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KWF
Kryptowählfernsprecher

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 cryptographic items. The unit is also known as NSN 5805-12-193-1611 and can be used as a standard phone. According to some reports, 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.
  
KWF with the handset on-hook

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 red LED, marked Klar (clear), lights up. In this mode, the handset's PTT switch acts as a voice silencer.
  
Line terminals

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 [2].

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).
 
Closed KWF box KWF with top lid removed KWF with the handset on-hook KWF with the handset off-hook Line terminals SECURE and CLEAR buttons Ringer ON/OFF switch and 'call missed' indicator

 
Controls
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.

KWF with the handset off-hook

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.
 
Interior
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 socket 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.
  
Removing the PCB

The handset is connected to the KWF by means of an NF10 connector, 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.
 
Bare KWF unit taken out of its case Interior of the KWF Interior of the KWF Removing the PCB KWF PCB with 64-pin DIN connector at the right Close-up of the line transformer KWF interior 64-pin DIN PCB socket
Bare KWF unit (wit top case shell) NF10 socket Buzzer (ringer) Hook switch CLEAR and CRYPTO indicators Handset with NF10 plug Dial Extra hook switch (modification)

 
Help required
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.
 
References
  1. Manfred Bischoff, ANT Kryptowählfernsprecher
    19 June 2013. Retrieved Augutst 2013.

  2. Malte Cornelius, Infos zur Kryptowählfernsprecher
    Telefon Forum für Historische Telefone (German).
    29 May 2009. Retrieved August 2013.

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