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One-Time Tape cipher machines

A mixer is a class of cipher machine that is based on the Vernam Cipher, in which plain text is mixed with a random key stream — hence the name mixer — by means of modulo-2 addition (XOR). If the tape contains evenly-spread truly random symbols and the device is used correctly, the system is guaranteed to be unbreakable. It is also known as a One-Time Tape (OTT) machine.

Mixers on this website
The Siemens T-43 mixer machine
British/Canadion one-time tape cipher machine used during and after WWII
British 5-UCO (BID/30) OTT cipher machine
ATCRRM mixer machine used on the Washington-Moscow hotline
British one-time tape cipher machine compatible with Rockex
PTT Colex (predecessor of Ecolex)
Philips Ecolex I
Philips Ecolex II
Philips Ecolex IV
Siemens Schlüsselgerät D
Siemens M-190 OTT cipher machine, used on the Washington-Moscow hotline
Hagelin TC-52
Hagelin C-446-RT, the OTP (OTT) version of the C-446
OTP/OTT version of the Hagelin CX-52
Russian M-105 (AGAT) mixer machine
DUDEK StG-1 (T-352 / T-353) one-time tape cipher machine developed in Poland
OTT cipher machine (mixer) for teletype networks (telex)
Mils Elektronik one-time tape cipher machine, developed in the mid-1970s.
Mils Elektronik one-time tape cipher machine, with key generator
Lorenz Mixer (Mi-544)
SELMA OKA-150, telegraphy cipher machine
Related equipment
KD-100 key tape disintegrator
It is often thought that, like most cipher machines, mixers are classified items. However, due to the way the mixer works, there is nothing secret about the machine at all. Besides, when the machine is used correctly, the code is unbreakable anyway. Most mixer machines were therefore unclassified, although circuit diagrams and user manuals may have been restricted at the time.

With machines of this class, it is the key tape that protects the secret. This is the reason why the key tapes were only used once and were destroyed immediately after use, so that they could not fall into the wrong hands. Operational key tapes were always classified. They often carried labels like NATO Secret. Placing a classified key tape on a machine, makes the entire system classified.

Most mixers, or OTT machines, use data from a teleprinter machine or from a paper-tape reader as input. Such data is generally stored in 5-bit digital format, commonly in ITA2 code (baudot), but other data formats are also possible. Plain text is either entered directly on the keyboard of the teleprinter (online), or is stored on a punched paper-tape first and replayed later (offline).

<i>Mixing of the <b>plain text</b> and the <b>key</b></i>

The above illustration explains how the mixer works. Each letter of the Plaintext is added to a letter from a Key tape, using an exclusive-OR, or XOR, operation. In mathematics this is known as modulo-2 addition. In cryptography it is known as the Vernam Cipher. It has the advantage of being reversible: by adding the key stream to the ciphertext, the original plaintext is retrieved.

 More about the Vernam Cipher.

Many companies and countries claim the invention of the One-Time Tape cipher machine (mixer). Although the Philips Ecolex was definitely not the first machine in this class, its 'inventor' was payed for his patents for many years. STK (now: Thales) claims that it was a Norwegian invention, but their patent of 1952 1 is predated by the Siemens T-43, the British 5-UCO and the British-Canadian Rockex, all of which were developed during WWII and were introduced in 1943.

Although all mixers are based on the so-called Vernam Cipher, an invention of Gilbert Sandford Vernam in 1918, and that Vernam is also the (co)inventor of the One-Time Pad (OTP), the first machine that was based on the Vernam Cipher (1926, Telekrypton) used a looped key tape and was therefore not a One-Time Tape machine. This means that, based on the currently available information, the Siemens T-43, the British Rockex and the 5-UCO should be recognised as firsts.

  1. Although this patent is frequenty mentioned in literature, for example in [1], we have not been able to find it. If anyone has access to this patent, please contact us.

  1. Norwegian National Security Authority (NSM), Årsmelding 2008
    NSM Annual Report 2008 (Norwegian). Noen kryptosuksesser. p. 15.
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© Crypto Museum. Created: Tuesday 04 August 2009. Last changed: Tuesday, 25 January 2022 - 23:38 CET.
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