Voice Encryption Unit
The MSC-2001 is a voice encryption unit,
developed and manufactured around 1978 by
Siemens in Munich (München, Germany).
Although it can be used in the audio line of virtually
any radio set, it was initially developed for use with the AN/PRC-77 radio.
Up to 8 different 50-bit crypto keys can be stored in the device, with the
active one selectable with a (1-8) rotary selector. 1
The rotary selector can also be used to select CLEAR mode
(no encryption) and for ZEROIZING the device (wipe all keys).
The unit is connected to the radio via a short cable between the TX/RX socket
and the audio socket of the radio.
New crypto keys are loaded into the device by means of a special
key fill unit that is connected to the REMOTE socket.
The device is suitable for the transmission and reception of
analogue audio (voice) and digital data, all connected via the TX/RX socket.
Speech is digitized at various bitrates by means of a built-in
The device is intended for simplex or half-duplex operation,
but can be used for full-duplex communication as well, by using two
devices. For encryption, a self-synchronising ciphertext feedback
(CFB) encryption algorithm is used, also known as an autoclave,
which synchronises in less than 0.5 seconds.
In the 1990s, MSC-2001 units started appearing on the surplus market
in Europe and the US, as a result of which many are currently
owned by collectors and hobbyists.
Below are copies of the brochure, a brief description, the user manual
and the technical manual.
Missing from the MSC-2001A.
- MSC-2001 · S42043-E9-B104
This version has a rotary MODE-selector on it's front panel, to the left
of the REMOTE and TX/RX sockets. Most MSC-2001 units found on the surplus
market today, are of this type. The device featured on this page, is
also of this type.
- MSC-2001A · S42043-E15-A104
This is basically the same device as the MSC-2001 descirbed above, but with
the MODE-selector missing from the front panel. With this version, the MODE
had to be selected on the externally connected REMOTE control unit.
This version was for airborne applications.
All controls and connections of the standard MSC-2001 are at its front panel,
as shown in the diagram below. At the right are two identical 14-pin female
sockets, one for connection to the host transceiver, and one for connection
of a remote control unit, or a key transfer device such as the
Siemens Key Gun.
At the left is a 10-position rotary switch that is used for selection of the
desired MODE of operation. It has 8 storage places for an encryption key (1-8).
Selecting any of them enables encryption.
Setting the switch to the next position after 8, selects CLEAR operation.
In case of an emergency, all cryptographic keys that are stored inside the
device, can be erased quickly by ZEROIZING the device.
This is usually a two-step procedure, as per NATO standard.
In the case of the MSC-2001, it involves breaking the white plastic ring
below the MODE knob and placing the MODE-selector in the position before
1, marked CODE ERASE. In practice this means
turning the knob with force past position 1.
Once this is done, the plastic ring must be replaced.
Suitable cryptographic keys for the MSC-2001 were distributed on 5-level
punched paper tape, similar to the tapes used with
telegraphic equipment (telex)
and punched in accordance with the ITA-2 standard,
also known as Baudot.
The key is 50 bits long and has an extra check character.
Loading suitable encryption keys into the key compartments of the MSC-2001,
was done with a special Siemens Key Gun, such as the one shown in the image below.
This universal key transfer device has a built-in memory, and is
loaded itself by means of a 5-level punched paper tape, that is pulled through
the built-in tape reader manually, at an arbitrary speed, similar to
The keys are loaded from tape into the device by means of a
special Siemens Key Gun, of which two versions
were available. In its basic form, they Key Gun is connected
to the REMOTE socket of the MSC-2001, and the paper tape is simply
pulled through the gun's built-in tape reader.
A more advanced version of the key gun – shown in the image on
the right – has an internal CMOS memory in which eight keys
can be stored. The keys are retained by an internal battery,
and can be transferred to one or more MSC-2001 devices later.
In case of an emergency, it can be cleared.
In the 1990s, MSC-2001(A) devices started to appear on the European and
American surplus markets, and became available to collectors and hobbyists.
As the supply of KEY GUNs was very limited — they were unavailable to
non-military users — several parties mounted an attempt to develop a
replacement. The Odyssee-2001 project is arguable the most successful of
➤ More about the Siemens Key Gun
The diagram below shows the format of the key tape, which conforms to
the ITA-2 standard.
The actual cryptographic key consists of 50 bits.
As each ITA-2 character is 5-bits long, this means that the key consists
of 10 characters. There are leading and trailing characters, plus
characters to identify the start and validity of the key. Furthermore
extra stunt characters 1 are inserted after each key, to ensure that the
teleprinter – on which the key tape is made – is put in a known state.
The tape starts with a leader that consists of approx. 40 x the letter 'V'.
This is followed by the start character – the letter 'A' in this case –
and the check character, which is always the letter 'J'. The next 10
characters are the actual key words, each of which has 32 possibilities
(25 = 32).
As the key words can be any code from the ITA-2 telegraph alphabet,
including a figures shift (FIGS), it is necessary to put the
teleprinter back in a known state at the end of the KEY. This is done
by inserting the reset sequence: letter shift (LTRS), carriage
return (CR) and line feed (LF).
There is no checksum and
the tape ends with a trailer that consists of approx. 40 x the letter 'V'.
If multiple keys (up to 8) are distributed on a single key tape, the trailer
is also the leader of the following key. In that case, the start character 'A'
– which is used for a single key – should be replaced by a different one that
assigns the key to a specific key compartment of the MSC-2001.
The following start characters can be used to address the key compartments
1 to 8:
X → 1
F → 2
Y → 3
S → 4
B → 5
D → 6
Z → 7
E → 8
In telegraph speak, 'stunt characters' is a common expression for
the control codes: LF, CR, LTRS, FIGS and SPACE.
In this case they are used to put the teleprinter in a known state.
They are not part of the key.
Below is a (much) simplified block diagram that explains the operation
of the MSC-2001. A the left is the host transceiver, of which the audio
input and output (i.e. microphone and speaker lines) are routed through
the MSC-2001. The handset of the transceiver is at the far right.
At the heart is an integrated cryptographic processor, or crypto logic,
that controls key stream generation, randomized initialisation sequency,
key selection, and generation of the encrypted signal. It has a built-in
device that safeguards the process, to ensure that no clear signal is
accidentally transmitted. The delta (de)modulator converts analogue audio
to the digital domain and vice versa. In this case, a variable slope
delta modulator (VSD) is used. For a more detailed block diagram and a
description of the circuit blocks, please refer to the technical manual [D].
The MSC-2001 is housed in a strong rectangular metal enclosure
that measures 180 x 100 x 40 mm and weights 924 grams.
It consists of a front panel – to which all internal parts are mounted –
a metal case shell, and a rear panel that holds the parts together.
Rubber gaskets between front panel and case shell, and
between rear panel and case shell, make the device water resistant.
The interior can be accessed by loosening the two screws at the rear panel.
After removing the rear panel,
the 3.4V AA-size backup battery becomes visible. The interior may be
removed from the case shell,
but pulling out the front.
Inside the case is a die-cast aluminium frame that is attached to the
front panel. The frame holds two PCBs that have their components facing
inwards. As a result, only the
solder side of both PCBs
visible. The PCBs are held in place by 6 or 8 screws around the edges,
and are inter-connected via a 13-way flex strip.
The overall design of the MSC-2001 is very good. The device is housed
in a strong and compact enclosure, and both PCB are mounted onto a
strong die-cast aluminium frame. The components are nicely
divided over the two PCBs, but the actual PCB layout it not very good.
The tracks are very thin and there is no ground plane whatsover.
This must have caused problem with the audio lines, which is confirmed
by a number of wire modifications
on the solder side of both PCBs.
Below is he pinout of the TX/RX socket and the REMOTE socket, both
of which are available at the front panel of the MSC-2001. For the wiring
of the control socket of the MSC-2001A and the sockets of the control
box, the interface unit and the radio, please refer to the service manual .
- System ground
- Connection to REMOTE (B)
- PTT (input)
- AF input
- Connection to REMOTE (E)
- (+) 27.5V
- Transmit relay (output)
- Microphone ground
- not connected
- Clear/cipher criterion (output)
- SQ + AF output
- (+) 12V
- Receive data (input)
- Transmit data (output)
- System ground
- Connection to TX/RX (B)
- PTT (input)
- AF input
- Connection to TX/RX (E)
- (+) 27.5V
- Key input check (output)
- Microphone ground
- (-) Reader voltage (output)
- Clock (input)
- not connected
- (+) 27.5V (input) for key input (L)
- Data (input)
- Read/write (input)
CryptoCiphertext Feedback (CFB) autoclave
Bitrate9.6, 12 or 16 kbit/s ➤ see table
Audio response300 - 2700 Hz
Key variations> 1015
Key loadingVia ➤ key loading device
Modehalf-duplex (full-duplex with two devices)
Data input20mV - 2V at 50 kΩ
Data output≤ 5V at 1 kΩ
Audio input1mV - 1V at 5 kΩ
Audio output≤ 2V at 400 Ω
Power10 - 15 V DC
Current135 mA (85 mA in standby)
Temperature-30°C to +60°C (storage: -40°C to +70°C)
Dimensions185 x 100 x 40 mm
The following radio sets are known to be compatible with the MSC-2001:
- ARC-34 1
- ARC-164 1
- ARC-159 1
- GRC-171 RADAR 1
- GRC-171 ATC 1
- TRAP-22 1
Requires modification. See service manual page 1-26 for further details .
- MSC 2001, Sales brochure
Siemens AG, June 1979. 8 pages (German). 1
- MSC 2001, Beschreibung und technische Daten
Device description and technical specifications (German)
Siemens AG, date unknown. Chapter 3, 3 pages (German).
- MSC 2001, Bedienungsanleitung
User manual (German). Siemens AG, December 1978. S42043-E9-P201/D.
- MSC 2001, Technical Manual, Volume I and II
Siemens AG, 22 May 1980.
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© Crypto Museum. Created: Sunday 01 August 2010. Last changed: Wednesday, 27 December 2017 - 18:09 CET.