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Gretacoder 603
Bulk encryption device

Gretacoder 603, or GC-603, was a bulk encryption device developed in the mid-1980s by Gretag in Regensdorf (Switzerland). It was intended for the encryption of fast multiplexed telephone and data channels at speeds between 32 kbits/s to 2 Mbits/s. Data is exchanged in full-duplex (FD) time-division multiplex (TDM). The device is based on the nearly identical military GC-601.

The device is housed in a heavy 4U 19" rack­mount enclosure, with heavy grips at either side. Power is connected to the socket at the right. All further connections and some controls are at the left. At the center is an empty space with two metal stubs for the separate key loading device.

The image on the right shows a typical GC-603, with the key entry unit missing. A typical setup consisted of two such devices: one at either side of a data link. Peripheral equipment should be connected to the MUX socket on the front panel, which accepts bundled (multiplexed) data traffic.

A physical key is used to turn encryption on or off, using the lock at the upper edge of the front panel. It is likely that the GC-603 was suitable for exchanging (computer) data and voice traffic (telephone) simultaneously. The military origin of the machine is clearly visible at the front panel, which is partly painted in a military green colour and has four military sockets for the peripherals.

At present, not much is known about this device. According to the former owner, it was used for communication via satellite between two bank sites: one in Switzerland and one in the far-east. This is supported by the DYMO labels that were present on the devices when we received them. You can help us to extend this page by  providing additional information or documentation.

The complete unit Front panel of the Gretacoder 603 Close-up of the control panel MUX cable
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The complete unit
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Front panel of the Gretacoder 603
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Close-up of the control panel
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MUX cable

Key loading
For maximum security it is necessary to alter the cryptographic key(s) at regular intervals. In a military environment, keys are generally swapped at least every 24 hours. The Gretacoder 603 can hold several keys, allowing a new key to be entered whilst the previous one is still in use.

Keys are entered into the GC-603 by means of a proprietary key transfer device, also known as a fill gun or filler. The image on the right shows the key transfer device that was used with the Gretacoder 603. It can be attached to the two metal stubs at the center of the front panel and should be connected to the socket marked KEY.

Another proprietary unit that was supplied with each GC-603, is a 3-position rotary switch, mounted on a military connector. It should be connected to the CONTROL socket on the front panel and is used to select the required MODE.

The switch has three positions: OP, LP1 and LP2. OP probably means: operate, whilst LP1 and LP2 are probably used to select which KEY is being loaded with the key entry device shown above. The red button - marked E – can be used to erase the KEYs in case of an emergency or compromise.

Key loading device Key selector switch
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Key loading device
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Key selector switch

Further information
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Crypto Museum. Created: Thursday 06 August 2009. Last changed: Saturday, 24 February 2018 - 08:49 CET.
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