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Siemens
  
← D
  
T-37/ICA
One-time tape cipher machine - this page is a stub

T-37i CA, 1 also written as T-37/ICA, is a one-time tape (OTT) cipher machine, also known as a mixer, comprising a Siemens T-37i teletypewriter (telex) with an embedded cipher extension, in­tro­duced around 1956 by Siemens & Halske in Berlin (Germany). It is an improved version of the Siemens T-37h SZ. 2 Both machines can be seen as a successor to the wartime T-43 (Sägefisch).

...

The device can be recognised by the large red and green lamps that are integrated with its metal cover. 		  

The T-37/ICA was introduced in 1956, and was approved for use by NATO at all levels of con­fi­den­tiality in 1960 [5]. In 1965 TEMPEST problems were encountered. Measurements had shown that information could leak out via the power line, allowing an adversary to reconstruct (part of) the plaintext. After adding a power line filter [6] the device was re-approved for NATO-use [7].

In 1957, the T-37/ICA was succeeded by the external Schlüsselgerät D (cipher machine D), with could be used with any teletypewriter. Nevertheless, the T-37/ICA was kept in production for many years, and remained in service well beyond 1980.

  1. The letters 'CA' stand for Cryptographic Application.
  2. T-37h SZ, also written as T-37/HSZ, is (nearly) identical to the T-37/ICA. The letters 'SZ' stand for Schlüssel-Zusatz (Cryptographic Extension). The service documentation for the T-37/HSZ is also applicable to the T-37/ICA [I].
Specifications
Documentation
  1. Afbeeldingen Verreschrijvers Type 37 en Type 34 en Ponsontvangers
    Detailed pictures and drawings of T-37 and T-34 (Dutch).
    PTT, 1951. 3rd print.

  2. Tgf 1011a, montage-, instel- en onderhoudsvoorschrift T typ 37 g t/m i
    Installation, alignment and maintenance manual (Dutch).
    PTT, November 1956, Issue 1.

  3. Tfg 1011b, Afbeeldingen
    Pictures, belonging to Tgf 1011a (Dutch).
    PTT, November 1956, Issue 1. 1

  4. Tgf 1024a, Beschrijving bladschrijver T typ 37 a t/m i
    Description of T-37a thru i teleprinters (Dutch).
    PTT, August 1959. Issue 2. CM303981.

  5. Tgf 1024b, Afbeeldingen
    Pictures, belonging to Tgf 1024a. (Dutch).
    PTT, August 1959. Issue 2. CM303981.

  6. De Verreschrijvers T-37/LP, T-68/LP
    Description and operating instructions for the Siemens T-37 and T-68 (Dutch).
    Belgian Army. March 1967.

  7. Afregelvoorschrift voor den Verreschrijver en den Reperforator Fabr. S. & H.
    TLX 23. Alignment procedure for the Siemens T-37 (Dutch). A5 size.
    CM303970. To be used with item (F) CM303971.

  8. Afbeeldingen van den Verreschrijver en den Reperforator Fabr. S. & H.
    TLX 70. Images of the Siemens T-37 (Dutch). A4 size.
    CM303971. To be used with item (F) CM303970.

  9. Blattschreiber T typ 37 h mit Schlüsselzusatz - Manual (German)
    Fs Bs 2179/80. Siemens & Halske, 23 May 1956. 2
  1. Document kindly provioded by Gert Stam [4].
  2. Document from MfS Abt XI 385 archive, obtained from BStU [3] and kindly provided by Jörg Drobick [2].
References
  1. Jerry Proc and contributors, T-37 ICA Fjernskriver (Teletypewriter)
    Accessed 24 january 2026.

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

  3. Bundesbeauftragte für die Stasi-Unterlagen (BStU) 1
    Federal Commissioner for the Stasi-Records.

  4. Gert Stam, restored Siemens T-37 with documentation — THANKS !
    November 2017.

  5. NATO, Siemens T-37i CA (approval)
    SGM-138-60. NATO, 9 March 1960.
    ➤ French version

  6. NATO, Siemens T-37i CA (power line filter)
    SGM-111-65. NATO, 23 March 1965.

  7. NATO, Siemens T37i CA (re-approval)
    SGLP 231/65, NATO, 30 March 1965.
  1. Full name: Bundesbeauftragte für die Unterlagen des Staatssicherheitsdienstes der ehemaligen Deutschen Demokratischen Republik (DDR) — Federal Commissioner for the Records of the State Security Service of the former German Democratic Republic (GDR) — officially abbreviated BStU.
Further information
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© Crypto Museum. Created: Saturday 24 January 2026. Last changed: Wednesday, 28 January 2026 - 21:16 CET.
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