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TST 7700
Voice and data encryptor

The 7700 was a voice encryption device, developed by Tele Security Timmann (TST) in Tützing (Germany) around 1984. The device is suitable for secure voice conversations, data transmissions and (optionally) fax transmissions. It provides high-end encryption with a choice of vocoders and encryption algorithms and is housed in a military-grade cream or green TEMPEST enclosure.
 
The TST-7700 was designed for use at the highest level of secrecy and was suitable for voice and data communication. It was completely operated from the connected telephone set (using DTMF tones) and offered voice encryption without loss of quality. Furthermore, it could be used on bad quality telephone lines, with graceful degrading of the voice quality.

When used over satellite links, the delays, that are typical for such connections, did not affect the encryption. For data communication, a 25-way RS-232 connector was available at the front.
  
TST-7700 voice and data encryptor

The TST-7700 is suitable for voice data at 2400, 4800 and 9600 baud and uses synchronous communication. Computer data can be transferred (asynchronous) at any speed between 1200 and 19200 baud. The device was normally connected between the telephone set and the line, but it was also possible to protect an entire office by placing it between the PABX and the line [2].

The TST-7700 was later replaced by the TST-7790 that was housed in the same enclosure and had the same physical appearance. Contrary to the TST-7700, which had to be powered externally, the TST-7790 had a suitable mains power supply bolted to the rear of the unit [2].
 
TST-7700 voice and data encryptor TST-7700 front panel TST-7700 rear panel Power socket at the rear TST-7700 voice and data encryptor Module identification table

 
Connections
The TST-7700 has no controls; it is fully controlled from the conected telephone set. At the front panel are three D-type sockets that are used for the connection of the telephone set, the line and (optionally) a computer via the standard RS-232 serial port (also known as the COM-port).

Connections at the front panel of the TST-7700

The coloured dots above the connectors indicate the RED and BLACK sides of the unit. The telephone set and the computer are both RED devices as they handle plaintext. The encrypted data is present at the telephone line, which is therefore said to be the BLACK side. Power is supplied to the unit via a 9-way connector at the rear. The pin-out is currently unknown.
 
Key loading
Cryptograpic keys were loaded into the TST-7700 by means of a dedicated key loader, known as the TST-0706, that was connected to the 25-way D-type socket at the front panel. The same connector that was used for connection to the RS-232 port of a PC. Photograph taken from [2].

The image on the right shows the TST-7790 with the TST-0607 key loader attached at the front. They key loader itself carries a TST-0502 memory card that holds the actual keys in encrypted format, as generated by the propiretary TST Key Production Center. The keys are fully encrypted themselves, in order to avoid compromise in case they are lost in transit.
  

 
TST-7790
The TST-7700 was later replaced by the slightly enhanced TST-7790. It had nearly the same features and had the same connections at the front panel, but had a mains power supply bolted on at the rear [2].

The image on the right shows a TST-7790 unit as is was printed on the front page of the brochure [2].
  

 
Interior
The TST-7700 is extremely well built. It is housed in a case that is similar to the enclosure of other TST devices, such as the TST-4043, but it is much better shielded against unwanted EMC/RFI emission (TEMPEST). RF shielding is further improved by the use of RF gaskets.
 
The unit can be accessed by releasing 4 long bolts at the corners of the front panel, after which the heavy front panel can be removed.

Inside the unit are five extremely well designed PCBs, three of which are slotted into a backplane that is mounted to the rear panel. The remaining two boards (i.e. the rightmost two boards) are held in place by metal brackets and are connected to the other boards by means of flat-cables. The leftmost three board are connected via the backplane, but also via additional flat-cables that are mounted at the front side.
  
Front panel removed and all PCBs half-way out

The leftmost board is the interface to the connected telephone set (i.e. the RED side). It is built around a so-called SLIC (Subscriber Line Interface Circuit); a pre-fabricated ceramic carrier board manufactured by Mitel that allows virtually any type of telephone set to be connected [3].
 
The next board is the (telephone) line interface, wich consists of an extended eurocard-size PCB with two daughter cards. One of the daughter cards holds the Rockwell modem chips, whilst the other one holds the physical line interface.

The third PCB is the digital board that connects to all other boards and to the front panel. At the front side of this board are five sockets for the flatcables. At the top left of this board is a TAMPER switch that is actuated by a bracket behind the front panel. It causes the crypto-keys to be deleted when the front panel is removed.
  
Vocoder board

The cryptographic keys are retained in a voltaile memory that is powered by an internal Li-ION battery. By breaking the power to the volatile memory (using the tamper switch), the keys are deleted. The rightmost two boards are the vocoders. As the device can be used for full-duplex, two identical vocoders are needed: one for sending and one for receiving. The vocoders are not slotted into the backplane at the rear, but are directly connected to the digital board.
 
Front panel removed Front panel removed and all PCBs half-way out Looking into the TST-7700 after the front panel has been removed Backplane with 3 slots for PCBs Front panel with sockets for the cables and a metal guide for the TAMPER switch Metal guide for actuating the TAMPER switch Telephone interface board Mitel MA88600 SLIC
Line interface Line board with the two daughter cards Line interface, bottom PCB Rockwel modems Physical line interface Digital board Li-ION battery on the digital board Vocoder board

 
References
  1. Helmut 'Jim' Meyer, HS0ZHK, My way to Ham - Radio and beyond
    Website QRZ.COM. Personal correspondence. Crypto Museum, July 2013.

  2. TST-7790 Sprach- und Datenverschlüsselungsgerät für analoge Telefonleitungen
    Tele Security Timmann (TST). Sales Brochure, 9 pages (German). 21 June 2005. 1

  3. Mitel, MA88600, Global Subscriber Line Interface Circuit
    Datasheet. April 1995. Retrieved July 2013.

  1. Brochure kindly supplied by Helmuth (Jim) Meyer [1].

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