Compared to earlier designs, the machine has a number of advantages.
It has 6 removable cipher wheels – the pin-wheels –
that can be configured outside of the machine.
Furthermore, they can be installed in any order and additional wheels
could be ordered.
This increased the number of possible KEY settings and hence cipher security.
With the right selection (and order) of the cipher wheels and the settings
of the lugs on the slide bars, the C-52 can be backwards compatible
with the earlier C-3 and C-4 machines, such as the
C-38 (M-209), the
BC-38 and the
The machine shares many parts with the more advanced CX-52,
such as the enclosure, the printer, the slide bars and the accessories, but is
significantly less advanced than the CX-52.
It has slightly different pin-wheels, with a regular stepping behaviour,
whereas the CX-52 features irregular stepping.
The latter is caused by the slide bars. Although the
slide bars of the CX-52 can be installed in a C-52,
only the last two can advance the wheels. This is explained below.
The machine was offered to existing customers who needed backwards
compatibility with C-3 and C-4 machines, but also to customers of non-friendly
nations, to which the more advanced CX-52 could not be sold.
Such countries were typically on the NSA's and CIA's proscribed list.
The diagram below provides a quick overview of the controls and settings of
the C-52. Note that a number of versions and variants exist, some of which
may look different. The machine is shown here with its
top lid open, so that the six pin-wheels (front) and the revolving bar drum
with its 32 slide bars are visible. At the front left is a double printer that acts as the
Setting the cryptographic KEY, involves setting of the pins on each of the pin-wheels, installing the wheels in a given order and setting the
start position. When the lid is closed, it is protruded by the pin-wheels.
A white line indicates the start position of the pin-wheels. A the left side
is a knob that should be set to C when ciphering, or D when
deciphering. A message is enciphered letter-by-letter, by rotating the large
knob at the left until the dial at the front points to the desired input letter.
It is then encrypted by pushing down the handle and releasing it again.
When operating the handle, the drum (cage) makes one full revolution, during
which the cams and lugs on its slide bars interact with the pin-wheels.
This causes an angular displacement of the print head and makes each of
the pin-wheels step by one position. Although this guarantees the maximum
possible cipher period – each wheel has a different (coprime) number of
divisions – the stepping motion is regular and therefore
easier to predict that with irregular stepping wheels.
- Regular stepping of the pin-wheels
- No fixed/variable (F/V) feature
- Different way of sensing the guide arms
- Slightly different pin-wheels
- Slightly larger diameter of the support discs inside the drum
- Different guide arm locking mechanism
- Not available in RT-version (one-time tape)
Although the C-52 was more advanced than the earlier C-3 and C-4 machines,
such as the C-38 (M-209)
and the C-446, it was still possible to make it
backwards compatible. This was a useful feature for existing customers.
In such cases, the following wheels had to be used in this order:
C-38 → 26 25 23 21 19 17
C-52 → 26 25 46 42 38 34
Note that the rightmost four wheels of the C-52 have twice the number of
steps of the C-38 wheels. With carefully chosen pin-settings it was
possible to make the two models interoperable.
In this mode, the first five bars of the cage (1-5) were not used,
so that the remaining 27 bars could act like the 27 bars of the
The basic version of the C-52 was made for the 26 letters of the Latin
alphabet (A-Z). It means that the indicator (dial) has 26 letters as well,
as does the double print head. For support of different languages, such
as Arabic or Russian, a special 30-character variant was also available.
The image on the right shows the Arab version of the C-52, which was used in
countries like Saudi Arabia and Iran.
The machine shown here comes from the personal
collection of Hans Bühler
— the Crypto AG sales representative who was arrested in Iran in 1992.
➤ More information
Although the interior of the C-52 is very similar to the CX-52,
and many of the parts are shared between the two machines, there are some
significant differences. First of all, the pin-wheels are slightly different
and so is the way their guide arm are sensed. A guide arm senses whether
a pin at a certain position on the circumference of the pin-wheel
is inactive (set to left) or active (right).
In the CX-52 the guide arms are locked in place by means of a moving U-shaped
locking bar at the start of a ciphering operation.
The tip of the guide arm is then caught under the locking bar if the pin was
active (i.e. set to the left) or stopped in front of it when the pin was inactive (right).
In the C-52 the guide arms are not really locked in place,
but their movement is restricted within a certain window, by a
six cut-outs, or slots, in the aluminium bar
at the top of the machine. It is shown in the image on the right and
is located between the pin-wheels and the drum (cage).
Due to the different construction of the pin-wheels, they are not
interchangeable with the wheels of the CX-52. Another difference with the
CX-52 is the construction of the drum. Although the bars are identical,
the drum of the C-52 does not accept
all of the slide bars that were available.
Unlike the CX-52, the drum of the C-52 does not accept bars that can cause
wheel stepping in the first 30 positions (1-30). This was done by increasing the
diameter of the six support discs inside the drum for most of the circumference.
The image on the right shows a close-up of part of the drum, in which two of
the support discs are visible. The outer edges of these discs reach the hight
of the cams on the bars.
As the discs are cleverly positioned in front of the locking pawl of
each of the pin wheels, they block the driving cog-wheels and inhibit wheel stepping.
This means that only bars with six never-stepping cams
— the so-called 0-cams — can be used in these positions. Only the last two
positions of the drum (31 and 32) can hold slide bars that control wheel
stepping, as the support disc is recessed at these positions. This is clearly
visible in the image above. Move the mouse over it to highlight the
raised and recessed parts of the discs.
Bars and support discs inside the drum of the CX-52
These drawings illustrate the difference between the C-52 and the CX-52.
The drawing above shows the pin-wheels and the drum of the CX-52, in which
slide bar 77 1 engages with the cog-wheel (red) and the blocking pawl
(blue) of the pin-wheels. Move the mouse over the image to see what happens
when the bar shifts to the left. The black vertical lines are the support
Bars and support discs inside the drum of the C-52
In the C-52 (above), the diameter of the support discs is slightly larger,
as a result of which they engage directly with the blocking pawl (blue),
thereby effectively replacing the cams on the slide bars. As a result,
the wheels are unable to advance (step). Only all-0-cam bars can
be used here. Stepping is only possible with the
last two slide bars, as that part of the support discs is recessed.
This means that, depending on the bars, the wheels can theoretically
step 0, 1 or 2 positions per cycle. In practice however,
only one bar controlled the stepping.
It has six always-stepping cams
(the so-called C-cams), so that each pin wheel makes exactly one step
on each ciphering cycle.
For a more detailed explanation of the slide bars, the angular displacement
of the print head and the wheel stepping mechanism, please refer to the
technical description of the CX-52
or read .
➤ More on the CX-52 page
➤ Overview of all known slide bars
➤ Check out the slide bar configuration of the C-52 machines in our collection
Note that bar 77 is a fictive bar, on which all combinations of cams are
present. This bar does not exist in real life. It is also used as an example
in Bart Wessels paper .
BC-52 Simulator for Windows
Belgian crypto-researcher Dirk Rijmenants, has created a very realistic
simulation of the BC-52 that runs on the Windows™ operating system.
The BC-52 is actually a C-52 that is seated on a B-52 keyboard, hence the
name BC-52. The software allows you to select between C-52 and CX-52 simulation,
and customize the machine in various ways. Full instructions are included
with the program.
The image on the right shows a screenshot of the BC-52 simulator
running on Windows. It can also be used on Linux (WINE) and Mac (Parallels).
➤ Download (off-site)
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© Crypto Museum. Created: Sunday 16 February 2020. Last changed: Friday, 26 February 2021 - 22:52 CET.