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Schlüsselgerät D
One-time tape cipher machine (mixer)

Schlüsselgerät D, or cipher machine D, was a One-Time Tape (OTT) cipher machine, introduced by Siemens & Halske in 1959 [1]. The machine is built around a Siemens T.send.77f double tape reader, and was used in combination with a Siemens T-37, T-37i, T-68 or T-100 teletypewriter. The machine can be seen as the successor to Siemens' wartime development of the T-43 mixer.
The image on the right shows a typical Cipher Machine D, consisting of a bottom section with the actual cipher machine, a double tape reader 77f in the middle, and a control unit at the top.

The rearmost tape reader is for the (random) key tape, whilst the one at the front is for the clear text tape or the cipher text tape, depending on the mode. The machine is intended for offline use and can be used stand-alone (i.e. connected to a 5-level tape puncher), as a transmitter (i.e. connected directly to the line), or as part of a data-entry setup (i.e. connected to a telex unit).
Siemens Schlüsselgerät D

The device uses the so-called Vernam Cipher in which each character of the plain text tape is mixed with one character of the key tape, by means of an XOR-operation. When the key tape contains true random data and the machine is used properly, this cipher is truely unbreakable.

Cipher Machine D was introduced in 1959 and was produced until the mid 1960s, after which it was succeeded by the M-190. According to date codes on the components, the device shown here was built in or after 1965. It has French labels 1 and was used by the French Army or the French Foreign Ministry, in combination with a T-37 or T-68 telex. The device is very similar to the Philips Ecolex II, which was built around the same Siemens T.send.77f double tape reader. Around 1965, Schlüsselgerät D was succeeded by the similar but more advanced M-190 mixer.
  1. The device was usually labelled in German, but the one shown here was made especially for the French Government and, hence, is labelled in French. As all labels are removable, Siemens could easily adapt the device for any country or customer.

Siemens Schlüsselgerät D Close-up of the controls and the two tape readers Controls Left view

The diagram below shows a view of the left side of the machine. The name tag Schlüsselgerät D is missing here and has probably been removed. It is usually present to the right of the other two tags. The leftmost plate is of the double tape reader (T.send.77f), which is used as the heart of the complete device. The actual mixer (i.e. the XOR circuitry) is located in the bottom section.

Controls and connections of Schlüsselgerät D

At the bottom of the left side are the connections for the mains network and a teletypewriter (telex). Also present at the left is a hinged door behind which a large socket with a connector is located. Apart from the power switches, all controls are located at the top section. The device has two identical 5-level punched paper tape readers. The rearmost (i.e. the one closest to the control panel) is for the key tape. The one at the front is for the clear tape or the cipher tape.
Left view Right side Controls and tape readers Controls Controls Behind the hinged door Connector removed from the socket Switching the machine ON
Interface panel Key tape reader Clear/cipher tape reader Opening the tape reader Close-up of the sensing pins Closing the tape reader Tape path (reader closed) Tape path (reader open)

Block diagram
The block diagram below shows how the device works. At the left are the two tape readers, one for the plain (or cipher) text and one for the key tape. The 5-bit outputs of the tape readers is fed to the inputs of a so-called mixer that produces the 5-bit XOR (modulo-2) of the individual bits of the inputs. The XOR circuits in the mixer are built with electric relays. Note that the inputs of the mixer are wired via connector H, which is available at the left side of the machine. A shorting plug must be present in the socket for normal operation. The pinout of the plug is given below.

From the mixer, the 5-bit parallel signal is fed into a serializer, which converts the data into a serial asynchronous protocol, allowing it to be sent over a standard telex line or via radio. The line interface converts the serial data signal into a regular 40 mA current loop telex signal.
The complete setup was known as Schlüsselgerätesatz D (cipher kit D), extended with the number of the teletypewriter with which it was issued. As a general rule of thumb, black units were commonly configured for 45 or 50 baud operations, whilst beige units were generally used for 75 baud operation, although there are exceptions to this rule. The following setups are known:
  • D/37
    Schlüsselgerät D with Siemens T-37 or T-37i teletypewriter. In this case, the cipher machine would be supplied in a black enclosure, just like the one featured here.

  • D/68
    Schlüsselgerät D with Siemens T-68 teletypewriter. As the T-68 telex machine was supplied both in black and beige, it is likely that the cipher machine was available in these colours as well.

  • D/100
    Schlüsselgerät D with Siemens T-100 teletypewriter. In this case the cipher machine was most likely supplied in a beige enclosure, in order to match the colour of the T-100.

Beige variant of Schlüsselgerät D Beige variant of Schlüsselgerät D

Double tape reader   T.send.77f
Cipher Machine D is built around a very popular double tape reader, known as the Siemens & Halske T send 77f. Although initially intended for retransmission equipment, it became a very popular component of some One-Time Tape (OTT) cipher machines in the early 1960s. The two tape readers are fully independent and are not driven by a common axle. Instead, stepping is controlled by a solenoid. Each tape reader delivers its 5-bit parallel output onto a connector at the rear. The actual mixing and conversion into a serial signal is done outside this unit.
  • NSN 5815-25-105-4214
    Distributor-Transmitter, Teletypewriter

  • Chiffrier-Zusatzgerät (Lochstreifenleser) [4]
    Siemens & Halske T send 77 f
    (als Zusatz zum Fernschreiber T 100 oder T-37 i)
    Siemens & Halske, Wernerwerk München, 1964.

The interior of the rather heavy Cipher Machine D is easily accessible. The bottom case shell can be removed after losening just 3 bolts: one at the left and two and the right side of the case. The die-cast aluminium top cover can be taken away after removing a large bolt at the top centre.
The image on the right presents a right-angle view of the machine, after the top and bottom case shells have been removed. The double tape reader can easily be identified towards the front of the device and appears to be in great shape.

Mounted towards the rear is a control panel with 4 push-buttons and a set of 10 electric relays. It is mounted in such a way that the buttons are reachable when the aluminium cover is fitted. The control panel is connected directly to the bottom section by means of a 14-pin connector mounted behind the double tape reader.
Interior - right view

The actual mixer, i.e. the XOR circuitry, is mounted in the bottom section that can be separated from the main unit. It is held in place by four recessed screws. The bottom unit consists of the Power Supply Unit (PSU), the line interface, 5 XOR relays, 1 large telex relay and two small relays.
The image on the right gives a good view of the bottom section, which is connected to the main unit via the two large black connectors at the top right. The power supply and the line interface are at the left, with the fixed rubber mains cable just visible at the top left of the image.

The relays, that are used at the heart of the XOR circuits are clearly visible at the front right. They are used in combiation with a set of rectifier bridges mounted behind them. They are just visible behind the rightmost metal bracket. The large telex relay can be found at the far right.
Mixer unit

At the left side of the device is a hinged door behind which a sloped 30-pin socket marked H is mounted. All parallel data signals are available on this connector, allowing it to be used 1 for testing, fault analysis and external processing of the data. A matching 30-pin shorting connector is fitted into the socket so that in normal use, the 5-bit output of both tape readers is connected to the two 5-bit inputs of the mixer. Looking into the socket, the plug is wired as follows:

Wiring of the shorting plug in socket H, which must be present for normal use

Once the two 5-bit data streams (text and key) are mixed, they are serialized, so that the data can be sent through a regular telex line or via radio. The serializer is mounted in the bottom section of the machine, inside a metal enclosure, close to the actual line interface. The serial output is available as a 50 baud 40 mA current-loop signal on the circular socket at the left.
  1. The shorting plug can also be used as a physical key for protection of cipher security. When the operator has to leave his machine unattended for a short period of time, he could remove the shorting plug from the socket, so that the machine could not be used by others. This is why the plug is easily accessible via the hinged door at the left of the machine.

Top case shell Bottom case shell Interior - right view Interior - left view Left view Control panel interior Mixer unit T.send.77f unit with control panel and connector

  1. MfS, 1 Abteilung XI, Lageeinschätzung des ZCO 1960
    20 January 1960. Tgb.Nr.: 180/60. BStU, MfS, HA II Nr. 033616. Obtained via [2].

  2. Jörg Drobick, Website Der SAS- und Chiffrierdienst (SCD)
    Retrieved March 2016.

  3. Museumsstiftung Post und Telekommunikation

  4. Friedrich L. Bauer, Enzifferte Geheimnisse: Methoden und Maximen...
    1993-2000. ISBN 978-3-642-63545-8. p. 476.

  1. MfS = Ministerium für Staatssicherheit, also known as Stasi.
    The opressive security service of the former DDR (East-Germany).

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© Crypto Museum. Created: Monday 21 March 2016. Last changed: Monday, 11 April 2016 - 07:57 CET.
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