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Ecolex I Mk 2 →
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Ecolex I   Mark 1
One-time tape cipher machine - wanted item

Ecolex I was an online/offline One-Time Tape (OTT) cipher machine for teletype communication, developed and built around 1950 by the Dutch Post Office (PTT). Between 1953 and 1956, it was built by Philips Usfa in Eindhoven (Netherlands). The device was based on Colex, the first Dutch mixer, but was built with thermionic valves (vacuum tubes) rather than electromechanical relays. It was an alternative to the British 5-UCO machine and was approved for NATO traffic at all levels.

Two versions of the Ecolex I existed: one that operated on the 26 letters of the Latin alphabet and was compatible with existing mechanical OTT machines, and one that operated on all 32 characters of the ITA2 telegraph alphabet. These two versions were known as Mark 1 and Mark 2 respectively. The image on the right shows the Ecolex Mark 1 with its front cover removed. As far as we know, it is the only surviving specimen of this model. It is currently held in an internal collection of the Dutch Department of Defense.

The Ecolex Mark 2, is a completely different machine that allows all characters of the ITA2 alaphabet to be encrypted, including carriage return, linefeed, etc., as it works on all 32 ITA2 characters. It is smaller and contains fewer switching elements than the Ecolex Mark 1.

The Excolex I Mark 1 — at the time simply known 1 as the Ecolex Mark I — was intended to be fully backwards compatible with the mechanical OTT machines of the era — in particular the Hagelin C-446/RT — and also with the manual One-Time Pad (OTP) methods of the era. This way, embassies that did not (yet) have an Ecolex, could still encrypt/decrypt messages (either mechanically or manually) as before, with the key provided on paper rather than on tape [8].

As a result of this, only the 26 characters of the Latin alphabet could be used, whilst the stunt characters (linefeed, carriage return, etc.) are passed straight through, so that cryptograms should still be printed in spaced 5-letter groups. One exception is the SPACE character, which is replaced by the letter 'X'. 2 Because of these limitations and exceptions, the Ecolex I Mark 1 is an extremely complex device, which is built around 68 switching elements, each built around an E90CC double-tetrode valve. The Excolex Mark 2 is simpler as it lacks these limitations [B].

The exact production quantity of the Ecolex I is currently unknown, but from Philips Usfa archives it is known that approx. 25 machines were built by Philips between 1953 and 1956 [2]. If we include the prototypes and early production machines made by PTT, it is likely that the total production quantity of the Ecolex I is somewhere between 30 and 40 units. In 1955, the machine was succeeded by the Ecolex II, which is actually the transistorised version of the Ecolex Mark 2.

 More about the Ecolex Mark 2
 More about the Ecolex II

  1. 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.
  2. In some languages, such as Dutch, the space was replaced by the 'X'. Different letters were used other languages, such as K (French, Italian, Spanish) or Z (English).

Ecolex I - interior front side
Complete setup (without the PSU)
Ecolex-I with tape reader
Complete setup (with the PSU)
Power Supply Unit
1 / 7
Ecolex I - interior front side
2 / 7
Complete setup (without the PSU)
3 / 7
Ecolex-I with tape reader
4 / 7
Complete setup (with the PSU)
5 / 7
Power Supply Unit
6 / 7
7 / 7

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 Vernam Cipher. The process is also known as mixing, which is why OTT machines are also known as mixers.

Modulo-2 addition of the characters from two tapes

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. In the Ecolex I Mark 1, the process is slightly more complicated, as only the 26 letters of the Latin alphabet are encrypted. Other characters, such as carriage return (CR) and linefeed (LF), appear unchanged in the output stream. Furthermore, the SPACE character is replaced by another letter.

 More about the Vernam Cipher

Key tapes
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

Ecolex I Mark 1
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).


Ecolex I Mark 2
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.

 More information


Double tape reader   T-send-77
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 by Siemens.

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 NATO (1949), 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 5-UCO and easily fitted a table top. After several improvements, SECAN approved the ETCRRM for use by NATO on 19 April 1954 [3].

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 Colex, 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).

Colex 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.

Philips Usfa
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 [4], 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 [5].

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 [6]. 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 [6]. 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.

  1. 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.

  • 2601
  • 2602
    Release version, 25 chars + space
  • US 8500/00
    26 letters, or 25 letters + space
  • US 8500/01
    with 4-pin connectors
  1. Estimated production quantity of Ecolex I Mark 1 and Mark 2 together.

  1. Maarten Oberman, Staatsgeheim, De Beveiliging van Overheidsberichten
    State Secret, Government Communications Security (Dutch).
    2022. ISBN 978-9-4644-8870-8.
  1. Ecolex I, User manual (French)
    Date unknown, but probably 1953. 1

  2. Ecolex Mark 1 and 2
    The Netherlands Government, 1 October 1954. SECRET. 1
  1. Document kindly provided by Maarten Oberman [7].

  1. Photographs from Philips Usfa
    Crypto Museum Archive. CM300624.

  2. Philips Usfa, Internal Memo L/5636/AvdP/JG
    23 August 1982, page 5. CM300428.

  3. NATO, Approval of Electronic Mixer ETCRRM
    SGM-311-54. 19 April 1954. NATO SECRET.
    Declassified by NATO on 17 November 1999 (IMSM-431-99).

  4. NATO, Netherlands On-Line Cipher Equipment
    SGM-1254-53. 17 August 1953. NATO SECRET.
    Declassified by NATO in 2006 (IMSM-0001-2006).

  5. NATO, Approval of Electronic Mixers ECOLEX Mk I and II
    SGM-556-54. 12 August 1954. NATO SECRET.
    Declassified by NATO in 2006 (IMSM-0001-2006).

  6. NATO, Automatic Crypto-Equipment Requirements for the Allied Command Atlantic
    SGM-560-55. 15 August 1955. NATO SECRET.
    Declassified by NATO on 24 November 1999 (IMSM-0431-99).

  7. Maarten Oberman, Personal correspondence
    June 2019. Crypto Museum, RMM Oberman archive.

  8. Maarten Oberman, Staatsgeheim, De Beveiliging van Overheidsberichten
    State Secret, Government Communications Security (Dutch).
    2022. ISBN 978-9-4644-8870-8.

  9. Survey of Versions of Ecolex I and Ecolex II equipments
    The Netherlands Government, 1 January 1959.
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Crypto Museum. Created: Thursday 13 June 2013. Last changed: Sunday, 19 February 2023 - 13:41 CET.
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