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NL OTT Philips PTT Ecolex I →
The machine is housed in a large metal case, probably 19" wide,
with metal carrying handles at the top. The ON/OFF switch is located
on the small control panel at the lower centre. A nearly identical enclosure
was used for the Ecolex I cipher machine,
which was built in the same era.
By using a random number generator (RNG) for the production of 5-level
baudot key tapes,
the machines using them -commonly called
mixers- can provide an
unbreakable cipher.
The data from a key tape is generally known as the key stream
and is mixed with the plaintext by means of the mathematical
modulo-2 or XOR operation. This operation is known
as the Vernam Cipher.
Although random number generators were also available from other manufacturers,
such as Reichert Elektronik,
the Dutch Government wanted a machine was not flawed and could not be manipulated, and tasked the
PTT Labs in The Hague with the development of the EROLET I.
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The image above shows the Us 8503 random number generator with the optional
Us 8903 checking unit or counter.
The latter consists of 33 counters, 32 of which are
used to count the occurencies of each of the 32 possible binary values
(25) .
If the system produces truely random noise, the data should be evenly spread
and the counters should all have a nearly identical value after several hours
of operation. The counters are numbered 0-31. The extra counter at the bottom
shows the total number of characters that have been produced since the last
reset.
Power was provided by a separate Power Supply Unit (PSU).
The EROLET was developed by the PTT in 1954/55 and was taken into limited
production by the Dutch defence contractor Philips Usfa
in Eindhoven (Netherlands) in 1955.
Between 1955 and 1959, no more than 10 units were built [2]. They were
used by the Dutch Government, but also by the Governments and Armies of
other countries like Denmark [3]. As far as we know, there are no surviving
examples of the EROLET.
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At the time, the PTT was the Dutch state-owned telecom monopolist.
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The EROLET was capable of producing random 5-bit characters at speeds
of 45.5, 50, 75 and 100 baud. At the default speed of 50 baud, this
means that approx. 400 characters are generated per minute. EROLET
can produce all possible binary values (0-31), or can be limited to any
subset by means of the switch panel.
The circuits are suitable for 40 mA and 60 mA single current loop.
The simplified block diagram above shows how EROLET works.
At the top left is the noise generator, for which a 'noisy' gas-triode
was used. This noise signal is used as the basis for the generation
of the random bit values. The output is at the bottom.
Counters can be added to verify the uniform distribution of the
produced values (0-31).
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The twin tape puncher is shown in the image on the right.
Blank tapes are entered into the puncher at the right, where the two
reels are mounted horizontally.
Both tapes are fed through the same tape puncher, so that both
copies are guaranteed to be identical.
Once punched, they are spooled onto two empty reels at the left.
Reichert
(and later its successor Mils) 1
also developed twin tape punchers with a built-in random number
generator (i.e. noise source), plus counters to verify the
uniform distribution of the generated 5-bit values.
➤ More
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The machine shown above was used around 1959 at Philips Usfa for the
production of key tapes.
Although the Reichert tape duplicator with built-in noise generator, was
probably just as good as the EROLET developed at the PTT, the Dutch Government
wanted to avoid the possibility of being manipulated and only trusted
national production. It was the only way to guarantee security.
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In 1965, Reichert moved his company to neutral Austria and renamed
it Mils Elektronik (later: Mils Electronic).
The double tape puncher is therfore also known as a
Mils Machine. Mils
went out of business in 2018.
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The complete EROLET system was designated Us 8020 and consisted of
the following parts:
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Us 8503/00 Random Number Generator Us 8501/00 Power Supply Unit Us 8502/00 Cable bewteen (A) and (B) Us 8902/00 Counter 1 Us 8903/00 Cable between (A) and (D) 1 Us 8901/00 Tuning Fork 125 Hz 1
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Optional device (recommended).
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Obtained in February 2016 from [3].
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© Crypto Museum. Created: Monday 13 June 2016. Last changed: Wednesday, 08 October 2025 - 14:15 CET.
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