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Rotor Enigma Tree D → ← B
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.
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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.
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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 rotors 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].
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Control panel of the standard Enigma C
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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 rotors 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 rotors 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.
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Differences with Enigma B
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Compared to its predecessor – the Enigma B
– the following differences can be observed:
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- Wooden case lid held by metal retaining bracket
- 45° bracket with spare light bulbs in case lid
- 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 rotors with numbers (01-26)
- UKW with two positions (four on some versions)
- Removable top lid
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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 rotors, each of which can be stepped by a key-press
like the odometer of a car.
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).
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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 rotors 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 rotors that were supplied with the
first 50 machines. When an unknown number of machines was ordered later,
their rotors were all engraved with the single-letter sequence of rotor I
in the above table. It is likely that the existing rotors were upgraded
at that occasion.
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Note that on rotor I, the letter 'X' is present, but 'Ö' is not.
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Note that on rotor V, the combination 'AX' ispresent, but 'AÖ' is not.
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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.
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Control panel of Funkschlüssel C
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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 rotor 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.
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Differences with standard version
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- 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 rotors (three in the machine)
- Different engravings on each rotor
- UKW with four orientations
- 12 internally fitted spare light bulbs
- Top panel removable in two parts
- Serial number prefix 'M'
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Below is the simplified circuit diagram of Funkschlüssel C. At the
left is the rotor stack, with three cipher rotors — 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.
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.
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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 rotor 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:
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:
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- General traffic
- Messages intended for an officer
- Messages intended for a staff officer
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Schlüsselzahl — Allgemein
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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].
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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.
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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 rotor order and ring settings had to be changed.
The following example is shown in the manual:
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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 rotors that were
all engraved with letters (A-Z), as previously only on rotor I.
It is likely that at the same time, the existing rotors (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:
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Grundeinstellung Base key Tagesschlüssel Daily key Das F.- und K.-Buch Indicator books
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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).
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 rotor order and the initial settings of the rotors. At the right
is the daily key.
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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].
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- 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 rotor 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.
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Document kindly provided by Frode Weierud [1].
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© Crypto Museum. Created: Sunday 13 September 2009. Last changed: Saturday, 17 February 2024 - 16:17 CET.
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![Enigma C with serial number A-150. Taken from original ChiMaAG images [6].](img/c_03_thumb.jpg "image # c_03_large.jpg")
![Enigma C with serial number A-150. Taken from original ChiMaAG images [6].](img/c_01_thumb.jpg "image # c_01_large.jpg")
![Enigma C with serial number A-150. Taken from original ChiMaAG images [6].](img/c_02_thumb.jpg "image # c_02_large.jpg"){kind=small}
![Enigma C with serial number A-150. Taken from original ChiMaAG images [6].](img/c_03_large.jpg)
![Enigma C with serial number A-150. Taken from original ChiMaAG images [6].](img/c_02_large.jpg)
