Pin-and-lug pocket cipher machine
CD-55 and CD-57 were hand-held mechanical
cipher machines, developed
by Boris Hagelin
and introduced by Crypto AG in Zug (Switzerland)
around 1957. The CD-57 is compatible with the
desktop cipher machine and was small enough to fit in the pocket
of, say, a coat.
The device measures just 137 x 80 x 40 mm and weights about 1 kg.
It consists of a robust two-part die-cast aluminium enclosure in which all
moving parts are mounted in the bottom half. The upper half is a
hinged lid that is opened by
pressing a small button 1 at the top
of the case.
The CD-55 and CD-57 are not interoperable, but each version
is at least compatible with one other
Hagelin desktop cipher machine.
The case was available in grey or green-ish hammerite and in a
military 'olive drab' green.
The latter is shown with the instruction manual on the right.
Both models were introduced around 1957 and remained in service until
the mid-1970s. In some countries even longer.
Due to their small size and the fact that they are compatible with another
(desktop) cipher machine,
the machine became popular with a number of Armies
in Europe and elsewhere. The Austrian and the Swiss Army used it for tactical
field messages for many years.
As the CD-57 could be concealed easily, for example in the pocket
of a coat, it became a popular encryption device during the Cold War
with a number of intelligence agencies,
in particular French intelligence.
A special version of the machine with a gold-plated dial was produced
as a luxury gift to loyal customers. It was given to the Shah of Persia 2
and was also supplied to the Vatican.
In the late 1950s and the early 1960s, the CD-57 was also built under
licence by the German Manufacturer Rudolf Hell
as the STG-61. 3
The CD-57 was succeeded in 1977 by the HC-520,
one of the first fully electronic cipher machines,
that marked the end of the mechanical era.
On some versions the lid is opened with a rotary knob rather than
Persia is known today as Iran. The Shah was overthrown in 1979.
It is currently uncertain whether the STG-61 is fully compatible with the
The CD-57 measures no more than 13 x 8 x 4 cm and weights just 710 grams.
It has a metal operating lever that extends from the left side of the case, as
shown in the images below. When in transit, the operating lever can be pushed
all the way in, and held in place by a locking lever. Input and output of
plaintext and ciphertext takes place via a circular window at the front top.
When operating the device, the outer letter ring should be
set to a fixed position,
as specified in the daily key settings. In the example above,
the outer letter ring is set to 'A', which means that the 'A' is lined up
with the index mark at the top edge of the case. When encoding a text, the
metal operating lever should be pressed-in and released once for each letter.
This rotates the inner letter ring to an arbitrary position.
The outer ring then serves as the input (plaintext), whilst the inner ring
represents the output (ciphertext). In the example below, A is translated
When a mistake is made, the keying mechanism can be turned back one or more
positions, using a small crank that is normally stored inside the top lid
of the case. The crank can be
inserted into a small hole in the front panel,
which is in fact the main axle of the cipher wheels. By
turning the crank
counter clockwise, the mechanism can be stepped back. Each ¼ circle represents
The interior can be accessed by
pressing a small button
at the top (or turning a knob in case of the
military variant). The diagram above shows the interior of a typical
CD-57. All moving parts are mounted in the bottom half of the case.
At the left is the stack of six cipher wheels, installed on the main axle.
The wheels are held in place by the drum locking lever.
A metal rod in front of the wheels marks the index at which the
current position of the ciper wheels is read.
The CD machines were available in four basic models and in a number of
(case) variants. The following basic models are known:
This version has 6 pin-wheels and features a simple keying mechanism
that works like the C-38,
It looks identical to the CD-57, but is much easier to break.
It was supplied to non-NATO countries, as agreed in a
secret arrangement with the NSA
The CD-55 is compatible with the C-52
(the non-NATO variant of the CX-52).
This version has 6 pin-wheels, but features a much improved keying
mechanism that is similar to that of the CX-52.
It was supplied to NATO and NATO-friendly countries.
This version could be converted into an unbreakable OTT cipher machine
The CD-57 could be converted into an unbreakable
one-time tape (OTT)
cipher machine in just a few seconds, by removing the wheel stack
and replacing it with a so-called RT/CD option.
The abbreviation 'RT' means Random Tape.
This was a slightly improved and modified variant, built under licence
by the German manufacturer HELL,
for use by the German Bundesgrenzschuts (BGS).
When Hagelin unfolded his plans for a new -much more secure- cipher
machine around 1951, it caused great upset at the
American AFSA. 1
The new CX-52
had an advanced keying mechanism that defeated all
current methods for automatically breaking Hagelin pin-wheel cipher machines.
This resulted in series of secret talks between AFSA and Boris Hagelin,
which eventually lead to a
between the two parties, in which it was agreed that Hagelin would develop
at least two versions of each new crypto device, a secure and a less-secure version, and that the secure version would only be sold to NATO and
NATO-friendly countries, i.e. the allies of the US.
The less-secure version featured the old C-38 keying mechanism
and would be presented to 'less-friendly' countries and countries that were
considered to be enemies of the United States.
In the case of the CX-52,
this led to the development of the
looked identical, but had the old keying mechanism.
It could easily be broken by the NSA with the existing methods.
When Hagelin developed a pocket variant of the CX-52, the CD-57,
the same strategy was followed.
The CD-55 is the less-secure version of the CD-57 and is interoperable
with the C-52.
In secret Hagelin/NSA terminology, the CD-55 was known as a
Class 1 machine. 2
As per agreement with the NSA, Class 1 machines
can not be converted into an OTT cipher machine.
➤ More about the secret agreement between Hagelin and the NSA
The AFSA was the predecessor of the NSA.
Class 1 machines feature the old 'simple' keying mechanism.
NATO friendly version
NATO and NATO-friendly countries were given access to the new technology
and, hence, the full-blown CD-57 with its new -advanced- keying mechanism
that features irregular stepping.
The machine has six removable pin-wheels that can be placed on the
axle in any of 270 orders.
Some machines were even supplied with a total of 12 cipher discs, of which
6 could be placed in the machine in any of 665280 possible orders. 1
Like the CD-55, the CD-57 is available in a few case variants.
The colour of the enclosure is grey (or green-ish) hamerite or
military olive drab.
The case is opened either with a
push-button at the top, or by
means of a rotary knob
in that position. Military users often preferred
the rotary knob as it prevented the case from opening accidentally
when carrying it in a pocket.
The image above shows a typical CD-57 ready for use, with its operating
lever fully extended. In secret Hagelin/NSA terminology, the CD-57 was known
as a Class 2 machine, which means that it features the new -advanced-
keying mechanism. The CD-57 is interoperable with the
Some CD-57 machines could be converted into a One-Time-Tape (OTT) machine,
in which case a metal fitting is present at the bottom. It was used to hold
a cassette with the (random) key tape.
Calculated as 12!/6! = 479001600/720 = 665280.
Random tape version
In cryptography, there is one system that is truly unbreakable:
the One-Time Pad (OTP),
or its tape variant: the One-Time Tape (OTT).
The latter works by adding a random value from a key tape to each letter
of the plaintext by means of an XOR-operation (modulo-2 addition).
At the receiving end, the same XOR-operation is applied to the ciphertext,
to reveal the plaintext again.
When correctly applied, the OTP/OTT technology provides an absolutely secure
communication, that can not be broken with existing methods.
Hagelin developed all Class 2 machines in such a way that they could be
converted into an OTT machine within seconds, simply by removing the cipher
wheels and fitting a drop-in OTT unit.
The image on the right shows Hagelin's RT/CD option, that fits in
the space that is normally taken by the wheels. Although it looks simple, it
is actually an extremely clever tape sensing unit.
Note that the ability to convert the machine to an OTT device was
only available on Class 2 machines, and that such machines were only
sold to NATO and NATO-friendly countries. Other customers did not have
access to the OTT technology. This was part of a
secret agreement between Hagelin and the NSA
that existed from 1951 onwards .
The complete machine (CD-57 with RT/CD option) was known as the
CD-57/RT or CDR-57.
More information about the RT variant will be added as and when it
After World War II had ended, the occupying Allied Forces in Germany
decided that Germany should not be allowed to develop high-grade technology,
such as advanced cipher machines. Nevertheless, cipher machines were needed
for the new German Army (Bundeswehr) and the new intelligence service that
had just been established in 1956:
the Bundesnachrichtendienst (BND).
The problem was solved by licencing the design of the CD-57 from Hagelin
in Switzerland, and allowing
Rudolf Hell's company
in Kiel (Germany) to build it. The design was approved by Dr. Erich
Hüttenhain 1 who suggested a few modifications.
After agreeing the modifications with Hagelin, HELL was allowed to
manufacture the improved CD-57 from 1961 onwards. It became known as
the HELL STG-61 and was primarily used by the Bundesgrenzschutz (BGS) 2
as a replacement for their bulky
The STG-61 was interoperable with the
H-54 cipher machine. 3
From a mechanical viewpoint, HELL was an extremely good company,
and it was not uncommon for them to make some mechanical improvements
to a design to make it work more reliably. In the case of the STG-61
several modifications were made, the most obvious of which is the
addition of a reset-wheel to the letter counter.
Apparently, this version can not be converted to an
OTT cipher machine
as the mounting for the key tape is missing from the case.
Surprisingly, the case is opened by means of a push-button,
rather than with the more secure rotary knob.
➤ More about the STG-61
During WWII, Dr. Erich Hüttenhain was the chief cryptologist of the Third
Reich. After the war he was employed by the new German intelligence service
Organisation Gehlen (OG).
In 1956, the OG was renamed to
In 2005, The Bundesgrenzschutz (BGS) was renamed
Bundespolizei (Federal Police).
was a functional copy of Hagelin's CX-52,
built under licence by Rudolf Hell
from 1954 onwards. The H-54
also featured Hüttenhain's modifications.
At the NSA the machine was known as the CX-52bk.
Apart from the different models listed above, there were a number of
case variants that could be applied to any of the models. The following
variants are currently known:
- Civil version
This is the most common variant of the CD-57. It is housed in a grey
or green-ish hamerite case. It is often called the 'civil version' but
was also supplied to military users.
- Military version
A special military version was available in a dark green olive drab case.
Although this version is fully compatible with the civil one, it was
sold exclusively to military customers.
- Limited edition
As a luxury gift to special customers, a limited number of bright
machines were produced with a gold-plated letter dial.
This version, also known as LUX, was also supplied to the Vatican.
It is believed that all LUX machines were based on the CD-55 model.
Although there is no functional difference between the civil version
and the military version of the CD-57, i.e. both variants are compatible
the device was available in two design variants: one painted in grey
hammerite and one with a typical military green (olive) finish.
The grey hammerite version is commonly known as the civil version,
although it was also issued to some military customers.
The case is opened by pressing the small push-button at the top.
The image on the right shows a typical grey hammerite
civil version of the CD-57. Some cases have a more green(ish) hammerite
finish, although it is not certain whether this is actually a different
colour, or simply the effect of long-term exposure to sunlight on the
The letter discs have white letters on a black background
and a course black setting ring.
Most civil CD-57 machines could be converted into an OTT cipher
machine by installing the RT/CD option
and fitting a key tape cassette
to the mounting at the bottom. If the OTT option was not required,
the mounting was commonly omitted, just like in the example above.
On the CD-55 machine, the mounting was never present, as it could
not be converted to an OTT device.
Especially for military customers, Hagelin developed a variant that
was adapted to the military requirements. This version was fully
compatible and interoperable with the civil version.
The case was typically finished
in a dark green 'olive drab' wrinkle paint. It made the machine far less
To prevent accidental opening of the device in a tactical environment,
for example when carrying it in the pocket of, say, the trousers,
most military machines have a different type of lock, whereby the
push button on top of the device is replaced by a knurled knob.
In order to open the device, the knob should be rotated clockwise.
Surprisingly, the rotary knob is not present on the
the CD-57 clone that was manufactured under licence by
in Kiel (Germany) in 1961, despite the fact that the STG-61
was also a military-grade machine.
To raise the contrast between the plaintext and ciphertext letter rings,
the latter is inverted (i.e. black letters on a white background).
All military machines could be converted to OTT machines and were fitted
with the key tape mounting. Again, this feature is missing from the
Especially as a gift to loyal customers, such as the Shah of Persia,
a limited run of 'special' CD-55 machines was made.
They were housed in a bright painted enclosure
and had a number of gold-plated parts, such as the big letter dial at
the front and the curved operating lever at the left.
The Limited Edition of the CD-55 was known as CD-55/LUX.
Although this case variant is fully operational, it was probably not
intended for daily use, but merely as a showpiece on a desk.
This version was also supplied to the Vatican, but it is currently
unknown whether it was actually used for encrypting messages.
The image on the right shows the extremely rare CD-55/LUX on a
dark blue background. Many thanks to Gerhard Sulger Buel for
supplying them and for permission to reproduce them .
In order to obtain the maximum cipher period (i.e. the number of steps
before the machine repeats itself), each coding wheel (or disc) has a
different number of divisions and pins. Each pin can be placed in an
active or inactive position. The following 12 wheels are known:
25 26 29 31 34 37 38 41 42 43 46 47
Of these 12 wheels, only 6 are present in the device at any time.
Quite often, the machines were supplied with just 6 cipher wheels, e.g.:
29 31 37 41 43 47
In some situations, two identical wheels sets were issued with a machine.
It allowed the operator to prepair the alternate wheel set for a new key,
whilst the old key was still in use. When the key was changed, e.g. at
12 o'clock midnight, all the operator had to do was swap the wheels sets.
The removed wheelset could then be used to repeat the procedure
for the following day.
In order to removed the wheels, it is important that the operating lever
is in the locked position (i.e. fully pressed-in and locked). Next, open
the case and swing the wheel locking spring to the side. The wheels can
now be lifted from their axle. Alternatively, turn the device over with
one hand and let the wheels drop into the other hand.
Setting of the cryptographic key, involves the following settings:
- Order of the coding discs
- Setting of the pins on each disc
- Start position of the wheels
The CD-57 was usually delivered in a simple grey carton box that was slightly
larger than the device itself. The instruction booklet and the quality-check
card (if present) were store underneath the device. In case the device
was delivered with accessories, such as additonal cipher wheels or an
RT/CD drop-in OTT unit, it usually came in a box that was about twice its
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© Crypto Museum. Created: Wednesday 05 August 2009. Last changed: Wednesday, 02 November 2016 - 06:44 CET.