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Lorenz Mi544
One-time tape cipher machine · 1956 - this page is a stub

The Lorenz Mischer 544 (English: Lorenz Mixer 544), also known as Mi544 or LoMi, was a one-time tape (OTT) cipher machine (mixer), developed around 1956 by C. Lorenz Aktien­gesell­schaft in Stuttgart (Germany). It was used by the German Army (the Bundeswehr), and within NATO. The machine was compatible with other OTT cipher machines — such as the Dutch Philips Ecolex II and Ecolex IV, and the Norwegian ETCRRM — but only if key tape was shifted by one position [1].

The Mi-544 is housed in an extremely large die-cast aluminium enclosure that measures 680 x 520 x 330 mm and weights no less than 56 kg.

The device is intended as an add-on for existing teleprinter installations and should be connected between the teleprinter and the line. It takes the 5-level output from the teleprinter and mixes it with the characters from a key tape, using XOR arithmetic, also known as modulo-2 addition. The image on the right shows the initial version of the Mi544 in its typical green hamerite finish. At the top is a resessed bay for the key tape.
  
Lorenz Mi544

The One-Time Tape principle with its XOR arithmetic, is described in US patent 1,320,729 and is known as the Vernam Cipher, named after the American inventor Gilbert Vernam who invented it in 1917, along with the One-Time Pad (OTP). When used correctly (i.e. the key tape contains truely random characters, is used only once and is destroyed after use) the cipher is unbreakable.

The Mi-544 was developed in the mid-1950s, and was first delivered to the Bundeswehr in 1956. It was first described by Lorenz employee G. Grimsen in SEG·Nachrichtenheft 4 of 1956 [4]. This version has the bay for the key-tape cassette at the top. In 1958, it was succeeded by the slightly improved Mi 544B, which had the key-tape bay at the front right. The price for a bare Mi-544B in 1963 was DM 11,260 (EUR 5,630), or DM 15,885 (EUR 7,942) when the SYZ-634 synchroniser was also required [3]. 1 In the 1970s, the Mi544 was superceeded by the Siemens M-190.

  1. These prices do not include the teleprinter to which it had to be connected.

At present, this page only acts as a placeholder for (future) information about the Mi544 (LoMi).
Lorenz Mi544
Meter
Fuses
Lorenz Mi-544B. Image kindly provided by Heinz Blumberg [2].
Interior of the Lorenz Mi-544B. Image kindly provided by Heinz Blumberg [2].
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Lorenz Mi544
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Meter
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Fuses
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Lorenz Mi-544B. Image kindly provided by Heinz Blumberg [2].
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Interior of the Lorenz Mi-544B. Image kindly provided by Heinz Blumberg [2].

Models
  • Mi544
    Primary version of 1956, with key-tape bay at the top
  • Mi554B
    Improved version of 1958, with key-tape bay at the right
Tape cartridge
The 5-level key tape was supplied in the metal container, or cassette, shown in the image on the right. The cassette can be locked with a cross-key, and has an internal paper feed that is constructed in such a way that the paper can only be pulled out and cannot be put back in.

At the top is a raised section the holds the locking mechanism and a window through which the user can check the tape supply. The cassette was installed in a bay at the top of the Mi-544, or vertically at the front right of the Mi-544B.

  
LoMi key-tape cassette

Lockable key-tape cassette for the Lorenz Mixer Mi-544, with two keys
LoMi key-tape cassette
LoMi key-tape cassette
Locking and unlocking the cassette
Open cassette
Tape guides, designed for one-way use (to prevent the tape from being fed back)
Locking mechanism
Acceptance stamp inside the cassette
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Lockable key-tape cassette for the Lorenz Mixer Mi-544, with two keys
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LoMi key-tape cassette
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LoMi key-tape cassette
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Locking and unlocking the cassette
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Open cassette
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Tape guides, designed for one-way use (to prevent the tape from being fed back)
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Locking mechanism
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Acceptance stamp inside the cassette

Interior
Interior
Mechanics detail
Cam-shafts
Cam-shaft and electromagnets
Detail
Mechanics - top view
Mechanics - detail
Opening the key tape reader
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Interior
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Mechanics detail
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Cam-shafts
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Cam-shaft and electromagnets
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Detail
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Mechanics - top view
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Mechanics - detail
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Opening the key tape reader

Nomenclature
  • Lorenz Mischer 544
  • Lorenz Mixer
  • Mi544
  • LoMi 544
  • LoMi
Specifications
  • Principle
    One-Time Tape (mixer)
  • Alphabet
    5-level ITA-2
  • Transmission
    50 baud (optionally 45.5 baud) 1
  • Speed
    428 cpm at 50 baud (368 cpm at 45.5 baud)
  • Interface
    2-wire single-current, 40 mA
  • Distortion
    40% max.
  • Tape diameter
    190 mm max.
  • Synchroniser
    Option
  • Mains power
    220V AC (50-60 Hz)
  • Internal
    60V DC, 120V DC
  • Consumption
    150W (without peripherals)
  • Lamps
    6V, 0.3A
  • Fuses
    F1: 0.1A, F2: 0.3A, F3: 2A
  • Dimensions
    330 x 520 x 680 mm
  • Weight
    56 kg
  1. Requires the swapping of a cogwheel.

Documentation
  1. Mischgerat Mi 544, Technical description (in German language) 1
    Nr. 041-560 LD. Lorenz AG, Stuttgart (Germany). Issue 3, July 1958.

  2. Mischgerat Mi 544, Operating instructions (in German language) 1
    Nr. 041-1020 LD. Lorenz AG, Stuttgart (Germany). Issue 1, July 1959.

  3. Mischgerat Mi 544, Einstell- und Ölvorschrift (in German language)
    Nr. 041-913 LD. Lorenz AG, Stuttgart (Germany), August 1958.
    Provisional release.
  1. Document kindly provided by Heinz Blumberg [2].

References
  1. Ecolex II User Manual, Supplement
    Description of interoperability with Lorenz Mixer (in Dutch language).
    Koninklijke Landmacht. 8A-II-IUB 504427. 15 April 1965.

  2. Heinz Blumberg, Lorenz Mi 544 documentation
    Received 24 January 2019.

  3. ITT Standard Austria, Verschlüsselungsgeräte 1
    Letter to Bundesministerium für Auswärtige Angelegenheiten.
    Vienna, 8 April 1963. 3 pages.

  4. G. Grimsen, Das Mischgerat Mi 544 1
    C. Lorenz AG. Seg·Nachrichten, 1956, Heft 4, pp. 181-188.
  1. Document kindly provided by Heinz Blumberg [2].

Further information
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Crypto Museum. Created: Monday 02 February 2015. Last changed: Tuesday, 25 January 2022 - 23:27 CET.
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