The Kryptowählfernsprecher, or KWF, was a compact Army field phone
with dial, for use on 2-wire analogue telephone networks, developed and built
by ANT in Backnang (Germany) in the mid-1980s.
Contrary to popular believe, the device does not include any
The unit is also known as NSN 5805-12-193-1611 and can be
used as a standard phone.
According to some publications, the KWF was suitable for the German
AUTOKO 90 network.
The KWF is basically a standard analogue telephone set with an old-fashioned
dial and some additional facilities to allow it to be used on secure
(encrypted) telephone exchanges.
The phone itself has a standard (civil) cream body
and is housed inside a ruggedized
green plastic transport case, with a
separate lid to protect the controls.
When closed, the device is watertight.
A canvas strap is supplied to allow
the phone to be carried on the shoulder, e.g. by a soldier in the field.
The image on the right shows a typical KWF with the top shell removed.
As stated before, the KWF does not contain any cryptographic circuits.
It was used on telephone exchanges that had facilities to place encrypted calls
to similar exchanges in the field. A call would be initiated in clear,
after which both parties would agree to go secure. Both parties would
then press the green button on the phone to cause the exchange to switch
to encrypted mode.
The diagram above shows how it worked. The two exchanges are both connected
over a standard analogue (2-wire) telephone line or via multiplexed
line-of-sight (LOS) radio links. Between the two exchanges, a standard or a
trunk encryption device (TED) takes care of the encryption and decryption,
whilst the traffic between the phones and their exchange remains unencrypted.
Standard telephone units (T) and KWF sets can be connected to either exchange,
but only KWF phones have a facility to instruct the exchange to 'go secure'.
For proper crypto operation, the KWF should be connected to the exchange via
three wires (rather than just two).
The three wires are connected to the La, Lb and
ground terminals at the front left corner of the KWF.
In normal use, when La and Lb are properly connected,
the call is not encrypted and the
marked Klar (clear), lights up.
In this mode, the handset's PTT switch acts as a voice silencer.
When the user presses the green button, the KWF sends a 16 kHz tone to the
exchange. The exchange then switches to encrypted mode and signals this back
to the KWF by reversing the polarity of the La/Lb lines. This causes the
green LED (Schl) in the top right corner to be lit
When in secure mode, the large black button on the inside of the handset
acts as a Push-To-Talk switch (PTT), which is needed for half-duplex
connections. For this, the 16 kHz pilot tone is used again. Switching
back to clear mode is done by pressing the red ground button (marked ET)
or by finishing the conversation (i.e. placing the handset in the cradle).
This resets the La/Lb polarity.
The KWF has a very ergonomic design. All controls and connections are at
the top panel, which is slightly sloped for easier operation. At the center
is an old-fashioned interruptor-style dial
that allowed the unit to be
used on virtually any existing analogue telephone network in the world.
The handset is connected to the body of the
telephone set via a standard NF10 connector.
When not in use, the handset is placed in the cradle,
covering the dial and depressing the hook switch
at the right. The 2-wire telephone line is connected to the
La and Lb terminals at the front left.
An extra terminal is provided for the connection of the earth wire (ground).
When connected, the red button acts as a ground-switch (ET), which is
used on some exchanges (PABX) to put a call on hold. In secure mode,
the red ET button can be used to switch back to clear mode again.
At the top left is a rather loud buzzer
that can be muted with the white button
at the front. To the right of the white button is a 'ring' indicator,
which can also be used as a 'call missed' indicator.
When it receives a call, it becomes red. As soon as the
handset is picked up, it turns white again.
The green and red buttons at
the front are for switching to SECURE and CLEAR mode respecitvely.
The KWF is housed in a sturdy green plastic case and is manufactured to
military standards. The interior can be accessed by releasing four large
black cross head bolts from the corners of the control panel, after which
the actual phone
can be removed from the green plastic shell.
The KWF is extremely well built and is very service friendly.
All electronics are placed on a
single eurocard (10 x 16 cm) consisting
of a high-grade double sided PCB with first class components.
At one end of the board is a 64-pin DIN connector that slots into a
that is mounted behind the front panel of the KWF.
All controls, indicators, switches, the handset an the external line,
are wired to the DIN socket,
so that the PCB with the electronics can be
swapped within seconds in case of a failure.
The noise-cancelling handset
is manufactured by Siemens.
The handset is connected to the KWF by means of an
commonly used by the German Army, so that it can be replaced by a
headset if necessary. Inside the grip of the handset is a
large Push-To-Talk switch (PTT)
that is used in case of a half-duplex (radio) link.
At present we have no further information about this device.
Although it is just a telephone, the German Government insists that it
remains a restricted device, despite the fact that it is widely available
on the surplus market, via auction sites such as eBay.
If you have any information about this device, such as a user manual,
circuit diagrams, or information about the equipment it was used with,
please contact us. As always, your help is much appreciated.
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© Crypto Museum. Created: Wednesday 07 August 2013. Last changed: Friday, 23 February 2018 - 21:25 CET.