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Enigma I
The Service Enigma Machine

Enigma I (Roman number 1) is an electromechanical cipher machine developed by Chriffrier­maschinen AG (later: Heimsoeth und Rinke) In Berlin (Germany) for the German Reichswehr (later: Wehrmacht) in 1927, and introduced in 1932. It is based on the chassis of the commercial Enigma D, but has a fixed reflector and a plugboard (Steckerbrett) at the front. The latter was a unique feature for the German Armed Forces. The machine was used throughout WWII and is known under various names. It is officially known as Enigma I and by its factory designator: Ch.11a.
Initially, the machine was supplied with three coding wheels, that could be inserted in any of 6 possible orders (3 x 2 x 1). In December 1938, two additional wheels were supplied, bringing the total number of possible wheel orders to 60 (5 x 4 x 3), a 10-fold increase in cipher security!

It is the Steckerbrett however that increases the total number of combinations dramatically. The image on the right shows a typical Enigma I that was used by the German Army. It was found as lost luggage in a train in Italy at the end of WWII and is shown here open top lid and open flap.
Click to enlarge

The Enigma I was used by both the Heer (Army) and the Luftwaffe (Air Force). It was later also adopted by the Kriegsmarine (German Navy) where it became known as the M1, M2 and finally the M3. The only obvious differency between the Army version and the Navy version is that the wheels of the latter have letters (A-Z) rather than numbers. About 20,000 machines of this type were manufactured by various manufacturers, but only a handful have eventually survived.
Enigma-I closed Enigma-I closed Enigma-I front view, flap open Enigma-I front view Enigma I with top lid and flap open Enigma-I top view Protection against the sun
Lifting the wheel cover Setting the start position (Grundstellung) Setting the brightness of the lamps Using an external 4V source View at the plug board (Steckerbrett) Keyboard and lamp panel Encrypting a letter Using the hinges in the alternate position

Below is the simplyfied circuit diagram of the Enigma I. At the right is the keyboard and the lamp panel. At the far right is the battery. When a key is pressed, the current flows from the battery through one of the switches (i.e. a letter key), the Steckerbrett and the cipher wheels, until it hits the reflector at the left. The current is then returned through the wheels and the Steckerbrett after which a lamp is lit. In the example below, the letter 'Q' is pressed after which the 'E' lights up.
Simplified circuit diagram of a 3-wheel Service Enigma

Each time a key is pressed, the rightmost wheels makes a single step which effectively alters the wiring of that wheel. After the wheel has made a full revolution, it will cause the wheel to its left to make a single step, much like an odometer in a car. In the description on this page it is assumed that you are familiar with the operating principle of the Enigma. If you are not, click here.

 Enigma working principle
The diagram below shows the various features of the Enigma I. The machine is shown here with the top lid and the front flap open, ready for use. The machine is powered by a 4.5V battery and is turned on with the large rotary switch to the right of the cipher wheels. To the right of this switch are two screw terminals that allow the machine to be powered by an external source.

Enigma I controls and connections

The machine has three electromechanical cipher wheels (selected from a set of 5 wheels), each with 26 contacts at either side. The layout of the keyboard and the lamp panel is in the standard QWERTZ order. As the Enigma I is a military machine, it has a plugboard at the front, covered by a wooden flap that has to be closed during operation to ensure that all plugs are fully inserted.

5 cipher wheels. Click for the wiring details.

Extra wheels   IV and V
From 15 December 1938 onwards, each Enigma I was supplied with five coding wheels instead of three, of which three were in the machine at any given time. Each day, the operator would place the three chosen wheels in the machine in a particular order, as instructed by the codebook.
The remaining two (unused) wheels were stored in a small wooden box. The image on the right shows a typical example of such a storage box with the two wheels each held by a spindle. Although other types of boxes are known to exist, this was the most commonly used model.

Click the image for a closer look. As you can see, wheels I and V are currentlty stored inside the wooden box, which means that the remaining wheels (II, III and IV) are currently fitted inside the machine. With 3 out of 5 used, the number of possible wheel orders is 5 x 4 x 3 = 60.
Click to enlarge

Each wheel has a single notch on its circumference. Whenever the wheel reaches the position of the notch, a pawl is engaged. This pawl then causes the next wheel (i.e. the wheel to the left of the current wheel) to make a single step. This movement is called Enigma stepping. The position of the notch is different on each wheel (see the wiring table below).

One of the disadvantages of having just one notch on each cipher wheel, is that wheel stepping will be very regular and can therefore more easily be predicted. Other machines, such as the Enigma G and the Tirpitz (Enigma T), featured multiple notches and had therefore an irregular (less predictable) wheel motion. Such machines lacked the additional Steckerbrett however.

In 1926, the German Army - then called the Reichswehr - adopted the lamp-based Enigma machine, or Glühlampenmaschine as it was called. In early 1927, the first machine with an single-ended Steckerbrett (plug board) was developed. It allowed each letter of the alphabet to be transposed any other letter and can be seen as the equivalent of a freely rewirable wheel that does not move. Experiments showed that it was too easy to make mistakes with these cables.

Later that year the final version, with an improved double-ended Steckerbrett was released. It was based on the chassis of the Enigma D and was given the internal designator Ch. 11a. The first batch of machines (approx. 600 units) were delivered on 10 December 1927. The Reichswehr called this machine the Enigma I. All further German Army Enigma machines that were built before and during the war, would be based on the Ch.11a.

 Full history

Enigma in use. Click to enlarge. Copyright unknown.

The photograph above shows an Enigma machine in operation, and was probably taken inside a radio van during WWII. The image was scanned from an extremely small 27 x 38 mm original and was digitally cleaned up and enhanced. More wartime Enigma photographs are available here...
In order to replace the battery or to change the daily key (Grundstellung) of the Enigma, the top lid of the machine needs to be opened. A rigged bolt with a red circle on top at either side of the lamp panel, should be loosened in order to raise the hinged top lid and access the interior.
After raising the top lid, the wheel mechanism and the lamp panel are exposed. The image on the right gives a good view at the drum. At the right is the entry wheel, or Eintrittswalze (ETW). Left of the ETW are the three cipher wheels and at the far left is the reflector, or Umkehrwalze (UKW), here marked with a red letter B.

Just visible at the bottom right is the interior of the power switch. It is activated by the knob that is attached to the top lid. Just above the power switch is a black box that holds the 4.5V battery. The box is closed with a lid and a small lock.
Close-up of the wheels and the lamp panel

Below the wheels is the lamp panel. It has 26 light blubs arranged in 3 rows. They are laid out in the same order as the keyboard. Note that there are two additional lamp sockets; one at either side of the middle row. The extra socket at the right is marked Lampenprüfung (lamp test). It can be used to quickly test a lamp without typing on the keyboard, simply by pressing it down.

The extra socket at the left is marked Kabelprüfung (cable test). If a lamp it fitted in this socket, it can be used to quickly test a patch cable. On the Steckerbrett an extra single-ended socket is present at either end of the middle row of plugs. The extra sockets are marked with a red dot. Now hold the tick pin of one plug to the leftmost socket, and the thin pin of the other one to the socket at the right. If the wire is OK, the cable-test lamp should light up. Then test the other wire.

Note that the light bulbs have an ordinary E10 fitting, but that the glass bulb itself is flattened. Lamps of this time were commonly used for bicycles and flash lights in those days. Although it is possible to replace them by ordinary round bulbs, this is not recommended as they will damage the letter-film that is mounted in the top lid. Never use ordinary light bulbs in an Enigma!

 More about the lamps
Enigma-I interior Close-up of the wheels and the lamp panel Power switch Lamp panel (interior) Activated light bulb on the lamp panel Lamp test Testing a lamp Cable test lamp

Setting the key
Sending a message with an Enigma machine involves the setting of two cryptographic keys: (1) the daily key and (2) the message key. Initially, the daily key was setup once a day at midnight and was valid for 24 hours, hence its name. Later in the war it was changed more frequently.
Setting the daily key (Grundstellung) involved a number of steps. First the top lid of the machine should be opened in order to access the interior. Next the UKW should be released and shifted aside, so that the wheel-set can be pressed together and removed from the machine.

Now that the wheel-set is out of the machine, the wheels are removed from the axle. Next, the user would select the appropriate three wheels for the new key and set the index ring as shown in the codebook. The index ring can be released by lifting a spring-loaded pin on its side.
Pressing the wheels together

The wheels are now put back on the spindle in the order given in the codebook, and then placed in the machine again. After locking the UKW again, the top lid can be closed. The wheels should now be set to the required start position, which is visible through the windows in the top lid.

Now that the wheels are setup correctly, the plugboard (Steckerbrett) needs to be configured as per codebook. This involves removing all patch cables and re-inserting them as indicated. The machine is now ready for use. For each new message, the operator had to select a unique message key, consisting of three randomly selected letters. The exact method for setting the message was changed several times during the war and is beyond the scope of this page.
Close-up of the wheels and the lamp panel Releasing the UKW Shifting the UKW aside Pressing the wheels together Removing the axle with the wheels Empty wheel-space Umkehrwalze (UKW) Eintrittswalze (ETW)
Wheelset The wheels removed from the spindle The three wheels removed from the spindle Wheel number 4 showing its 26 spring-loaded contacts Locating the Ringstellung Releasing the ring Setting the start position (Grundstellung) Removing a plug (Stecker)

The double-ended Steckerbrett had the advantage that it would swap the letters in pairs and that the sockets had a built-in switch. If no swapping was required, the cable could simply be left out. This greatly improved setup time and reduced the chance of mistakes when setting the daily key.

Each Enigma comes with 12 cables: 10 to be used on the Steckerbrett and two spares that are stored in the top lid of the case. Each patch cable as a 2-pin plug at either side. Each plug has a thick and a thin pin, so that it can not be inserted the wrong way around. The cable crosses the connection between the plugs. The thick pin of one plug is connected to the other's thin one.
The image on the right shows a double-ended plug with a thick and a thin pin. Swapping the letters in pairs means that if A is transposed into Z, the reverse is also true: Z is transposed into A. This is called self-reciprocity. Compared to the single-ended Steckerbrett, this reduces the total number of possible wire combinations.

The same self-reciprocity was exploited by Gordon Welchman when improving Turing's Bombe, resulting in shorter Bombe-runs when breaking the Enigma's daily keys. It effectively eliminated the Steckerbrett from the equasion.
Double-ended plug (Stecker)

With 26 letters, and hence 26 sockets on the Steckerbrett, a maximum of 13 patch cables could be installed. Any number of cables between 0 and 13 was possible and the maximum number of combinations would be reached when the number of patch cables was different each day. In practice however, the German operation procedure generally instructed the use of 10 cables.

 More about the Steckerbrett
View at the plug board (Steckerbrett) Close-up of the Steckerbrett Removing a plug (Stecker) Placing a plug (Stecker) Testing a cable Rightmost test socket Patch cable Double-ended plug (Stecker)

Transport case
Most Enigma I machines came in a (oak) wooden transport case, such as the one shown in the images above, in which they were bolted to the bottom. Such cases could be caried by a leather strap that was fitted at the rear.

Please note that not all wooden boxes had a lether carrying strap though. Some cases were fitted with a metal grip, similar to the ones used with the Enigma M4. The wooden boxes supplied by Enigma manufacturer Ertel Werk in München, had a canvas strap fitted to the right side of the box, as shown in the image on the right.

Furthermore, most machines supplied by Ertel Werk were fitted inside a so-called Panzerholz case, consisting of a metal base plate with the Enigma, and a removable black cover.
Rare oak wooden transport case fro Ertel Werk, with canvas carrying strap.

Below is the wiring for each wheel, the ETW and all three known UKWs. UKW-A was used before WWII [1]. UKW-B was the standard reflector during the war and UKW-C was only temporarily used during the war. The wiring of all 5 wheels is identical to the wiring of the first 5 wheels of the Enigma M3 (used by the Kriegsmarine) and the U-Boot Enigma M4.

  1. Wiring recovered by Philip Marks and Frode Weierud in 2000 [1].

Norway Enigma
In 1945, immediately after WWII, some captured Enigma-I machines were used by the the former Norwegian Police Security Service: Overvaakingspolitiet. They modified the wheel wiring and the wiring of the Umkehrwalze (UKW, reflector). The wiring of the Eintrittzwalze (ETW, entry wheel) and the position of the turnover notches on the wheels were left unaltered. A machine that is modified in this way, is often called Norway Enigma or Norenigma as suggested by Frode Weierud in 2001 in order to discriminate between the standard and the modified wiring [2].

Enigma Uhr
During WWII, the Germans made several attempts to make the Enigma more secure.

In July 1944, the German Luftwaffe came up with a smart device called Enigma Uhr, which was introduced without any prior warning whatsoever. It was attached to the side of an Enigma I and connected directly to the Steckerbrett.

 More information

The machine is known by the following names:
  • Reichswehr Enigma
  • Wehrmacht Enigma
  • Heeres Enigma
  • Ch.11a
  • Enigma I
  • Army Enigma
  • Service Enigma
  • Army/GAF Enigma
  • 3-wheel Enigma
  • 3-wheeler
The official name is Enigma I (Roman number 1) or Ch.11a.
  1. Philip Marks and Frode Weierud,
    Recovering the Wiring of Enigma's Umkehrwalze A

    Cryptologia, January 2000, Volume XXIV, Number 1, pp. 55-66.

  2. David Hamer and Frode Weierud, Wiring details.
    Personal correspondence. 2001.

  3. Bedienings-Aleitung zur Chiffriermaschine 'Enigma'
    17 Pages at A5 size. 2 April 1945. German.

Further information

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Crypto Museum. Last changed: Friday, 15 May 2015 - 20:04 CET.
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