Specification

Before going into more detail, it has to be noted that there is a variety of machines with different characteristics that can render them incompatible to each other. This does not mean that the working principle is affected. All the machines work under the exact same principle, even with G31 variants which appears to apply some apparent irregularity. They also share the same set of components with the exception of I variants that also incorporates the Plugboard. For the purpose of illustrating, a standard three rotor machine has been selected for this example. It features the most common characteristics found in a wide range of machines. The Enigma machine combines electrical and mechanical parts.

Initially the enciphering process will be explained; it presents the method of converting alphabetic characters using the Enigma encryption methodology. It provides an insight as to what distinguished the machine from its rivals. This is followed by the mechanism construction which accounts for the stepping procedures generating a different encoding of each character. The mechanism is presented as the heart of the device, and further discussion is extended over the other vital components that connect to it.

Components

The machine features components that remained throughout its production. Out of all the components only the rotors are removable, in some occasions the reflector is removable as well. A number of rotors are interchangeable between some machines. Although each variant has exact specifications that are noticeable to a trained eye, there are many examples of the same machines with different manufacturing characteristics. This was due to different companies manufacturing similar machines. In some cases, that was also due to the time period they were manufactured in. Initially the machine mostly incorporated die-cast components and was very robust. As the war raged on, Germany started having a limited recourse of raw materials. There are a number of variants found depicting this, on these machines many components are constructed from Bakelite, an early plastic. This is reflected to all extends where same variants have been found with components constructed out of a different material.

Stator

The stator is a type of static wheel which was originally named the Entrittswalze (ETW). It is the entry and exit point of the signal passing through the rotors and the reflector. It does not move and is placed on the rightmost side of the mechanism, before the first rotor placed on the right position.

The only purpose of the stator is simply to transfer the signal into the rotors from the wires connected to the keys and mechanism. It has twenty-six contacts on its left side. Most Enigma's have the same ordered stator. It is ordered in the same way as the keyboard from Q to Z. Some models like the M4 have the stator arranged in an alphabetical order. The Tirpitz Enigma is the only model which has the stator ordered in a randomised fashion.

Rotor

The rotors are the most significant part of the Enigma. They are the actual components that carry out most of the cryptographic procedure. Its actual name was Walze which can be defined as Wheel or Rotor. Most machines where supplied with three rotors. Some machines have a variety which ranges up to eight rotors. In cases the rotors would be rewired to increase security.

Each rotor has some distinctive characteristics. It has twenty-six positions from A to Z. On either side there are also 26 contacts aligned in a circular fashion. They are interconnected to the other side in a specific randomised fashion. The index of the rotor is engraved along the side. It is either marked as A to Z or 01 to 26. In addition, each rotor has a ring that can be used to offset the wiring arrangement. It is regarded as creating an offset in the opposite direction. A final component is the notches which are fixed on the side of the rotor and cannot be altered.

Most rotors present exactly the same characteristics. The obvious differences are the offset ring construction and index labelling. They had the same size and notch construction to accommodate ratchets and pullers. In Type G machines the notch construction is altered to endorse the cog-wheel mechanism. The Enigma G-312 had a smaller body which implies that the rotors were smaller as well. This renders them incompatible with any other variant. A final difference is found on the Enigma M4 where there is a fourth thin rotor. As the name suggests it is slimmer than any other type of rotor. These rotors are not exchangeable with any other rotor or position in the mechanism.

Reflector

The reflector is a component which is similar to a rotor but on most cases it does not move. Its original name was Umkehrwalze (UKW). It completes the cryptographic process by reflecting the signal back to the rotors extending the encryption cipher. Another characteristic is that it only has twenty-six contacts on the right side. It does not feature contacts on the other side. This means that thirteen contacts are connected and reflected back through the remaining thirteen contacts. So when a character is entered through the leftmost rotor it is reflected back into it. This gives a vital characteristic to the machine. As it is now self-reciprocated. Some models allow the reflector to be set to one of twenty-six positions but it does not move during enciphering. Some machines also allowed the reflector to be replaced by another compatible one.

In 1944, UKW-D was released. It is based on the same principle as all other reflectors. The major difference came with the ability to rewire the reflector. This was an accessory that proved to be very powerful by introducing further complexity. It did not have a great effect since it was introduced very late, almost at the end of the war.

The two known models that were compatible with the UKW-D were the Enigma KD and M4. It came as a standard with KD, where the original reflector and the left most rotor was replaced by it. Enigma M4 is a similar story; in order to fit the UKW-D, the original thin reflector and thin rotor are removed and exchanged by it. In this sense M4 can now be regarded as a three rotor machine.

Comparing the functionality of common reflectors to the UKW-D it is obvious that they have exactly the same functionality. The differences appear as the wiring can actually be altered without the need of tools. The reflector has a cover that can be removed. Inside that there are twelve tightly packed cables that interconnect the twenty six contact points of the right side of reflector.

Although there are twenty six contacts, only twenty four contacts are available to wire. This is due to the fact that there are space restrictions. Two contacts are missing and on their place are retainers for holding the cover. The contacts that are missing are J and Y which are permanently interconnected. The contacts are arranged from A to Z in the ordinary Enigma fashion with the two contacts described, simply missing. On this base, the reflector can be wired in many ways and therefore create a far more complex and unpredictable machine.

Cryptographic Process

The mechanism incorporates a range of components to carry out the cryptographic procedure. When a key is pressed, an electric signal passes through all components. In this process, the character is translated from point to point, until it reaches the lamp panel where it is displayed. The actual cryptographic components required to complete the process are three. The electric signal enters the Stator where it is translated to a contact; it then passes through all the Rotors, reaching the Reflector. The Reflector then passes the signal back to the opposite direction where it makes a trip through the same components in reverse. In Type I, there is one further component to this process. This component is the Plugboard that is placed right before the Stator in the process discussed above.

In order to create a communication channel, the same settings are required on both sides of the correspondence. This is to say that when messages are due to either be encrypted or decrypted, the machines require the same Key. This theoretical key is needed because both machines must be set up in the exact same way at the start of each procedure.

Characters are mostly encoded by a number of Rotors. These Rotors are rotatable and carry twenty-six contact points on each side. Each contact belonging to one side is connected by random fashion to another contact of the other side. When each key is pressed, the right most rotor advances by one step, resulting in a different mapping of the wires and subsequently the way a character is translated. The advancing of the rotors occurs before the letter is enciphered. On every key press, the first action is for the rotors to advance and then to allow a trace of electricity to pass inside the machine.

A key press can be further explained as it has two basic states. The first state can be referred to, as a half press and the second half, as a full press. Upon a key press, the first half will complete all the stepping procedures required and when a full press is achieved the circuit will close, allowing electricity to flow. The signal containing the character passes to the static rotor where it is simply transformed into a contact point. From this position the signal transfers through all the rotors where it is enciphered according to the current arrangement of the wiring. It then reaches the reflector where the signal is sent back through the rotors and out to the static rotor. From that point the signal containing the ciphered character transfers to the lamp panel. A specific lamp switches on to display the translated value of the character that was entered.

The machine incorporates two very important characteristics that are core values of the Enigma's internal design. The first characteristic is that a character can never be encoded to itself. In this way, A cannot be encoded to A, which reduced the strength of the device. Another crucial characteristic is self-reciprocity; mostly accredited to the reflectors functionality in the enciphering process. This implies that messages can be encrypted or decrypted using the same settings. The basic working principle behind the Enigma has been discussed in great extend by Crypto Museum (2009).

Stepping Process

The stepping of the Enigma is similar to the one of an odometer. The keys control the rotor movement. On a key press the rightmost rotor advances by one position before the circuit closes and the electric current is turned on. For this reason the rightmost rotor is also referred to as the quick or fast rotor. If a turnover point is present on the next position, it will move the rotor to its left. In the same way the middle rotor can also be described as the normal rotor. Finally, the left rotor can be described as the slow rotor. This is not the case with Type G where rotors have multiple turnover points and individual cog-wheels. An anomaly has also been observed on traditional mechanisms which further distinguish it from an odometer.

All of the machines use three rotors. The naval M4 is the only exception having a unique fourth rotor which was slimmer and incompatible with all others. A number of models have a settable reflector that acts like an extra rotor. This can create the misconception that they are indeed a four rotor machine. The reflector is not movable during the stepping procedure though, its functionality is to provide twenty six settable positions. Type G variants are the only machines that allow the rotation of the reflector during stepping procedures. It can therefore be considered as the only four rotor machine since all rotors are moved as part of the process. To sum up, whether the reflector is movable or not they are still a three rotor machine constituting the M4 as the only true example of a four rotor machine.

Upon inspecting the mechanism, similarity to the principle of an odometer mechanism is evident. As a basic principle, when the rightmost rotor has completed a full revolution around itself, the rotor to its left will step along with it. The same process is further extended between the middle and left rotor. Some rotors have multiple turnover points that alter that process. These are accountable for the rotor movement. The rotors advance in a counter clockwise fashion. If letter A was visible in the window then the letter B will be the next visible character.

Most machines operate traditionally by pawls and ratchets. This is the basic part of the stepping process that uniquely identifies the Enigma. The first characteristic is visible when a rotor has more than one turnover positions. This produces some irregularity to the stepping process. The rotor to the left advances more frequently for each full revolution of the rotor to its right. Type G works in the same principle but operates with a gearbox mechanism that used cog-wheels rather that the standard pawls and ratchets to operate the rotor stepping.

These alterations distinguish the process of the Enigma between Type G and the other two categories. Added to these characteristics there is machine anomaly that have been observed and recorded. It only affects mechanisms operating with pawls and ratchets and can be considered as a factor which reduces the strength of the enciphering process. This affects all machines operating under that principle. Type G does not suffer from this anomaly but presents another effect which does not reduce the strength of the machine.

Double-Stepping Anomaly

The Double Stepping anomaly is observed on machines that are controlled by pawls and ratchets. It does not occur in Type G machines which are controlled by cog-wheels and a gearbox like mechanism. When this event occurs, the middle rotor will step twice on two consecutive key presses. This is to say that when the middle rotor is on its own turnover position it will step twice on two consecutive key presses. The example below illustrates the occurrence and its outcome, as it would have been visible through the rotor windows of an Enigma.

IIIIII
ADO
ADP
ADQ
AER
BFS
BFT
BFU

This mechanical anomaly is explained in great detail by David Hamer (1997).

To achieve this effect, the rotors must be arranged in a specific way. Rotor I, is placed on the fast right position and causes the next rotor to step when it rotates from Q to R. Rotor II is placed on the normal middle position and causes the next rotor to step when it changes from E to F. Finally, Rotor III is placed on the slow left position. When the right rotor advances from Q to R, it causes the middle rotor to advance from D to E. On the next step, the right rotor advances from R to S and the middle rotor advances again from E to F. At the same time, the middle rotor causes the left rotor to advance by one step. This is caused due to the internal mechanic functionality of the stepping levers, controlling the ratchets and pullers.

This is a mechanical anomaly that lies in the laws of the manufacturing principles of the mechanism. As a digital world entity, all real life obstacles are removed. Double stepping presents a mechanical anomaly that is not reflected in theory. The simulator replica implemented does not present this anomaly.

Although such event presents and improvement that increases the cryptographic strength, it is not a positive result as it distinguishes it from the actual operation of the Enigma. In order to asses this issue, a method has been implemented that recreates the anomaly. This is another step to implementing the simulator as close as possible to a true Enigma.

Lobster Effect

The Lobster Effect describes an event that occurs only in cog-wheel driven machines such as the Zählwerk Enigma and the rest of Type G variants. It does not occur on traditional Enigma's. When this event occurs, all four rotors will advance by one step. The example below illustrates the occurrence during stepping sequences and its outcome. It is displayed on an Enigma G-312 and is compared to the adjacent outcome of an Enigma M4 which does not present this effect.

G31 G–312M4
N—Z—A—MN—Z—A—M
N—Z—A—NN—Z—A—N
N—Z—A—ON—Z—A—O
N—A—B—PN—Z—A—P
N—A—C—QN—Z—A—Q
O—B—D—RN—Z—B—R
O—B—D—SN—Z—B—S
O—C—E—TN—Z—B—T
O—C—E—UN—Z—B—U
O—C—F—VN—Z—B—V
O—D—G—WN—Z—B—W

This effect is explained in great detail by David Hamer (2000).

Procedure

In order for the cryptographic process to work, the same settings are required on both sides of the correspondence. This is to say that when messages are due to either be encrypted or decrypted, the machines require the same Key. This theoretical key is needed because both machines must be set up in the exact same way at the start of each procedure.

For the machine to be set up, the following steps must apply:

  1. Set the position of the 3 rotors (rotor sequence) from left to right on the rotor shaft
  2. Set the number or letter rings on one of the 26 possible settings on each rotor(code wheel)
  3. Set the starting position of the 3 rotors. The number or letter should be visible through the 3 windows at the start of coding.
  4. Insert Double-plug Cables into the plugboard (Patch panel).Set the plug- in wire connections from 0-13 wires into any of the 26 sockets on plugboard

The machine is set to go!