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One-Time Tape cipher machine - this page is a stub

Colex, the abbreviation of Code-Telex, was a One-Time Tape (OTT) cipher machine, also known as mixer, developed in 1946 by Ir. Roelof Oberman at the Dr. Neher Laboratory in Leidschendam (Netherlands), at the time part of the Dutch state-owned telecom monopolist PTT. The device was developed at the request of the Dutch Government, who – based on wartime experience – did not trust foreign cipher equipment, as it might contain weaknesses or even a deliberate backdoor.

Part of original circuit diagram of the Colex cipher machine. Copyright Crypto Museum.

The device consists of two parts: a mixer and a key generator. The mixer adds each letter of the plaintext to a character from the key tape, using a mix and unmix operation. This operation later became known as exclusive-OR (XOR) or modulo-2 addition. It is based on the Vernam Cipher, invented in 1917 by Gilbert Sandford Vernam. The image above shows part of the original circuit diagram, from the Crypto Museum collection [A]. Due to shortages in electronic components in the period immediately after WWII, it was constructed with approx. 100 electromechanical relays.

Oberman's signature on the original Colex circuit diagram

A critical part of the system is the key generator, which Oberman named Roulette. It comprises a 400 kHz oscillator of which the output is divided by 2 in five stages. Each stage produces one of the five bits of the ITA2 telegraph alphabet (Baudot), whilst a relay is used to sample the output at ~225 ms intervals. The uncertainty of the movement of the electromechanical relay is used as the source of randomness (noise). To check whether the noise was evenly distributed, a set of five electromechanical counters was connected to the machine, allowing the user to check whether each of the five output bits appeared on the key tape an (approximate) equal number of times.

Colex was developed during the course of 1946 and 1947, and was eventually inaugurated on 6 April 1949 by the Dutch Prime Minister Willem Drees. It was used for highly secure communi­cations between the Foreign Office in The Hague and its embassies in London (UK), Paris (France), Washington (USA) and also in Jakarta (Indonesia).

In the image on the right, Colex is just visible at the bottom left whilst the Prime Minister enters a message on the keyboard of a T-37 telex. At the occasion, Professor Oberman was decorated as Officier in de Orde van de Nederlandse Leeuw. 1
Dutch Prime Minister Willen Drees at the commissioning of the first Colex machines on 6 April 1949

In 1953, Colex was succeeded by Ecolex – short for Electronic Codetelex – in which the electro­mechanical relays were replaced by thermionic valves (vacuum tubes). Ecolex was later renamed Ecolex I, and was eventually succeeded by Ecolex II, 2 in which transistors replaced the valves. In 1956 or 1957, the production of the Ecolex machines was transferred to Philips Usfa.

As far as we know, there are no surviving samples of the Colex machine.

  1. Officer in the Order of the Dutch Lion.
  2. The first version of the Ecolex II was built with valves (Ecolex IIa), whilst a later version (Ecolex IIb) was built with the first geneneration transistors.

  1. Colex Circuit Diagram
    Dutch: Schema van codeer-inrichting.
    R. Oberman, 5 February 1947. #CM303151/C

  2. Noise Generator, circuit diagram
    Anton Snijders, 29 May 1951.
  1. Maarten Oberman, Personal correspondence
    December 2018 — December 2020.
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Crypto Museum. Created: Tuesday 08 December 2020. Last changed: Monday, 11 January 2021 - 20:14 CET.
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