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Ecolex III →
One-time tape cipher machine
Ecolex-II is an online/offline
One-Time Tape (OTT) cipher machine
for digital teleprinter communication,
developed in 1958 by the Dutch Post Office (PTT)
and built by Philips Usfa NV
in Eindhoven (Netherlands) between 1958 and 1963, as the
transistorized successor to the valve-based
Ecolex-I of 1953.
The machine is also known by the internal Philips designator Us 8011.
The machine was usually connected to a double tape reader
(a modified Siemens T-send-77f), a
(commonly a Siemens T-68 telex)
and the telex line (or a suitable radio interface).
The image on the right shows a typical Ecolex II with model number
Us-8011/07, which reveals that quite a few
variants were produced over
the years. In this case, the /07 suffix tells us that the device
has non-standard military connectors.
Furthermore it has a modified telex socket at the rear panel, as
the circular 6-pin socket
has been replaced by a contemporary 8-pin ADo8 socket.
The device measures 52 x 23.5 x 14 cm
and weights ~14 kg. It is constructed in such a way, that the
double tape reader
could be placed on the top surface,
but is was also possible to place it aside the base unit.
The device is based on the so-called Vernam Cipher,
in which two inputs (from the two tape readers) are added by means of
a binary XOR 1 operation. This is commonly known as mixing, which is why
the machine is also known as a mixer.
The main advantage is that the same
operation is used for deciphering. When correctly applied,
the cipher is unbreakable.
XOR = exclusive-or, also known as modulo-2 addition.
The diagram below shows the various features of the Ecolex II. All user controls
are on top of the device, close to the front edge. All connections
are at the rear, with fixed cables for LINE and AC mains power.
The machine is constructed in such a way, that the
Siemens T-send-77f double tape reader
can be placed horizontally on the top surface.
It has its own AC mains power cord.
The 30-pin socket on the left side of the double tape reader,
is connected to the 30-pin socket at the rear of the Ecolex.
It carries the 5-bit parallel digital data from both tape readers and
supplies it to the Ecolex. It is also used by the Ecolex to send stepping
commands to the tape readers.
The simplified block diagram above explains how the Ecolex II works.
At the left is the Siemens T-send-77f with its two tape readers. Tape 1 holds
the plaintext. Tape 2 delivers the key stream. The outputs from the two
tape readers are used as inputs to the Ecolex mixer, which
does an XOR operation on the individual bits of the input signals.
Note that all signals are processed in serial form (not parallel), so that
only one XOR circuit is needed. The resulting signal is sent straight
to the line.
In online mode, the signal from Tape 1
is replaced by the signal from a teletypewriter.
When receiving an encrypted message, the data lines are rerouted so that the
teleprinter and the line are swapped. In online mode, the line signal
is used as one of the inputs to the mixer. The other input is provided by the key tape
reader of the T-send-77f (Tape 2). In offline mode,
the incoming data is first punched to paper tape, which is then played back
on Tape reader 1.
The required MODE of operation is set with the
MODE selector at the front edge of the device.
It consists of five so-called radio buttons plus
an ALARM push button. The leftmost button (RESET) puts the machine in the
default state (plaintext on line). The following modes are available:
In this mode, precautions are take to ensure that no plaintext is sent to
Local machine stays in current mode (with yellow or green lamp ON).
Red lamp will be ON at remote end only. Press RESET to release the alarm state.
The diagram below shows the standard setup of an Ecolex II cipher system.
The teletypewriter at the right — which is usually connected directly to the
line — is rerouted via the Ecolex II. The double tape reader is connected to
the 30-pin socket at the back of the Ecolex, and acts as the input device
for the plaintext tape and key tape. It should be seen as one unit with the
The double tape reader was usually placed on top of the Ecolex II, so that
the complete system did not require much space. The system is suitable for
any type of teletypewriter, but was often used in combination with the
compact Siemens T-68,
which was a popular machine at the time.
The above setup is the most flexible one, as it allows the machine to be used
in a an online and offline configuration. When deciphering a message,
the teletypewriter is used as a printer.
When used online, the plaintext is entered directly on the keyboard of
the teletypewriter and is encrypted and transmitted immediately by the
Ecolex II. In this situation, the teletypewriter acts as the input device.
Online operation is considered to be less secure than offline use, as the
typing characteristics of the operator might reveal useful information to
a potential eavesdropper.
When used offline, the plaintext is first stored on paper-tape (using the
teletypewriter), and then played back on the plaintext reader of the Ecolex
(i.e. the frontmost reader of the T-send-77f). In this configuration the
tape reader acts as the input device and does not reveal any characteristics,
as it sends the characters at a fixed speed. Offline is therefore
considered to be the safest option. Note that in this mode the teletypewriter
is still connected to the Ecolex, as it is used as a printer.
Double tape reader
A crucial part of the Ecolex II cipher machine is the double tape reader
that is used for the two input streams of the Vernam Cipher. In this case,
the Siemens T-send-77f was used, which was a standard component at the time.
It was modified by Philips to make it suitable for the Ecolex II.
The T-send-77f was also used on other
OTT cipher machines (mixers), such
as Siemens' own Schlüsselgerät D
(cipher machine D), which is shown
in the image on the right. It is placed on top of the actual cipher machine,
which can be seen as the Siemens equivalent of the Ecolex II.
The main difference between the Siemens and Philips cipher machines is that
in the Siemens Schlüsselgerät D the XOR circuits are all relay-based,
whilst in the Philips Ecolex II this is done by means of electronic XOR
circuits, making the Ecolex more robust and also potentially faster.
Furthermore, the Siemens machine does not have an appropriate alarm function
and is only suitable for offline use.
The double tape reader is connected to the 30-pin socket at the rear of the
Ecolex II. The rearmost reader is used for the key tape which provides the
key stream. This is usually a
One-Time Tape (OTT)
that is filled with truly random characters.
This reader has a built-in knife that destroys the tape immediately after use.
The tape reader at the front is used for the plaintext message (when encrypting)
or for the received ciphertext message (when decrypting).
Shortly after WWII,
in the early days of the Cold War,
mixers became popular items for protection of TOP SECRET
information, such as the radio an telephone traffic between the Foreign Office
and Embassies abroad, and strategic communication between the various
countries. Because of their size and lack of suitable components,
they were constantly in short supply at the time.
One of the first affordable and reasonably-sized mixers to arrive on the
scene in 1952 was the ETCRRM
of the Norwegian company STK.
Philips followed a year later, in 1953, with the
Both the ETCRRM
and the Ecolex I
were made with thermionic valves
(radio tubes). By 1955, the ETCRRM was produced at a rate of 200 machines
per month, at a unit cost of just US$ 1200. In 1963, the
ETCRRM was even used on the
In contrast: only 25 Ecolex I machines were ever built,
and the price for a single unit was between US$ 3000 and US$ 6000.
In 1957, the Dutch PTT's research lab started the development
of the Ecolex II, which would be built with the new transistor
technology that had just become available. Like its predecessor –
the Ecolex I –
it would be built by Philips Usfa NV.
Various experiments were carried out and quite a few prototypes
were made, such as the one shown in the image above that
uses a single 5-level paper tape reader made by Creed in the UK.
In the final design it was decided to replace the single reader
by a double one made by Siemens
in Germany, as shown in the image on the right.
A total of 120 Ecolex-II units were built by Philips Usfa
They were sold to the Dutch Army, the Dutch Ministry of Foreign Affairs
and the German Ministry of Foreign Affairs .
On 23 May 1958, the Ecolex II was approved for NATO traffic of all
Just before the Ecolex-II was introduced in 1959, Philips had already
started its own development of OTT cipher machines,
resulting in the
Ecolex III 1
and the Ecolex IV,
both of which offered full signal synchonisation.
Within NATO, the Ecolex III was known as Ecolex II-B.
The machine consists of three basic mechanical parts: a bottom plate,
a metal case shell, and a frame that holds the actual cipher machine.
After rotating the key clockwise,
a metal pin – located below the lock –
pops out and unlocks the case.
The upper case shell can now be removed,
after which the interior is exposed,
as shown in the image on the right.
The machine is built on a strong metal frame, which can be divided into
three sections: a power supply unit (PSU)
at the rear, a control panel at the
front, and the digital electronic circuits
with their wiring at the center.
The electronic circuit consist of 10 modules in a so-called cordwood 1
construction, in which the components are mounted vertically between
two printed circuit boards. Each cordwood module measures approximately
185 x 55 x 25 mm
and is mounted from left to right at the metal frame.
The wiring to the 10 modules is located at the top
and is bundled towards the right side of the machine, so that each
cordwood unit can easily be lifted from the frame in case of a repair.
The bottom side of each cordwood module can be accessed from the bottom
of the machine, after lifting the frame
from the metal bottom plate.
The image on the right shows a close-up of the cordwood modules, as seen from
the left side of the machine. The brown parts are resistors, and the
black ones are OA85 Germanium diodes. All transistors are held in
a brown isolation sleeve.
The complete mixer consists of 30 sub-circuits
— spread over the 10 cordwood
modules — that are built from 62 Germanium transistors (OC76) and no less
than 330 Germanium diodes (OA85). An overview of the 30 sub-circuits is given
in the table below.
The sub-circuits represent logical functions (e.g. AND,
OR, XOR, etc.), switches and analogue power circuits. The circuit diagram
plus a detailed technical description of each sub-circuit is
available in the technical manual [D].
Cordwood modules, or cordwood constructions, refers to a manufacturing
method for electronic circuits, in which conventional parts
like resistors, capacitors, transistors and diodes, are mounted vertically
between to parallel printed circuit boards, with the aim to save space.
Cordwood structures are used for example in the
NSA's FLYBALL modules -
used extensively in the KW-7 cipher machine –
and CIA bugs like the SRT-52.
The block diagram below shows the operation of the complete
machine, including the double tape reader. Central to the system are
the MODE selector — used to select the desired input — an
equalizer — used for re-shaping the signal — and the mixer (XOR).
The output of the mixer is delivered to the outgoing TX line
and to the receive circuit of the local teleprinter at the far right.
At the left is the double tape reader, of which the KEY tape reader
(Tape 2) is always connected to the mixer. The MODE switches are used
to select the other input to the mixer: the incoming line,
the output of the local teleprinter or the other tape reader (Tape 1).
In the latter case, Tape 1 is either the plaintext tape (when ciphering
a message) or the ciphertext tape (when deciphering).
The timing of the entire system is controlled by the selected
input signal (via the MODE selector).
Below is an overview of the functions of the 30 sub-circuits
that are spread over the 10 cordwood modules. The first column
shows which transistors are used for these circuits. In
November 1960, small changes were made to some of the circuits, and the
entire circuit diagram was reorganised. The second column gives the
transistor numbers for Ecolex II versions Us 8011/08 and /09.
Tr 1-2Text signal
Tr 3-4Tape reader control
Tr 5-6Delay circuit (alarm)
Tr 7-9Alarm circuit
Tr 10-11Check tape feed
Tr 62-63Break trigger
Tr 12-13Start/stop command
Tr 42-43Tape reader 1 command (plaintext)
Tr 51-52Tape reader 2 command (key)
Tr 60-61Speed check
Tr 27-28Selector command
Tr 29-30Square wave generator (clock)
Tr 31-32Generator buffer
Tr 33-341st divider
Tr 35-362nd divider
Tr 37-383rd divider
Tr 23-24Regenerator (equalizer)
Tr 22Regenerator time pulse
Tr 25-26Transmit relay driver
Tr 29-41Teleprinter driver
Tr 44-45Tape reader 1 buffer (plaintext)
Tr 46-475th element store (plaintext)
Tr 48-50Tape feed 1 (plaintext)
Tr 53-54Tape reader 2 buffer (key)
Tr 55-565th element store (key)
Tr 57-59Tape feed 2 (key)
Tr 14-155 ms delay
Tr 16-1720 ms delay
Tr 18-19Send/receive switch
Tr 20-2140 ms delay (or 170 ms)
- LINE PLAIN on/off
- LINE CRYPTO on/off
- Invert output
- Teleprinter current 40/60 mA
- Line current single/double 5a
- Speed 45.5 or 50 baud
- 250 Hz frequency adjust
This switch controls the possibility to send plaintext straight to the line.
In the UP position, with the MODE selector set to TAPE PLAIN,
the plaintext from reader 1 is sent straight to the line. In the DOWN
position, this operation is blocked. It is always possible to send
plaintext directly from the keyboard.
This switch controls the possibility to send ciphertext straight from the
keyboard of the connected teletypewriter.
In the UP position, with the MODE selector set to LINE CRYPTO,
ciphertext can be sent straight from the keyboard. In the DOWN position,
this operation is blocked.
This switch can be used to invert the output of the mixer.
In the DOWN position, the machine uses the XOR operation for mixing the
individual bits from the plaintext and keystream. In the UP position, the
output of the mixer is inverted (NOT-XOR). This was the default setting
for most Ecolex II machines.
This switch controls the line current that is used for the connected
teletypewriter. In the UP position, 40mA is used. In the DOWN position,
this is 60mA.
This switch controls the line configuration. The UP position selects
single-current operation. The DOWN position selects
From version Us 8011/08 onwards, switch 5 is used to delay the LINE CHECK
by 5 ms.
In the UP position, the machine is suitable for 50 baud signals. In the
DOWN position, a speed of 45.5 baud is selected, making the machine
compatible with American teletypewriters.
This is a TEST switch that should be used when adjusting the generator
frequency. In the UP position, a neon lamp is enabled. Using a
125 Hz tuning fork,
the generator frequency can be adjusted with the potentiometer
to the right of the switch. Once the frequency is correctly adjusted,
the switch should be returned to the DOWN position (more about this
Adjusting the 125 Hz generator
The internal 125 Hz frequency generator of the Ecolex II
can be adjusted easily and accurately,
with the help of a 125 Hz tuning fork.
After placing switch 7 in the UP
position, a neon lamp (to the right of the switch) will light up at 125Hz.
By looking at the neon lamp through the
slit of an exited 125 Hz tuning fork, the
potentiometer at the right should be adjusted so that only one of the
lamp's electrodes is lit. When the light alternates between the
two electrodes, the 125 Hz generator has not been adjusted correctly.
➤ More about tuning forks
An extra switch
is located on the internal PSU from model Us 8011/05 onwards.
When set to the ON position, the Ecolex II is suitable for connection to a
TH-5/TG telegraph/telephone terminal. Note that in this situation, the
alarm function of the Ecolex II can only be used when it is connected in a
4-wire configuration [A].
The image on the right shows the TH-5/TG as it is shown on the front cover
of its manual [F]. The device is housed in a green metal enclosure and was
used extensively by the US Army during the Vietnam war.
➤ Download TH-5/TG manual
We currently have an Ecolex II on loan, and we are aiming to get it up and
running in due course. It would be great if it could be made to communicate
with an Ecolex IV.
This depends on the state of the components, the wiring
and the ability to secure a double tape reader.
In the meantime, restoration of the base machine has been started.
The following has so far been restored:
- Case lock removed and replaced.
- Case realigned and paint restored.
- Locking mechanism repaired and realigned.
- Grommets of fixed cables at the rear replaced.
- Strain relief of power cable repaired.
- Rubber pads added at bottom to avoid scratches.
- One fuse holder repaired.
- Mains power switch replaced.
With the early Ecolex machines, the fixed line cord has a 6-pin
circular plug at the end. It is wired as shown in the diagram below.
At the rear of the machine is a female socket for connection of a
teletypewriter, with the same layout. Below is the pinout when looking
into the socket from the rear of the machine. Note that the socket has a
micro-switch behind contact 1.
When inserting a plug into the socket, pin 1 engages this switch,
which interrupts the internal 200Ω terminator.
- TX relay A (common)
- TX relay Z (mark)
- TX relay T (space)
- 120Ω RX relay (with 6)
- Alarm contact (with 5)
- Alarm contact (with 4)
- 120Ω RX relay (with 3)
Single-current: use TX contacts 1 and 2, set RX current to 40 mA.
Double-current: power supply via ballast lamps to pins 2 and 4, set line current to 20 mA.
2-wire operation: TX and RX contacts connected in series.
4-wire operation: TX and RX contacts connected separately.
The alarm contact is used to activate an external alarm circuit (e.g. lamp or bell).
On Ecolex II units, the existing 6-pin telex plug and socket (see above)
was replaced by a more common 8-pin ADo 8 one. The diagram below shows
the pinout for that connector type.
At present the wiring of the alarm lines is unknown.
- TX a (a1)
- TX b (b1)
- RX a (a2)
- RX b (b2)
- Bridge to 6 1
- Bridge to 5 1
This is a bridge inside the socket (not the plug).
Alternative 10-pin line plug
In some cases the fixed cord for connection to the (telex) line
does not have a standard 6-pin or ADo8 plug at the end.
Instead, it might be fitted with a military 10-pin male plug,
of which the wiring is given below, when looking into the socket.
This plug was standard on the Us 8011/07.
- not connected
- not connected
- not connected
Most Ecolex II units have a standard plug for connection to the
AC mains wall socket. Some versions however, notably the Us 8011/07,
have a 6-pin military plug fitted to its mains cable. The pinout of
this plug is given below, when looking into the socket.
- AC mains (A)
- not connected
- AC mains (B)
- not connected
Power110, 127 or 220V AC 1 (selectable with internal voltage caroussel)
Fuses2 x 500 mA (internal)
ModeOffline, online, 2-wire, 4-wire, single current, double current
Tape readerDouble, Siemens T-send-77f
Speed50 baud, 45,5 baud
Line current40 or 60 mA
Extra taps are available on the mains transformer to adjust the
selected voltage by ±5.5V.
Us 8011Philips Ecolex II cipher machine ➤ Versions
Us 8900/02Modified version of Siemens T-send-77f double tape reader (40mA)
?Line connection and filter unit
Us 8011/02Release version
Us 8011/05Addition of switch for TH-5/TG interface on the PSU
Us 8011/07Military connectors (and modified Telex socket)
Us 8011/08Upgraded circuits and new circuit diagram
Us 8011/09Same as /08 but with 4-pin telex socket (rather than 6-pin)
Crypto Museum are still looking for an Ecolex II unit for its collection.
If you can help in any way, please contact us. At present,
we have an Ecolex II machine on loan, but unfortunately it is not complete.
We are currently looking for the following items:
- Siemens T-send-77f double tape reader (Philips designator Us 8900/02).
40-pin to 40-pin cable for connection of the T-send-77f to the Ecolex II.
- Full service documentation (the one in [D] is incomplete).
- Line connection and filter unit.
Obtained via Google Books, March 2018.
- Philips Usfa, Photographs of Ecolex II
Crypto Museum Archive #CM300637.
- Philips Usfa, Internal Memo L/5636/AvdP/JG
23 August 1982, page 5.
- NATO, ECOLEX MARK II, Transistorized Version
SGM-318-58. 23 May 1958.
Declassified by NATO in 2006 (IMSM-0001-2006).
- Anonymous, Philips Ecolex II, Model Us 8011/07, S/N 205
Accessed March 2018.