Ecolex II →
← Ecolex I
Two versions 1 of the Ecolex I existed: Mark 1, which
operated on the 26 characters of the Latin alphabet and was compatible with
existing mechanical OTT machines, and Mark 2, which operated
on all 32 characters of the ITA2 telegraph alphabet.
[B] The image on the right shows the Ecolex I Mark 2 without its cover.
It is believed to be the only surviving specimen of this model,
and is currently held in an internal collection of the Dutch
Department of Defense.
The machine is built around 22 switching elements and 250 rectifier cells
The switching elements are implemented as circular plug-in units with
an E90CC special quality (SQ) double-tetrode valve on top. At the rear
are the transformers and a cooling fan. The device is much simpler than
its bigger brother, the Ecolex I Mark 1, which operates on the
26 characters of the Latin alphabet only, and consists of no less than
68 switching elements and 600 diodes.
The Ecolex I Mark II had PTT model number 3201 and was produced in
a small prototype series. The unit shown above has serial number 102.
If we assume that numbering started at 100, this means that it is the
second prototype. No further 3201 units were made, as the next prototype
– 3202 – was built with transistors . After a few protypes made by PTT,
production of the fully transistorised version was taken over by
Philips Usfa, where it was known as
US 8011 or Ecolex II.
➤ More about the Ecolex I Mark 1
➤ More about the Ecolex II
The suffix 'I' (one) to the model name Ecolex I was
added once the development of the Ecolex II had started.
Before that time, the device was simply known as Ecolex.
The two versions were known as Mark 1 and Mark 2, which,
confusingly, was also written as Mark I and Mark II.
One-time tape machines like the Ecolex I are in theory
unbreakable if, and only if, the keystream tape is truly random, the tape
is used only once, and the tape is destroyed immediately after use.
In the machine, plaintext (from the teleprinter or the first tape
reader) is added to the random tokens from the second tape reader by
means of modulo-2 addition, also known as exclusive-OR (XOR).
This principle, invented in 1970 by Gilbert Vernam, is known as the
The process is also known as mixing, which is why
are also known as mixers.
The advantage of using modulo-2 addition (XOR) is that the same
procedure can be used with the ciphertext to reveal the plaintext.
The process of unmixing is identical to the process of mixing.
➤ More about the Vernam Cipher
For generation of the keytapes it is important that it contains truly random
characters, which is why they are also known as noise tapes.
In practice however, many governments created the key tapes with a
pseudo-random number generator (PRNG), which is much weaker, as in that
case the 'noise' is deterministic. For this reason, PTT
developed its own random noise generator: EROLET.
➤ More about the EROLET keytape generator
The image on the right shows a complete setup with the Ecolex I Mark 1.
The machine itself is at the far right. At the left is a
Siemens T-37 teleprinter which is used
as an input/output device.
At the centre is a Siemens T-send-77f double tape reader on which the
plaintext and the key stream tape are loaded. The large
Power Supply Unit (PSU) is hidden under the table.
The machine is built around 68 valve-based switching circuits and 600
rectifier cells (diodes).
➤ More information
The Ecolex I Mark 2 is much smaller than the Mark 1, and works on any of the
32 characters of the ITA2 telegraph alphabet. Contrary to
the Mark 1 however, the Mark 2 is only suitable for point-to-point traffic,
in which an identical machine is used at both ends.
The device consists of 22 switching circuits built with E90CC valves and 25
rectifier cells (diodes). It is likely that the machine shown in the image
on the right is the only surviving specimen.
The machine described on this page is of this type.
Double tape reader
Regardless the version, a double punched paper tape reader had to be
connected to the dive, so that plaintext and keystream could be read
simultaneously and mixed in the Ecolex.
The image on the right shows (a variant) of the Siemens T-send-77,
which had been developed especially for this purpose. It was also
used with other cipher equipment like
Schlüsselgerät D, a similar OTT machine made
In the years following WWII, the countries of the
Western Union (WU)
had a growing need for secure military and diplomatic communications
equipment. After the WU had been dissolved into the newly established
the need for secure communication became even more important. Initially,
only the British 5-UCO,
a left-over from WWII, was available for this,
but the British were unable to supply it in sufficient quantities to
fulfill the needs. Apart from that, the 5-UCO
was a rack-based solution
that was way too large to be of any practical use in the field.
In 1953, the Norwegian company STK
tried to fill the gap by developing the ETCRRM,
a valve-based OTT machine that was much smaller than the
and easily fitted a table top. After several improvements, SECAN
approved the ETCRRM
for use by NATO on 19 April 1954 .
Several years earlier, in 1946, the
Dr. Neher Laboratory of the
Dutch Post Office (PTT) had started the development of a One-Time Tape machine
at the request of the Dutch Government.
Due to shortages in electronic components in the first years after WWII,
it was built with approx. 100 electromechanical relays. The machine was named
and was a development of
Dr. Ir. Roelof Oberman,
who would later become Professor
at Delft University. A small number of these
Colex machines were commissioned
on 6 April 1949 for
communication between the Dutch Foreign Office and its embassies in
London (UK), Paris (France), Washington (USA) and Jakarta (Indonesia).
was not a fast machine – it could only handle three characters per
second – but its success resulted in the development of a successor:
ECOLEX — which is short for Electronic COde-teLEX.
Development was started in 1949, immediately after the introduction of the
Colex, and the first prototypes were ready in early 1950,
just 10 months after the release of the relay-based Colex.
As PTT was not interested in the commercial exploitation of the
machine, the production was transferred to
Philips Usfa, where it became
known as Ecolex I.
The machine was ready in mid-1953 and existed in
two variants: 1 Ecolex Mark 1 and Mark 2. A description was sent to
NATO on 4 August 1953 , but it was not until 12 August of the next
year that the two machines were approved for COSMIC traffic
and for NATO traffic higher than CONFIDENTIAL .
The price for a single Ecolex Mark 1 unit in 1955 was US$ 6000
and for an Ecolex Mark 2 unit 'just' US$ 3000, with a lead time of 12 and 11
months respectively .
The ETCRRM on the other hand was produced at a
rate of 200 units per month and was available immediately at a unit price of
just US$ 1200 .
ETCRRM therefore won the race.
Ecolex I was in production until 1958,
and a total of just 25 units were built before it was succeeded
by the smaller transistorised Ecolex II.
Mark 1 and Mark 2, also written as Mark I and Mark II,
should not be confused with Ecolex I and Ecolex II
which are two entirely different machines. Mark 1 and Mark 2 are two
variants of the Ecolex I which was simply called 'Ecolex' back then.
PTTPrototype with valves
PTTPrototype with transistors
PhilipsProduction version: Ecolex II
Philipswith 4-pin connectors
PhilipsImproved line impedance matching
Philipswith 4-pin connectors
PhilipsPartly with printed wiring
PhilipsModified for use with Creed tape reader 92
- Ecolex Mark II
- Ecolex Mark II A
- Ecolex I Mark 1
Document kindly provided by Maarten Oberman .
- Photographs from Philips Usfa
Crypto Museum Archive. CM300624.
- Philips Usfa, Internal Memo L/5636/AvdP/JG
23 August 1982, page 5. CM300428.
- NATO, Approval of Electronic Mixer ETCRRM
SGM-311-54. 19 April 1954.
Declassified by NATO on 17 November 1999 (IMSM-431-99).
- NATO, Netherlands On-Line Cipher Equipment
SGM-1254-53. 17 August 1953.
Declassified by NATO in 2006 (IMSM-0001-2006).
- NATO, Approval of Electronic Mixers ECOLEX Mk I and II
SGM-556-54. 12 August 1954.
Declassified by NATO in 2006 (IMSM-0001-2006).
- NATO, Automatic Crypto-Equipment Requirements for the Allied Command Atlantic
SGM-560-55. 15 August 1955.
Declassified by NATO on 24 November 1999 (IMSM-0431-99).
- Maarten Oberman, Personal correspondence
June 2019. Crypto Museum, RMM Oberman archive.
- Maarten Oberman, Staatsgeheim, De Beveiliging van Overheidsberichten
State Secret, Government Communications Security (Dutch).
2022. ISBN 978-9-4644-8870-8.
- Survey of Versions of Ecolex I and Ecolex II equipments
The Netherlands Government, 1 January 1959.
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© Crypto Museum. Created: Saturday 18 February 2023. Last changed: Thursday, 23 February 2023 - 10:10 CET.