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One-Time Tape machines   Mixers
Teleprinter version of the OTP

One-Time Tape, or OTT, is an automated digital variant of the so-called One-Time Pad, or OTP. When correctly applied, OTT-encrypted messages are unbreakable. OTT is best explained as a practical implementation of the OTP system for use with telegraph equipment, also known as Teleprinters or Telex, using a 5-bit digital code (often ITA-2, baudot) for the exchange of data.

A key tape is generated by recording the data of a random number generator (noise). The Vernam Principle is then used to combine each plaintext character with a single character from the key tape. As a simple XOR-operation is used for this process of 'mixing', the same key tape can be mixed again with the ciphertext at the receiving end, in order to recover the plaintext again.

Machines that use OTT, are commonly called mixers. A good example is the ETCRRM that was used for many years during the Cold War on the Washington-Moscow teleprinter hotline.

Another good example of a mixer, that was also used on the Washington-Moscow Hotline, is the Siemens M-190. Over the years, a variety of mixers have been developed and produced world-wide by various manufacturers. They were generally used at the top level of a command chain, for messages that had to remain secret indefinitely. Contrary to popular belief, mixers are (and have always been) unclassified devices. It is the combination of the machine with a keytape that is classified to the level of the keytape.

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)
Ecolex I, developed by PTT, manufactured by Philips Usfa
Ecolex I Mark 2, developed by PTT, never produced in quantity
Ecolex II, developed by PTT, manufactured by Philips Usfa
Ecolex III (or Ecolex IIB), the synchronised variant of the 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 items
KD-100 key tape disintegrator
The unbreakable One-Time Pad (OTP)
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 its operation. 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 enemy hands. Operational key tapes were always classified. They often carried labels like NATO Secret. Placing a classified keytape on a machine, makes the entire system classified to the level of the keytape.
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).

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 (Telekrypton, 1926) 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 [5], we have not been able to find it. If anyone has access to this patent, please contact us.

OTT generation
When using OTT equipment, or mixers, a sufficient supply of key tapes was mandatory in order to keep up with the constant flow of messages. Key tapes were initially produced manually with so-called manual tape punchers, but this had numerous drawbacks, such as the long time it took to produce a single tape and the lack of randomness in the human mind when pressing the buttons.

To overcome these drawbacks, the process was automated and machines were developed for creating the (pseudo) random key streams and punching them onto 5-level paper tape. Initially, mechanical methods were used for producing the key stream, but as these too lacked sufficient randomness, noise generators were introduced.

The image on the right shows the 5224, one of the first key tape generators that were available on the market, made by Reichert Elektronik in Germany (now: Mils in Austria). It has a built-in white noise generator and produces two tapes.

In order to ensure that both key tapes are identical, they are punched simultaneously in a single tape puncher. Futhermore, the machine has 10 counters, to keep track of the number of zeros and ones that are generated. In a truely random system, they should be distributed evenly.

Although noise generators were already in use during WWII for creation of one-time keys for the SIGSALY transatlantic secure voice link between the UK and the US, most OTP and OTT systems that were used during the war and shortly thereafter, used mechanically generated (and therefore deterministic) pseudo random keys.

The Siemens T-43 mixer, for example, used two Siemens T-52 Geheimschreibers in series for the generation of its key tapes. If the codebreakers at Bletchley Park had known this, it would have been relatively easy for them to break the cipher.

In 1952, Dr. Werner Liebknecht, a developer at C. Lorenz AG in Stuttgart (Germany) was the first one to publicly file a patent for a Random Number Generator (RNG) based on a white noise source. It produced evenly spread non-deterministic numbers that were idealy suited for the generation of One-Time Tapes. The patent was bought by Willy Reichert in Trier (Germany) who used it to build the first commercially available OTT generator on the market: the Würfel (Dice).

 More about the 5224

OTT distribution
Like with the manual OTP cipher, OTT systems suffer from the same key distribution problem. Especially in applications where communication takes place on a large scale, such as in the Army, one has to ensure that a sufficient supply of fresh (unused) key tapes is available at all times.

Although this may seem a simple requirement, it often caused logistics problems with Army units in remote locations and aboard ships. Key distribution would be seriously hampered, or indeed be completely impossible, in the event of a war, e.g. when operating behind enemy lines.

Key-tape shortages have led to several security incidents and compromises in the past. Some operators re-used an old key-tape, or used it in reverse direction. There are even stories about operators who taped a one-metre piece of key-tape together and used it as an endless loop...

For this reason, most Armies abandonned the use of OTT machines and replaced them by cipher machines with a built-in key generator. Such key generators generally consisted of a (pseudo) random number generator (PRNG) that was seeded by a much shorter KEY. Good examples of such machines are the Philips Ecolex X, the Philips Aroflex and the KL-51 (RACE). Although such machines are often advertised as a more practical implementation of the OTP, they do not meet the requirements of an OTP and are never absolutely safe. They are just much more practical.

Nevertheless, OTT machines remained in service for many years, for messages that had to remain secret indefinitely, such as at the highest level at NATO, in diplomacy and on the Washington-Moscow Hotline. Today, the principle of the OTP/OTT is often implemented with computers, but this poses a real security threat as there is no such thing as a secure personal computer.

  1. AIVD, One-Time Pad and OTP concealment
    Dutch General Intelligence and Security Service. October 2010.

  2. Detlev Vreisleben, Personal collection of One-Time Pads
    Photographed by Crypto Museum. Köln (Germany), 20 March 2010.

  3. Wikipedia, One-time pad
    Retrieved January 2013.

  4. Dirk Rijmenants, Secure Communications with the One Time Pad Cipher
    Paper (English) 2009-2014. Version 6.2, 18 December 2014.

  5. Norwegian National Security Authority (NSM), Årsmelding 2008
    NSM Annual Report 2008 (Norwegian). Noen kryptosuksesser. p. 15.
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© Crypto Museum. Created: Saturday 08 September 2012. Last changed: Sunday, 19 February 2023 - 12:05 CET.
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