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Siemens M-190
One-time Tape cipher machine (mixer)

The M-190 was a cipher machine for teleprinter communication (telex), built by Siemens in the early 1960s. Its operation is based on the Vernam Cipher and the machine therefore belongs to the so-called mixer-class or One-Time Tape (OTP/OTT). The M-190 can be seen as one of the successors to the wartime Siemens T-43, that was built on the chassis of a T-37 teleprinter. In the German Army the machine was also known as LOMI after its predecessor, the Lorenz Mixer.

The principle of a mixer machine is that a plain-text character from one paper tape is mixed with a random character from a key tape, resulting in a cipher-text character. As the mixing is done by means of an exclusive-OR (XOR) operation (also known as modulo-2 addition), the same key tape character is mixed with the cipher-text at the receiving end, in order to recover the plain-text.

The image on the right shows a typical Siemens M-190 cipher machine, with both the clear-text tape and the key-tape loaded. An identical key-tape has to be used at both ends of the link.

It is also important that the key-tape is at the same starting position at the sending and receiving end. The M-190 can be used for both offline and online operation. For offline use, the plain-text is previously recorded onto a blank paper tape, using a standard teleprinter, such as the Siemens T-100. The tape is then played back, using the frontmost paper tape reader (perforated tape).

Two M-190 machines in a transmission link

When used online, the teleprinter is connected directly to the M-190, and the frontmost paper tape reader left unused. Instead, the key-tape is automatically advanced by one position, each time the operator types a character on the keyboard. Online use of cipher machines is often considered unsafe, as it may reveal operator's characteristics, such as typing speed. Furthermore, it is prone to mistakes as an operator may accidently leave the machine in plaintext mode.

In order to avoid plaintext revealing mistakes, the M-190 has two large coloured lamps at the front right of its control panel. When the RED lamp is ON, the system is operated in plaintext mode. Only when the GREEN lamp is ON, encryption is enabled and transmission of text is safe. Although the M-190 can be used with virtually any teleprinter brand and model, it was commonly used in combination with the equally coloured contemporary Siemens T-100 teleprinter (telex).

The M-190 was used for many years by NATO and was compatible with other NATO mixer machines, such as the ETCRRM and the Philips Ecolex IV. It was also used for several years on the Washington-Moscow Hotline from 1980 onwards, until it was superceeded by computers in 1988.

M-190 with both paper tapes present
M-190 with both paper tapes present
M-190 with both paper tapes present
Siemens M-190 mixer machine
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M-190 with both paper tapes present
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M-190 with both paper tapes present
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M-190 with both paper tapes present
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Siemens M-190 mixer machine

From the mid-1980s onwards, electro-mechanical mixer machines like the M-190 were gradually replaced by fully electronic teleprinters and eventually by computers. Although some teleprinter links remained operational into the early 2000s, they have now been phased out completely. Most machines have subsequently been destroyed and are now highly wanted collectors items.

At the exhibition Secret Messages held between October 2008 and February 2009 in Museum Jan Corver in the Netherlands, we had the unique opportunity to connect two fully operational M-190 machines together in a small network. The setup was used to demonstrate the practical use of OTT/OTP equipment to the audience.

The image on the right shows former intercept-operator Louis van Erck (at the rear) and Crypto Museum curator Marc Simons (at the front) each sitting behind a Siemens T-100, exchanging messages at typing speed using two M-190s.

For the demonstration, the two teleprinter systems were connected together in a so-called current loop, using a line simulating device. The two M-190 machines were loaded with identical key-tapes that had been prepared prior to the event. Both machines were used in online mode. Before starting, both key-tapes had to be set to the same starting position. When typing text on one machine, the same text would be printed at the other end. It was also demonstrated what would happen if the key-tape on one machine was shifted by just one position.

Washington-Moscow Hotline
During the Cold War, the Americans and the Russians installed the so-called Washington-Moscow Hotline (sometimes referred to as the Red Line), allowing direct communication between the two parties in an attempt to avoid conflicts [1]. The Hotline was established in 1963 and initially consisted of two pairs of data links, using the ETCRRM one-time pad cipher machine, built by the Norwegian company STK, in combination with Siemens T-63 teleprinters with Cyrillic alphabet.

In 1980, the aging equipment was replaced by newer teletype units and Siemens M-190 cipher machines. The image above shows the Hotline terminal room in the NMCC at the Pentagon, with the M-190 clearly visible in the foreground. The picture was taken on 27 August 1985, shortly before the Hotline was superceeded by IBM computers as shown in the image below [2].

The image above shows the other side of the Hotline terminal room in the NMCC at the Pentagon on 14 November 1985 [3]. Four IBM PCs are visible in the background, complete with facsimile units (fax). In the foreground, the Siemens M-190 is still visible. Although the Hotline functionality was taken over by Fax, teleprinter lines were used as backups until 1988 [1].

 More about the Hotline

Like most cipher machines of the mixer-class, the M-190 is not (and never was) classified. It is the combination of the machine and a NATO-issued cipher tape, that was classified as NATO SECRET. Because of the fact that one-time pads (OTP) were used with the M-190, it is impossible to use the machine to break old intercepts, as all original OTP-tapes were destroyed immediately after use. Only a few M-190s have survived as most of them were destroyed in the early 2000s.

  • Data rate
    45.45, 50 or 75 baud (368, 400 or 600 char/min)
  • Line voltage
  • Line current
  • Power
    -24V and -120V
  • Motor
    220V AC, 50-60Hz, 80W, asynchronous, 2500 RPM
  • Dimensions
    27 x 33 x 61 cm
  • Weight
  1. Siemens & Halske AG, Mischgerät M-190, Beschreibung
    Description (German). A22261-A190-A1-0-18. August 1962.

  2. Siemens & Halske AG, Mischgerät M-190, Betriebsanleitung
    Operational and technical description (German). A22261-A190-A1-0-30. July 1965.
  1. Peter Koop, The Washington-Moscow Hot Line
    Retrieved January 2013.

  2. AP, Photograph of Siemens M-190 and teletypes in the Hotline room
    27 August 1985. Retrieved November 2012. Image kindly supplied by [3].

  3. Scott Davis, Images of US-Soviet Hotline
    © Time Inc. LIFE magazine, 14 November 1985, Arlington, VA, USA.
    Retrieved January 2013. 1
  1. Website no longer available in 2017.

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© Crypto Museum. Created: Wednesday 05 August 2009. Last changed: Friday, 23 February 2018 - 20:35 CET.
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