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Enigma C
Glow lamp Enigma machine · 1925

Enigma C, was an electro­mechanical rotor-based cipher machine, introduced in mid-1925 by Chiffriermaschinen Aktiengesellschaft (ChiMaAG) in Berlin (Germany). Like its predecessors, the Enigma A and Enigma B, it used light bulbs for its output, and was therefore known as a Glüh­lampen­chiffrier­maschine (glow lamp cipher machine). It was succeeded in 1926 by Enigma D.

Enigma C is very similar to the Enigma B Mark II, and has the common layout of the control panel, with the keyboard at the front, the lamp panel just behind it, and the rotors towards the back.

Like on Enigma B, the keyboard is not arranged as on a regular typewriter, but in the order of the alphabet. It would be the last Enigma machine with this layout. Several versions of the Enigma C are known to have existed, including a variant with 28 contact points (rather than 26), and one with 29 keys of which the 'X' was passed through unencrypted. It was known as Funkschlüssel C.
Enigma C with serial number A-150. Taken from original ChiMaAG images.

The image above shows one of the few surviving pictures of the standard Enigma C [3], as it was probably used by ChiMaAG for the operating instructions. It shows that the keyboard is divided over three height levels (like a staircase) and that the top cover is held in place by four knurled bolts (numbered 12 in the photograph, A in the drawing below). The machine was supplied in an oak-wood box, with a hinged top lid, a lock at the front and a leather grip at the left. It is the first machine with a power switch at the top, allowing selection between the internal 4.5V dry battery and an external 4V source, that can be connected to the two black screw terminals at the right.

Enigma C with serial number A-150. Taken from original ChiMaAG images [6].
Enigma C with serial number A-150. Taken from original ChiMaAG images [6].
Enigma C with serial number A-150. Taken from original ChiMaAG images [6].
Original image of Funkschlüssel C (low-resolution)
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Enigma C with serial number A-150. Taken from original ChiMaAG images [6].
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Enigma C with serial number A-150. Taken from original ChiMaAG images [6].
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Enigma C with serial number A-150. Taken from original ChiMaAG images [6].
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Original image of Funkschlüssel C (low-resolution)

Standard version
Although, as far as we know, there are no surviving examples of Enigma C machines, it is known that it had 26 keys, arranged in three rows in the order of the alphabet, 26 lamps – in the same arrangement, located behind the keyboard – three moving cipher wheels with 26 contact points each, and a reflector that could be installed in two (in some models four) different positions.

The rightmost rotor is the fast-moving one, which means that it makes a single step on each key press. Each rotor has a single notch, that may cause the rotor to its left to make a single step as well, similar to the operation of a car's odometer. This method of advancing the rotors is known as regular stepping. Like most other Enigma machines, it exhibits a double-stepping anomaly [2].

'Educated guess' of the exterior of the Enigma C
Control panel of the standard Enigma C

The drawing above shows an 'educated guess' of what the standard version of the Enigma C might have looked like. At the front is the keyboard with 26 keys arranged in the order of the alphabet. Behind the keyboard is the lamp panel, with the same layout. Towards the back are three cipher wheels that protrude the top lid — each with 26 contact points at either side.

The reflector (UKW) is fitted internally. To the right of the cipher wheels is a rotary switch that allows selection between the internal 4.5V battery, and an external 4V power source that can be connected to the terminals at the right. It also allows the brightness level of the light bulbs to be changed from hell (bright) to dunkel) (dark). It is the first model with this rotary switch.

Differences with Enigma B
Compared to its predecessor – the Enigma B – the following differences can be observed:

  • Power/brightness selector
  • Terminals for external power
  • Staircase keyboard
  • Top cover held in place by 4 knurled bolts (A)
  • 2 spare light bulbs on lamp panel (internal)
  • All wheels with numbers (01-26)
  • UKW with two positions (four on some versions)
  • Removable top lid
Circuit diagram
The simplified circuit diagram below, shows how the Enigma C operated. At the right are the keys (switches), the lamps and the battery. The current is supplied to the contacts of the right side of the first rotor, via the entry disc (German: Eintrittswalze, or ETW). The current then passes the three cipher wheels, each of which can be stepped by a key-press like the odometer of a car.

Simplified circuit diagram of the Enigma C

At the far left is the reflector (German: Umkehrwalze, or UKW), which returns the current through the rotors. The current leaves the rotors via the ETW again, and causes a lamp to be lit. Note that the process is symmetric, or reversable, meaning that the same settings can be used for coding and decoding. This is a property of the reflector. Unfortunately it has the unwanted side effect that a letter can never be encoded into itself. This is true for all glow lamp Enigma machines.

At the bottom right is the four-position rotary selector that acts as the power switch. It allows selection between BRIGHT (hell), DARK (dkl), OFF (aus) and EXTERNAL POWER (Sammler). When set to DARK, a series resistor (R) reduces the current through the lamps. When EXTERNAL POWER is selected, the voltage from an external battery or transformer can be applied to the terminals (P).

Funkschlüssel C
A special version of Enigma C was known as Funkschlüssel C. It was similar to a regular Enigma C (as described above), but had 28 contact points on each rotor, and 29 keys on the keyboard, including Ä, Ö and Ü, whilst the letter 'X' was unencrypted. In December 1924, ten prototypes had been ordered by the German Navy (then known as the Reichsmarine) for testing, followed – in January 1926 – by a large batch of 50 machines. The machines were supplied with five differently wired rotors — numbered I, II, III, IV and V — three of which could be installed in the machine [1].

This was done to increase the key space of the system by a factor of 10, as the three wheels can be installed in 60 different orders (5 x 4 x 3) rather than just 6 (3 x 2 x 1). To avoid confusion, each rotor was engraved with a different set of letters or numbers, as shown in the table below.

                               I    II   III  IV   V
                               ---  ---  ---  ---  ---
                         01    A    01   31   61   AA
                         02    Ä    02   32   62   AÄ
                         03    B    03   33   63   AB
                         04    C    04   34   64   AC
                         05    D    05   35   65   AD
                         06    E    06   36   66   AE
                         07    F    07   37   67   AF
                         08    G    08   38   68   AG
                         09    H    09   39   69   AH
                         10    I    10   40   70   AI
                         11    J    11   41   71   AJ
                         12    K    12   42   72   AK
                         13    L    13   43   73   AL
                         14    M    14   44   74   AM
                         15    N    15   45   75   AN
                         16    O    16   46   76   AO
                         17    P    17   47   77   AP
                         18    Q    18   48   78   AQ
                         19    R    19   49   79   AR
                         20    S    20   50   80   AS
                         21    T    21   51   81   AT
                         22    U    22   52   82   AU
                         23    Ü    23   53   83   AÜ
                         24    V    24   54   84   AV
                         25    W    25   55   85   AW
                         26    X    26   56   86   AX
                         27    Y    27   57   87   AY
                         28    Z    28   58   88   AZ

The wiring of these weels is currently unknown. Note that the uncommon engraving was only present on the wheels that were supplied with the first 50 machines. When an unknown number of machines was ordered later, their wheels were all engraved with the single-letter sequence of wheel I in the above table. It is likely that the existing wheels were upgraded at that occasion.

  1. Note that on wheel I, the letter 'X' is present, but 'Ö' is not.
  2. Note that on wheel V, the combination 'AX' ispresent, but 'AÖ' is not.

The layout of Funkschlüssel C is believed to be very similar to the layout of the Swedish Enigma B that was delivered to the Swedish SGS in early 1925, which also had 28 contact points on its rotors. Contrary to the Swedish Enigma B however, Funkschlüssel C had 29 keys — of which the letter 'X' was wired straight through — and had a reflector (UKW) that could be fitted in four different orientations – denoted α, β, γ and δ – whilst in the Enigma B it was fixed in place.

Layout of Funkschlüssel C
Control panel of Funkschlüssel C

The top panel can be removed in two parts, by removing the four knurled bolts near the sides (marked A in the image), allowing the wheel order to be altered and any broken light bulbs to be replaced. Note that extra fittings are present inside the machine, to hold 12 spare light bulbs.

Differences with standard version
Compared to the standard version of the Enigma C, the following differences are observed:

  • Rotors with 28 contact points
  • 29 keys on the keyboard and lamp panel
  • Letter 'X' wired straight through (unencrypted)
  • No internal battery
  • Socket for external power
  • Five wheels (three in the machine)
  • Different engravings on each wheel
  • UKW with four orientations
  • 12 internally fitted spare light bulbs
  • Top panel removable in two parts
  • Serial number prefix 'M'
Circuit diagram
Below is the simplified circuit diagram of Funkschlüssel C. At the left is the rotor stack, with three cipher wheels — with 28 contact points each — in between the reflector (UKW) and the entry disc (ETW). Each rotor can be set to any of its 28 positions. The UKW can be set in 4 positions. At the right are the keys and lamps. Note that the 'X' is wired straight through, which means that it will always be encoded into itself, contrary to the other letters which can never become theirselves.

Simplified circuit diagram of Funkschlüssel C

Funkschlüssel C does not have an internal battery. Instead it has a two-pin socket for connection of an external 4V power supply, such as the on-board voltage of a ship. As the lamps require a 3.5V power supply, the external voltage is reduced by a resistor (R) that is fitted inside the case.

1926 key procedure
The cipher principle used in the Enigma C – or in any other cipher machine with a reflector – is symmetric, which means that both sides have to use the same settings, or key. In this case, the key tells the operator which weels to use in each positon, the offset of the letter ring, the start position of each wheel and the position of the UKW (the latter could be placed in 4 orientations).

According to the key instructions of the German Navy of 1926 [A], the key consisted of two parts: a basic-number (Grundzahl) and a key-number (Schlüsselzahl). The Grundzahl was only changed periodically, and was usually valid for several months, as specified in the key sheet, for example:

Basic settings for Funkschlüssel C

The rightmost column specifies the date at which the new Grundzahl will come into effect (usually at 12:00 hours) — in this case: 14 April 1928. The leftmost column specifies the configuration of the rotors. In the example, Alpha (α) specifies how the UKW should be installed. The rest of the line (B 10 31 65 AZ) gives the Ringstellung (ring setting) of each of the rotors I, II, III, IV and V.

The Schlüsselzahl was changed more frequently and consists of five or six characters that specify which three rotors (of the total set of five) must be used, the order in which they should be placed (from left to right) and the start position of each rotor (i.e. the index that should be visible in the windows). As each rotor has a different engraving, a Schlüsselzahl of C 58 21 implies that rotors I, III and II must be placed in the machine and that C 58 21 should be visible in the windows.

Depending on the secrecy level of the message, the following types of Schlüsselzahlen existed:

  • Funkschlüssel C — Allgemein
    General traffic
  • Funkschlüssel C — Offizier
    Messages intended for an officer
  • Funkschlüssel C — Stab
    Messages intended for a staff officer
Schlüsselzahl — Allgemein
For regular messages, the Schlüsselzahlen were taken from a table with several columns, plus a reference number to the so-called Kenngruppen (message indicators) that were specified in a separate table (Kenngruppenbuch). For Funkschlüssel C — Allgemein, the table looked like this:

At any given time, only one column of the table (A, B, C, D, E or F) was in effect. As an example, assume that we are using the first line (1) and first column (A) — This specifies the Schlüssel­zahl 54 AD 09. As each rotor has a different engraving, this implies that rotors III, V and II are placed in the machine (in that order), with the markings 54 AD 09 visible in the windows. The values in each column are chosen in such a way, that the rotor order and rings settings did not have to be changed between messages. It was mandatory to use different settings for each message [A].

Schlüsselzahl — Offizier
Messages for an officer (that had to be kept secret from the crew) were encrypted with a different Schlüsselzahl that was valid for several days. The settings were taken from a secret table that was in the posession of the officer. For convenience and speed, they were chosen in such a way that the rotor order and ring settings were identical to those of the current Schlüsselzahl-Allgemein.

Schlüsselzahl — Stab
For messages of the highest level — for example messages directly from Naval High Command to a U-Boat Captain — a different key — Schlüsselzahl Stab (staff) — had to be used, that was valid for a longer period of time, generally in the order of one month. The settings were taken from a secret table that was usually kept in a safe, and were deliberately chosen in such a way that the wheel order and ring settings had to be changed. The following example is shown in the manual:

1933 key procedure
In May 1933, a new key procedure was issued, which became effective on 15 July 1933 [B]. This was probably around the same time as new Funkschlüssel C machines were delivered, with new cipher wheels that were all engraved with letters (A-Z), as previously only on rotor I. It is likely that at the same time, the existing wheels (or at least their engraved rings) were replaced.

As a result, the key procedure had to be changed as well. There were still three security levels — Allgemein (general), Offizier (officer) and Stab (staff) — but the following items were now used:

  • Grundeinstellung
    Base key
  • Tagesschlüssel
    Daily key
  • Das F.- und K.-Buch
    Indicator books
For general messages (Allgemein) and officer's messages (Offizier), the same base key (Grund­einstellung) was used, which was changed at irregular intervals, generally at least once a week. The required base keys were printed on a single sheet, which held the settings for one month, such as in the example below. The new key comes into effect at 12:00 hrs at the given day.

In addition to the Grundeinstellung — which was valid for several days — the user had to use a Tagesschlüssel, which was taken from a key sheet that held all daily keys for a full month. Note that this table holds the daily keys for both general messages (left) and officer's messages (right).

Example of a daily key sheet, with general keys and officer's keys

For staff messages (Stab) a different Grundeinstellung and Tagesschlüssel were used, which was valid for one full month. The key is valid from 00:00 hrs on the first day of the month, to 23:59 hrs on the last day of the month. Below is an example, taken from the manual. At the left is the UKW setting, the wheel order and the initial settings of the rotors. At the right is the daily key.

Example of a staff key sheet

Preparing a message
Preparing a message for transmission, was a complex task that involved the use of Kenngruppen (message indicators) that were taken from the Geheime Marinefunknamenliste und Kenn­gruppen­buch (secret navy callsign list and message indicator book), abbreviated to F.- und K.-Buch.

Users were encouraged to use as many abbreviations as possible, as long as the text remained unambiguously. This was done to keep the messages short and to hide the typical characteristics of the German language. In addition to its regular occurences in the text, the letter 'X' — wich was passed unencrypted — was also used as a dummy character (German: Blindbuchstabe). It had to be inserted randomly throughout the message, probably to confuse enemy code breakers.

The full message preparation procedure is described in great detail in the original instruction manuals of 1926 [A] and 1933 [B].

Related patents
  • DE460457 / 11 March 1926
    This patent introduces the Umkehrwalze (UKW) and the removable rotor-set, or drum, invented by Willi Korn. It describes how the drum can be removed by using a lever to shift the UKW aside. This was done to allow the wheel order to be changed easily in the field.

    It also claims that the top lid can only be closed when the UKW lever is locked in position. The UKW is a basic element for all glow lamp Enigma machines. The drawing below was taken from patent DE460457 and is very similar to the mechanism of the Enigma C.

    Click to view patent DE460457
  1. Der Funkschlüssel C 1
    B.Nr.A IIIh 177 geh. Berlin, 9 February 1926.

  2. Der Funkschlüssel C (Vorschrift) 1
    M. Dv. Nr. 21. Berlin, May 1933 (effective 15 July 1933).
  1. Document kindly provided by Frode Weierud [1].

  1. Frode Weierud, Enigma History
    Personal correspondence, February 2019.

  2. David Hamer: Actions involved in the 'double stepping' of the middle rotor
    Cryptologia, January 1997, Volume XX, Number 1.

  3. David Hamer, Personal correspondence
    Images taken from copies at PRO (national Archives), Kew, UK.
    December 2007.
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© Crypto Museum. Created: Sunday 13 September 2009. Last changed: Tuesday, 30 March 2021 - 09:51 CET.
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