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Mixer machines
One-Time Tape cipher machines

Mixers are a class of cipher machines that are based on the Vernam Cipher in which Plain text is mixed with a random key stream (hence the name mixer). If the mixer is used with a key tape that contains evenly-spread truely random characters, the cipher is guaranteed to be unbreakable. Such a key tape is commonly referred to as a One-Time Pad (OTP) or a One-Time Tape (OTT).
 
One-Time Tape machines on this website
The Siemens T-43 mixer machine T-43 British/Canadion one-time tape cipher machine used during and after WWII Rockex British 5-UCO (BID/30) OTT cipher machine 5-UCO ATCRRM mixer machine used on the Washington-Moscow hotline ETCRRM British one-time tape cipher machine compatible with Rockex Noreen Philips Ecolex I Ecolex 1 Philips Ecolex II Ecolex 2 Philips Ecolex IV Ecolex 4
Siemens M-190 OTT cipher machine, used on the Washington-Moscow hotline M-190 Hagelin TC-52 TC-52 Hagelin C-446-RT, the OTP (OTT) version of the C-446 C-446-RT OTP/OTT version of the Hagelin CX-52 CX-52-RT Russian M-105 (AGAT) mixer machine M-105 DUDEK StG-1 (T-352 / T-353) one-time tape cipher machine developed in Poland DUDEK Mils Elektronik one-time tape cipher machine, developed in the mid-1970s. ME-620 Mils Elektronik one-time tape cipher machine, with key generator ME-640

Most of these machines use data from a teletype unit or from a paper-tape reader as input. Such data is generally stored in 5-bit digital format (e.g. in Baudot code), but other data formats are also possible. Plain text is either entered directly on the keyboard of a teletype machine (online), or is stored on a punched paper-tape first and sent later (offline).

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

The above illustration explains how the mixer works. Eacht letter from the Plaintext is added to a letter from a Key tape, using an exclusive-OR (XOR) operation (sometimes called 'modulo-2 addition). The advantage of this operation is that it is reversable: adding the key stream to the cipher text again, reveals the original plaintext.

 More about the Vernam Cipher.
 
Unclassified
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 in 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.
 
Invention
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 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.
 
Futher information

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