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Gretacoder 519
Facsimile Cipher Unit

The Gretacoder 519, or GC-519, was a high-security fax encryption device, developed and introduced in 1984 by Gretag in Switzerland. It was intended for sending classified documents through a fax machine over standard (analogue) telephone lines with speeds up to 9600 baud.
 
The GC-519 was suitable for a limited range of approved fax machines and required a special interface (and possibly additional firmware) for each model that was available on the market at the time. For this reason the user had to quote the model number when ordering a GC-519.

Generally speaking, the GC-519 was connected between the (analogue) modem and the actual fax machine via a V.24 interface (RS-232). In some cases it was possible to use the internal modem of an existing machine, if the connection to the internal modem could be isolated.
  
GC-519 in desktop enclosure

The GC-519 was available in two case variants: as a 19" rackmount unit, or as a desktop unit. In the latter case, the 19" rackmount unit was built inside a heavy metal enclosure that could be placed under the actual fax machine. In order to protect the unit against tampering, it is locked inside the enclosure with a physical key. Without this physical key the case can not be opened.

Furthermore, the GC-519 was available in two software variants: a Closed User Group version (CUG) and an Open User Group configuration (OUG). in the former case, all users share the same set of cryptographic keys, whilst in the latter case separate keys were used for each user pair.
 
Controls
In everyday use, controlling the GC-519 is pretty straightforward. All connections are at the rear and all controls are at the front. The image below shows the front panel of the unit. Most of the front panel is taken by the 10 LED indicators that show the current state of the machine.


To the right of the indicators is a blue Key Module. It contains an EEPROM with the personal keys and key pairs. At the far right is a physical lock that prevents unauthorised removal of the key module. It also protects the case against unauthorised opening. All connections are at the rear:


At the far right is the mains socket with a selector for the correct mains voltage. At the left are three D-type sockets. The one at the left is for the connection to the fax machine, whilst the rightmost one is connected to the modem. At the centre is a 15-way D-type socket to which the remote control unit (RCU) is connected. A label indicates which type of interface is present.
 
The unit has no power switch. Once it is connected to the mains, it is operational and ready to receive fax messages. Depending on the software version (CUG or OUG, see above), two different remote control unit were available.

For Closed User Groups (CUG) a simple remote control unit (RCU) with 4 push-buttons and a mechanical key was used. As all partners share the same crypto key, there is no need to select a suitable key in this configuration. For Open User Groups (OUG) however, the more advanced RCU with LCD display, shown on the right, was used.
  
Remote control unit

The RCU was connected to the 15-way D-type socket at the rear of the unit. It was used for entering a password (in case a password had been configured) and selecting the appropriate key for the remote station. Each key is identified by a 4-digit number and is stored in an EEPROM inside the blue Key Module that is inserted at the front of the GC-519. It has room for 56 keys.

The actual keys can be generated automatically by the GC-519, but they can also be created manually and entered via the keypad. In that case, each key consists of 40 decimal digits that are entered as 5 groups of 8 digits each. Each group is followed by a 2-digit checksum in order to avoid typing mistakes. The manual describes in detail how to calculate the checksum [1].
 
GC-519 in desktop enclosure Front panel Rear panel Remote control unit Replacing the key module Key module Key module (bottom view) Manual

 
Interior
The GC-519 is a very robust rather large unit that is housed inside a 1U 19" rackmount cabinet. In order to get access to the interior of the machine, the physical key has to be turned fully counter clockwise. If the unit is mounted inside the 1U desktop case, the four black cross-head bolts at the front can now be removed and the actual unit can be pulled out from the front.
 
The case of the actual device itself can be opened by loosening the two spring-loaded bolts at the outer edges of the rear panel. Slide the top panel toward the rear and lift it off. Again, the top panel can only be removed if the physical lock is turned fully counter clockwise.

The interior is pretty straightforward and well organised. Most of the space is taken by the main board which is the actual encryptor. At the rear is a narrow board with the V.24 interface to the modem and the fax machine. At the left (in the read area) is the mains transformer.
  
Interior

The Power Supply Unit (PSU) circuitry is at the front left of the main board. It delivers +5V, +12V and -12V to the main board and the interface. The encryptor is built around a NEC D8085AHC microprocessor [2] running at 6.144 MHz. In the front right corner of the main board is a rather mysterious rectangular metal enclosure that probably houses the internal key generator.
 
Interior Interior PSU section Processor and key module section, with tamper switch at the top Micro processor section Tamper switch Encapsulated module Interface board

 
Physical key
At the front panel of the GC-519 is a physical lock that prevents the case from being opened by an unauthorised person. It also locks the blue key module in place. In order to remove the key module or open the case, the physical key has to be rotated fully counter-clockwise.
 
In locked state, a heavy metal bar is extended at the right side of the case. When built inside a 19" rack, or inside the 1U desktop unit shown here, this bar prevents the GC-519 from being removed from the enclosure. A very effective way of slowing down a tampering attempt.

As an extra safety measure, a MEDECO security lock is used. Unlike a common cylinder lock, this variant has tumbler pins which may be tilted sideways by the key, depending on the shape of the individual cuts of the key. This makes lockpicking more difficult and time consuming.
  
The physical keys

This is called a bi-axial lock and for a long time, MEDECO advertised it as one of the safest locks on the market. And even if the lock was successfully picked, a built-in tamper switch ensured that the internal cryptographic keys were destroyed immediately when the lock was turned.
 
The physical key placed in the MEDECO bi-axial lock GC-519 being removed from the cabinet, after rotating the physical key counter clockwise Removing the top cover Physical lock in 'locked' position Physical lock in 'unlocked' position Removing the key module The MEDECO biaxial lock Perspective view of the key, clearly showing the angled bitting cuts

 
References
  1. Gretag AG, Gretacoder 519 Facimile Cipher Unit, Operating Manual
    Order number 98.15.90. Version 3, December 1985. First released October 1984.

  2. Rochester Electronics, D8085AH Datasheet
    24 April 2013. Aftermarket reproduction.

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