Ecolex I →
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
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
A critical part of the system is the key generator, which
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
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.
Officer in the Order of the Dutch Lion.
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.
- Colex Circuit Diagram
Dutch: Schema van codeer-inrichting.
R. Oberman, 5 February 1947. #CM303151/C
- Noise Generator, circuit diagram
Anton Snijders, 29 May 1951.
- 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.