position for the rotors (the Grundstellung, meaning ground setting), say, AOH. The operator turned his rotors until AOH was visible through the rotor windows. At that point, the operator chose his own arbitrary starting position for that particular message. An operator might select EIN, and these became the message settings for that encryption session. The operator then typed EIN into the machine, twice, to allow for detection of transmission errors. The results were an encrypted indicator—the EIN typed twice might turn into XHTLOA, which would be transmitted along with the message. Finally, the operator then spun the rotors to his message settings, EIN in this example, and typed the plaintext of the message.

At the receiving end, the operation was reversed. The operator set the machine to the initial settings and typed in the first six letters of the message (XHTLOA). In this example, EINEIN emerged on the lamps. After moving his rotors to EIN, the receiving operator then typed in the rest of the ciphertext, deciphering the message.

The weakness in this indicator scheme came from two factors. First, use of a global ground setting—this was later changed so the operator selected his initial position to encrypt the indicator, and sent the initial position in the clear. The second problem was the repetition of the indicator, which was a serious security flaw. The message setting was encoded twice, resulting in a relation between first and fourth, second and fifth, and third and sixth character. This security problem enabled the Polish Cipher Bureau to break into the pre-war Enigma system as early as 1932. However, from 1940 on, the Germans changed procedure.

During World War II, codebooks were only used each day to set up the rotors, their ring settings and the plugboard. For each message, the operator selected a random start position, let's say WZA, and a random message key, perhaps SXT. He moved the rotors to the WZA start position and encoded the message key SXT. Assume the result was UHL. He then set up the message key, SXT, as the start position and encrypted the message. Next, he transmitted the start position, WZA, the encoded message key, UHL, and then the ciphertext. The receiver set up the start position according to the first trigram, WZA, and decoded the second trigram, UHL, to obtain the SXT message setting. Next, he used this SXT message setting as the start position to decrypt the message. This way, each ground setting was different and the new procedure avoided the security flaw of double encoded message settings.[19]

This procedure was used by Wehrmacht and Luftwaffe only. The Kriegsmarine procedures on sending messages with the Enigma were far more complex and elaborate. Prior to encryption the message was encoded using the Kurzsignalheft code book. The Kurzsignalheft contained tables to convert sentences into four-letter groups. A great many choices were included, for example, logistic matters such as refueling and rendezvous with supply ships, positions and grid lists, harbor names, countries, weapons, weather conditions, enemy positions and ships, date and time tables. Another codebook contained the Kenngruppen and Spruchschlüssel: the key identification and message key.[20]

The Army Enigma machine used only the 26 alphabet characters. Signs were replaced with rare character combinations. A space was omitted or replaced with an X. The X was generally used as point or full-stop.

Some signs were different in other parts of the armed forces. The Wehrmacht replaced a comma with ZZ and the question sign with FRAGE or FRAQ.

The Kriegsmarine replaced the comma with Y and the question sign with UD. The combination CH, as in "Acht" (eight) or "Richtung" (direction), was replaced with Q (AQT, RIQTUNG). Two, three and four zeros were replaced with CENTA, MILLE and MYRIA.

The Wehrmacht and the Luftwaffe transmitted messages in groups of five characters.

The Kriegsmarine, using the four rotor Enigma, had four-character groups. Frequently used names or words were varied as much as possible. Words like Minensuchboot (minesweeper) could be written as MINENSUCHBOOT, MINBOOT, MMMBOOT or MMM354. To make cryptanalysis harder, messages were limited to 250 characters. Longer messages were divided into several parts, each using a different message key.[21][22]

## History

The Enigma family included multiple designs. The earliest were commercial models dating from the early 1920s. Starting in the mid-1920s, the German military began to use Enigma, making a number of security-related changes. Various nations either adopted or adapted the design for their own cipher machines.

 A selection of seven Enigma machines and paraphernalia exhibited at the USA's National Cryptologic Museum. From left to right, the models are: 1) Commercial Enigma; 2) Enigma T; 3) Enigma G; 4) Unidentified; 5) Luftwaffe (Air Force) Enigma; 6) Heer (Army) Enigma; 7) Kriegsmarine (Naval) Enigma—M4.

### Commercial Enigma

Scherbius's Enigma patent—U.S. Patent 1,657,411, granted in 1928.
A rare 8-rotor printing Enigma.

On 23 February 1918, German engineer Arthur Scherbius applied for a patent for a cipher machine using rotors and, with E. Richard Ritter, founded the firm of Scherbius & Ritter. They approached the German Navy and Foreign Office with their design, but neither was interested. They then assigned the patent rights to Gewerkschaft Securitas, who founded the Chiffriermaschinen Aktien-Gesellschaft (Cipher Machines Stock Corporation) on 9 July 1923; Scherbius and Ritter were on the board of directors.

Chiffriermaschinen AG began advertising a rotor machine—Enigma model A—which was exhibited at the Congress of the International Postal Union in 1923–1924. The machine was heavy and bulky, incorporating a typewriter. It measured 65×45×35 cm and weighed about 50 kilograms (110 lb).

In 1925 Enigma model B was introduced, and was of a similar construction.[23] While bearing the Enigma name, both models A and B were quite unlike later versions: they differed in physical size and shape, but also cryptographically, in that they lacked the reflector.

The reflector—suggested by Scherbius's colleague Willi Korn—was introduced in Enigma C (1926).

Model C was smaller and more portable than its predecessors. It lacked a typewriter, relying on the operator; hence the informal name of "glowlamp Enigma" to distinguish it from models A and B.

The Enigma C quickly gave way to Enigma D (1927). This version was widely used, with shipments to Sweden, the Netherlands, United Kingdom, Japan, Italy, Spain, United States and Poland.

### Military Enigma

Enigma in use on the Russian front
Heinz Guderian in the Battle of France, with an Enigma machine

The Navy was the first military branch to adopt Enigma. This version, named Funkschlüssel C ("Radio cipher C"), had been put into production by 1925 and was introduced into service in 1926.[24]

The keyboard and lampboard contained 29 letters—A-Z, Ä, Ö and Ü—which were arranged alphabetically, as opposed to the QWERTZU ordering.[25] The rotors had 28 contacts, with the letter X wired to bypass the rotors unencrypted.[6]

Three rotors were chosen from a set of five[26] and the reflector could be inserted in one of four different positions, denoted α, β, γ and δ.[27] The machine was revised slightly in July 1933.[28]

By 15 July 1928,[29] the German Army (Reichswehr) had introduced their own version of the Enigma—the Enigma G, revised to the Enigma I by June 1930.[30] Enigma I is also known as the Wehrmacht, or "Services" Enigma, and was used extensively by German military services and other government organisations (such as the railways[31]), before and during World War II.

The major difference between Enigma I and commercial Enigma models was the addition of a plugboard to swap pairs of letters, greatly increasing cryptographic strength. Other differences included the use of a fixed reflector and the relocation of the stepping notches from the rotor body to the movable letter rings. The machine measured 28×34×15 cm (11 in×13.5 in×6 in) and weighed around 12 kg (26 lb).[32]

By 1930, the Army had suggested that the Navy adopt their machine, citing the benefits of increased security (with the plugboard) and easier interservice communications.[33] The Navy eventually agreed and in 1934[34] brought into service the Navy version of the Army Enigma, designated Funkschlüssel ' or M3. While the Army used only three rotors at that time, the Navy specified a choice of three from a possible five.[35]

In December 1938, the Army issued two extra rotors so that the three rotors were chosen from a set of five.[30] In 1938, the Navy added two more rotors, and then another in 1939 to allow a choice of three rotors from a set of eight.[35] In August 1935, the Air Force introduced the Wehrmacht Enigma for their communications.[30]

A four-rotor Enigma was introduced by the Navy for U-boat traffic on 1 February 1942, called M4 (the network was known as Triton, or Shark to the Allies). The extra rotor was fitted in the same space by splitting the reflector into a combination of a thin reflector and a thin fourth rotor.

There was also a large, eight-rotor printing model, the Enigma II. In 1933 the Polish Cipher Bureau detected that it was in use for high-level military communications, but that it was soon withdrawn, as it was unreliable and jammed frequently.[36]

The Abwehr used the Enigma G (the Abwehr Enigma). This Enigma variant was a four-wheel unsteckered machine with multiple notches on the rotors. This model was equipped with a counter which incremented upon each key press, and so is also known as the "counter machine" or the Zählwerk Enigma.

#### Other countries

Other countries used Enigma machines. The Italian Navy adopted the commercial Enigma as "Navy Cipher D". The Spanish also used commercial Enigma during their Civil War. British codebreakers succeeded in breaking these machines, which lacked a plugboard.[37] Not only militaries used the Enigma, they were also used by diplomatic services.

The Swiss used a version of Enigma called model K or Swiss K for military and diplomatic use, which was very similar to commercial Enigma D. The machine was cracked by Poland, France, the United Kingdom and the United States (the latter codenamed it INDIGO). An Enigma T model (codenamed Tirpitz) was used by Japan.

An estimated 100,000 Enigma machines were constructed. After the end of World War II, the Allies sold captured Enigma machines, still widely considered secure, to developing countries.[38] As these countries did not know that the machine had been broken, their supposedly secure communications were being read regularly by the major Western intelligence agencies.

## Surviving machines

US Enigma replica on display at the National Cryptologic Museum in Fort Meade, Maryland, USA.
A four–rotor Kriegsmarine (German Navy, 1935 to 1945) Enigma machine on display at the US National Cryptologic Museum

The effort to break the Enigma was not disclosed until the 1970s. Since then, interest in the Enigma machine has grown. Enigmas are on public display in museums around the world.

The Deutsches Museum in Munich has both the three- and four-rotor German military variants, as well as several civilian versions. Enigma machines are exhibited at the National Codes Centre in Bletchley Park, the Government Communications Headquarters, the Science Museum in London, the Polish Institute and Sikorski Museum in London, the Polish Army Museum in Warsaw, the Armémuseum (Swedish Army Museum) in Stockholm, the National Signals Museum in Finland, and at the Australian War Memorial and in the foyer of the Defence Signals Directorate, both in Canberra, Australia.

In the United States, Enigma machines can be seen at the Computer History Museum in Mountain View, California, and at the National Security Agency's National Cryptologic Museum in Fort Meade, Maryland, where visitors can try their hand at enciphering and deciphering messages. Two machines that were acquired after the capture of U-505 during World War II are on display at the Museum of Science and Industry in Chicago, Illinois. The San Diego State University Library has a machine. A four rotor device is on display in the ANZUS Corridor of the The Pentagon on the second floor, A ring, between corridors 9 and 10. This machine is on loan from Australia.

In Canada, a Swiss Army issue Enigma-K, is in Calgary, Alberta. It is on permanent display at the Naval Museum of Alberta inside the Military Museums of Calgary. A 3-rotor Enigma machine is on display at the Military Communications and Electronics Museum at Canadian Forces Base (CFB) Kingston in Kingston, Ontario.

Occasionally, Enigma machines are sold at auction; prices have in recent years ranged from US$40,000[39][40] to US$203,000[41] in 2011. Replicas are available in various forms, including an exact reconstructed copy of the Naval M4 model, an Enigma implemented in electronics (Enigma-E), various simulators and paper-and-scissors analogues.

A rare Abwehr Enigma machine, designated G312, was stolen from the Bletchley Park museum on 1 April 2000. In September, a man identifying himself as "The Master" sent a note demanding £25,000 and threatening to destroy the machine if the ransom were not paid. In early October 2000, Bletchley Park officials announced that they would pay the ransom, but the stated deadline passed with no word from the blackmailer. Shortly afterward, the machine was sent anonymously to BBC journalist Jeremy Paxman, missing three rotors.

In November 2000, an antiques dealer named Dennis Yates was arrested after telephoning The Sunday Times to arrange the return of the missing parts. The Enigma machine was returned to Bletchley Park after the incident. In October 2001, Yates was sentenced to 10 months in prison and served three months.[42]

In October 2008, the Spanish daily newspaper El País reported that 28 Enigma machines had been discovered by chance in an attic of Army headquarters in Madrid. These 4-rotor commercial machines had helped Franco's Nationalists win the Spanish Civil War because, though the British cryptologist Alfred Dilwyn Knox in 1937 broke the cipher generated by Franco's Enigma machines, this was not disclosed to the Republicans, who failed to break the cipher. The Nationalist government continued using its 50 Enigmas into the 1950s. Some machines have gone on display in Spanish military museums,[43][44] including one at the National Museum of Science and Technology (MUNCYT) in A Coruña. Two have been given to Britain's GCHQ.[45]

The Bulgarian military used Enigma machines with a Cyrillic keyboard; one is on display in the National Museum of Military History in Sofia.[citation needed]

## Derivatives

The Enigma was influential in the field of cipher machine design, spinning off other rotor machines. The British Typex was originally derived from the Enigma patents; Typex even includes features from the patent descriptions that were omitted from the actual Enigma machine. The British paid no royalties for the use of the patents, to protect secrecy. The Typex implementation is not the same as that found in German or other Axis versions.

A Japanese Enigma clone was codenamed GREEN by American cryptographers. Little used, it contained four rotors mounted vertically. In the U.S., cryptologist William Friedman designed the M-325, a machine logically similar, although not in construction.

A unique rotor machine was constructed in 2002 by Netherlands-based Tatjana van Vark. This device makes use of 40-point rotors, allowing letters, numbers and some punctuation to be used; each rotor contains 509 parts.[46]

Machines like the SIGABA, NEMA, Typex and so forth, are deliberately not considered to be Enigma derivatives as their internal ciphering functions are not mathematically identical to the Enigma transform.

Several software implementations exist, but not all exactly match Enigma behavior. The most commonly used software derivative (that is not compliant with any hardware implementation of the Enigma) is at EnigmaCo.de. Many Java applet Enigmas only accept single letter entry, complicating use even if the applet is Enigma compliant. Technically, Enigma@home is the largest scale deployment of a software Enigma, but the decoding software does not implement encipherment making it a derivative (as all original machines could cipher and decipher).

A user-friendly 3-rotor simulator, where users can select rotors, use the plugboard and define new settings for the rotors and reflectors is available.[47] The output appears in separate windows which can be independently made "invisible" to hide decryption.[48] Another includes an "autotyping" function which takes plaintext from a clipboard and converts it to cyphertext (or vice-versa) at one of four speeds. The "very fast" option produces 26 characters in less than one second.[49]

## In popular culture

• The play Breaking the Code by Hugh Whitemore focuses on the life and death of Alan Turing, who was the central force in continuing to break the Enigma in the United Kingdom during World War II. Turing was played by Derek Jacobi, who also played Turing in a 1996 television adaptation of the play.
• Robert Harris's 1995 novel Enigma is set against the backdrop of World War II Bletchley Park and cryptologists working to read Naval Enigma in Hut 8. The book, with substantial changes in plot, was made into the 2001 film Enigma, directed by Michael Apted and starring Kate Winslet and Dougray Scott. The film was criticized for historical inaccuracies, including neglect of the role of Poland's Biuro Szyfrów. The film—like the book—makes a Pole the villain, who seeks to betray the secret of Enigma decryption.[50]
• An earlier Polish film dealing with Polish aspects of the subject was the 1979 Sekret Enigmy, whose title translates as The Enigma Secret.[51]
• Wolfgang Petersen's 1981 film Das Boot includes an Enigma machine which is evidently a four-rotor Kriegsmarine variant. It appears in many scenes. The plot of U-571, released in 2000, revolves around an attempt by American, rather than British, forces to seize an Enigma machine from a German U-boat.
• Neal Stephenson's novel Cryptonomicon prominently features the Enigma machine and efforts to break it, and portrays the German U-boat command under Karl Dönitz using it in apparently deliberate ignorance of its penetration.

## References

### Notes

1. ^ Singh, Simon (1999). The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography. London: Fourth Estate. p. 127. ISBN 1-85702-879-1.
2. ^ Lord, Bob (1998–2010). "1937 Enigma Manual by: Jasper Rosal - English Translation". Retrieved 31 May 2011.
3. ^ Gordon Welchman, who became head of Hut 6 at Bletchley Park, has written: "Hut 6 Ultra would never have gotten off the ground if we had not learned from the Poles, in the nick of time, the details both of the German military version of the commercial Enigma machine, and of the operating procedures that were in use." Gordon Welchman, The Hut Six Story, 1982, p. 289.
4. ^ Much of the German cipher traffic was encrypted on the Enigma machine, and the term "Ultra" has often been used almost synonymously with "Enigma decrypts". Ultra also encompassed decrypts of the German Lorenz SZ 40 and 42 machines that were used by the German High Command, and decrypts of Hagelin ciphers and other Italian ciphers and codes, as well as of Japanese ciphers and codes such as Purple and JN-25.
5. ^ a b
6. ^ a b
7. ^ a b Miller, A. Ray (2001). The Cryptographic Mathematics of Enigma. National Security Agency.
8. ^ Bletchley Park veteran and historian F.H. Hinsley is often cited as an authority for the two-year estimate, yet his assessment in Codebreakers is much less definitive: "Would the Soviets meanwhile have defeated Germany, or Germany the Soviets, or would there have been stalemate on the eastern fronts? What would have been decided about the atom bomb? Not even counter-factual historians can answer such questions. They are questions which do not arise, because the war went as it did. But those historians who are concerned only with the war as it was must ask why it went as it did. And they need venture only a reasonable distance beyond the facts to recognise the extent to which the explanation lies in the influence of Ultra." F.H. Hinsley, "Introduction: The Influence of Ultra in the Second World War," Codebreakers: The Inside Story of Bletchley Park, edited by F.H. Hinsley and Alan Stripp, Oxford University Press, 1993, pp. 12–13.
9. ^ "Code Breaking – World War 2 on History". History.co.uk. Retrieved 2012-07-17.
10. ^ a b c Rijmenants, Dirk; Technical details of the Enigma machine Cipher Machines & Cryptology
11. ^ a b c Hamer, David (January 1997). "Enigma: Actions Involved in the 'Double-Stepping' of the Middle Rotor" (zip). Cryptologia 21 (1): 47–50. doi:10.1080/0161-119791885779. Archived from the original on 2011-07-19.
12. ^ Sale, Tony. "Technical specifications of the Enigma rotors". Technical Specification of the Enigma. Retrieved 15 November 2009.
13. ^ "Lückenfüllerwalze". Cryptomuseum.com. Retrieved 2012-07-17.
14. ^ Philip Marks, "Umkehrwalze D: Enigma's Rewirable Reflector — Part I", Cryptologia 25(2), April 2001, pp. 101–141
15. ^ Reuvers, Paul (2008). "Enigma accessories". Retrieved 22 July 2010.
16. ^
17. ^
18. ^ Friedman, W.F. (1922). The index of coincidence and its applications in cryptology. Department of Ciphers. Publ 22. Geneva, Illinois, USA: Riverbank Laboratories. OCLC 55786052.
19. ^ Rijmenants, Dirk; Enigma message procedures Cipher Machines & Cryptology
20. ^ Rijmenants, Dirk; Kurzsignalen on German U-boats Cipher Machines & Cryptology
21. ^ "The translated 1940 Enigma General Procedure". codesandciphers.org.uk. Retrieved 16 October 2006.
22. ^ "The translated 1940 Enigma Officer and Staff Procedure". codesandciphers.org.uk. Retrieved 16 October 2006.
23. ^
24. ^ Kahn 1991, pp. 39–41, 299.
25. ^ Ulbricht 2005, p. 4.
26. ^ Kahn 1991, pp. 40, 299.
27. ^ Bauer 2000, p. 108.
28. ^ Stripp 1993, plate 3.
29. ^ Kahn 1991, pp. 41, 299.
30. ^ a b c Kruh & Deavours 2002, p. 97.
31. ^ Michael Smith Station X, Channel Four books (Macmillan) 1998, Paperback 2000, ISBN 0-7522-7148-2, Page 73
32. ^ Stripp, 1993
33. ^ Kahn 1991, p. 43.
34. ^ Kahn 1991, p. 43 says August 1934. Kruh & Deavours 2002, p. 15 say October 2004.
35. ^ a b Kruh & Deavours 2002, p. 98.
36. ^ Kozaczuk 1984, p. 28.
37. ^ Smith 2006, p. 23.
38. ^ Bauer 2000, p. 112.
39. ^ Hamer, David; Enigma machines – known locations*
40. ^
41. ^ Christi's; 3 Rotor enigma auction
42. ^ "Man jailed over Enigma machine". BBC News. 19 October 2001. Retrieved 2 May 2010.
43. ^ Graham Keeley. Nazi Enigma machines helped General Franco in Spanish Civil War, The Times, 24 October 2008, p. 47.
44. ^ "Taller de Criptografía - Enigmas españolas". Cripto.es. Retrieved 2013-09-08.
45. ^ Posted on March 26, 2012 at 6:38 AM • 23 Comments (2012-03-26). "Schneier on Security: Rare Spanish Enigma Machine". Schneier.com. Retrieved 2013-09-08.
46. ^ van Vark, Tatjana The coding machine
48. ^ Enigma at Multimania
50. ^ Laurence Peter, How Poles cracked Nazi Enigma secret, BBC News, 20 July 2009
51. ^

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