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OTT NL PTT Oberman Ecolex I →
One-Time Tape cipher machine
- this page is a stub
Colex, the abbreviation of Code-Telex, was an
electromechanical
One-Time Tape (OTT) cipher machine or
mixer, developed in 1946
by Dr. 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
(backdoor).
The system 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 is known as
modulo-2 addition or exclusive-OR (XOR).
It is based on the Vernam Cipher,
invented in 1917 by Gilbert Sandford Vernam.
The image above shows part of the original 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 is a uniform distribution,
a set of five electromechanical counters was connected to the machine, allowing
the user to verify that each of the five output bits
appeared on the key tape an (approximate) equal number of times.
The first version of Colex was developed in 1946.
Due to shortages in electronic parts immediately after WWII,
it was decided to build the machine with electromagnetic relays;
the ones used in telephone exchanges.
After optimising the binary logic, it required 60 relays.
This turned out to be a bad idea however,
as the optimisation made it more difficult to alter the
design. As a result, a newer version was finished three months
later. It comprised 98 identical relays.
As the rack enclosure had space for 100 relays, the two empty
sockets were populated with spare relays. It allowed anyone to repair
the machine in the field with ony limited technical experience.
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Over the course of 1948, Colex was duely tested.
It was eventually
inaugurated on 5 April 1949 by the Dutch Prime Minister Willem Drees. 1
It was used for highly secure communications between the Foreign Office
in The Hague and the four Dutch embassies in London (UK), Paris (France),
Washington (USA) and 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 Oranje-Nassau 2 [2].
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For the production of the Colex machines, 1200 telephone relays had been
allocated, enough to build 12 machines. In practice however, only 6 Colex
machines were built, as newer designs with valves (tubes) had meanswhile
been considered. The surplus of 600 relays was given to Willem van der Poel
– a student at Delft University – who used it for the construction of the
first Dutch computer, called ARCO, short for Automatic Relay Calculator for
Optics calculations. ARCO was finished in 1952. It was no
faster than a human calculator, but was much more reliable 3.
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, 4
in which transistors replaced the valves.
In 1956, production of the Ecolex machines was transferred
to Philips Usfa.
As far as we know, there are no surviving specimen of the Colex machine.
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At the time, Dutch Prime Minister Willem Drees was also Minister of
Foreign Affair ad-interim. He attended the opening ceremony of the
Colex in his capacity as Minister of Foreign Affairs.|
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Officer in the Order of Orange-Nassau — named after the Dutch Royal Family .
➤ Wikipedia
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Due to its low operating speed — 30 seconds for an addition
and 45 for a multiplication — ARCO was nicknamed TESTUDO (turtle).
Nevertheless it was extremely reliable and was used for 12 years by
TNO for lens calculations. Furthermore, it paved
the way for later electronic computers like PTERA and ZEBRA [4][5].
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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.
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- The Hague, Netherlands (2 units)
- London, UK
- Paris, France
- Washington, USA
- Djakarta, Indonesia
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- Colex Circuit Diagram
Dutch: Schema van codeer-inrichting.
R. Oberman, 5 February 1947. #CM303151/C
- Noise Generator, circuit diagram
Anton Snijders, 29 May 1951.
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© Crypto Museum. Created: Tuesday 08 December 2020. Last changed: Tuesday, 06 January 2026 - 21:15 CET.
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![Part of original circuit diagram of the Colex cipher machine. Copyright Crypto Museum [A].](../../../people/oberman/img/rmm_colex.png)
