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Gretacoder 101/102
Speech scrambler

The Gretacoder 101 and Gretacoder 102, were speech scramblers for telephone and radio links, developed and built by Gretag AG in Regensdorf (Switzerland) in 1975. The devices use a two-dimensional type of voice scrambling and are suitable for half-duplex voice communication.

The GC-101 uses two-dimensional scrambling, consisting of time-domain and frequency-domain scrambling, each with its own adjustable parameters. By changing these parameters in a pseudo-random sequency every 100ms, a relatively secure cipher was established.

The image on the right shows the Gretacoder 101, which has a modular design. The device basically consists of four modules, three of which are visible at the front panel. The rear section of the unit is taken by the built-in mains power supply, with the mains entry at the front.
Gretacoder 101

For connection to a telephone line, the separate PA-101 adapter was available. The Gretacoder 102 (GC-102) was housed in a ruggedized military-grade green metal case. It was compatible with the GC-101 and was based on the same hardware components. The design of the GC-101 and GC-102 was based on the earlier speech scrambler SC-101 (1973) [1] that had similar specifications but was not modular. It is currently unknown whether it was compatible.

Development of the GC-101 started in 1973 and took most of 1974, with the first protypes arriving in October 1974. During the first half of 1975, the manuals, the maintenance schedules and the training schemes were developed. The first production units were available by the end of July 1975 [8]. The GC-101 became a popular unit in Switzerland, Austria, Germany and in many other countries. The GC-102 was mainly sold to the Swiss Army.

Several years later, in the early 1980s, the GC-101 and 102 were replaced by the Gretacoder 103 and the Gretacoder 104. Contrary to the GC-101, these units were not developed by Gretag, but by Telsy in Italy with whom Gretag had a good relationship. They were sold as OEM products.

Gretacoder 101 Gretacoder 101 with handset Operating the Push-To-Talk switch (PTT) of the handset GC-101 front panel with mains PSU Selecting single-dimensional (frequency-domain) scrambling Switching between CLEAR and SECURE Line interface GC-101 with PA-101 telephone adapter
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Gretacoder 101
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Gretacoder 101 with handset
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Operating the Push-To-Talk switch (PTT) of the handset
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GC-101 front panel with mains PSU
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Selecting single-dimensional (frequency-domain) scrambling
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Switching between CLEAR and SECURE
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Line interface
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GC-101 with PA-101 telephone adapter

All controls and connections of the Gretacoder 101 are at the front panel, which consists of three units: the main cipher unit at the top, and two smaller plug-in units at the bottom. The leftmost plug-in unit is the interface to the line and the handset, whilst the power input is as the right.

Controls and connections at the front panel of the GC-101

The image above shows the front panel of a typical Gretacoder 101 which is powered from the mains. A small red switch at the bottom right allows selection between 110V and 220V AC networks. A similar switch at the top left allows switching between two-dimensional scrambling (F+T) and frequency-domain-only scrambling (F). The latter was used on bad phone lines.

Front panel of the GC-101 with DC power input

Due to the modular design, it was possible to convert the Gretacoder 101 for DC-operation. In that case the mains AC plug-in at the bottom right was removed and replaced by a DC plug-in, allowing the unit to be powered by any DC voltage between 10V and 32V (e.g. in a vehicle).

Two-dimensional scrambling
The Gretacoder 101 uses a two-dimensional analogue scrambling technique, of which the parameters are digitally controlled in a pseudo-random pattern that is affected by the crypto key(s). One technique is known as time-domain scrambling, or T-scrambling, and is illustrated by the simplified diagram below. At the left is the scrambler; at the right the descrambler.


Using a sharp cross-over filter, the clear voice signal is split into two frequency parts: a high part and a low part. In the transmitter, the low part is delayed by means of a delay line, after which the two signals are added again. At the receiving end, the same cross-over filter is used, but this time, only the high part is delayed, after which the two components are added together again.


The second technique that is used in the Gretacoder 101 is frequency-domain scrambling, or F-scrambling, which consist of two steps. In the first step, the frequency spectrum of the clear voice signal (f) is inverted (fi), whilst in the second step the inverted spectrum is shifted by an offset Δfi. The part that 'falls off' at the 3kHz boundary, is added back in at the lower end (i.e. cyclic). The frequency shift Δfi can vary between a number of discrete values.

Both techniques combined

Both techniques have a number of parameters that control their behaviour. These parameters are all varied continuously in a pseudo-random manner, under control of the crypto logic. In order to get the best possible security, both techniques (T-scrambling and F-scrambling) are used simultaneously. However, if the telephone line or the radio channel that is used for the conversation is too bad (e.g. noisy), it is possible to turn off T-scrambling and use only F-scrambling. This is controlled by the small red switch at the top left of the front panel.

Cryptographic strength
The parameters of the T-scrambling and F-scrambling units inside the Gretacoder 101, are controlled by a digital pseudo-random generator that is affected by three fundamental elements: a structure, an elementary key and an auxiliary key. The structure is determined by two secret pluggable modules that are custom-wired and can be modified by the user.

The three keys that affect the pseudo-random number generator (PRNG)

The elementary key is entered by the user at the beginning of a session. It consists of eight octal digits (1-8) that are entered on the keyboard. The auxiliary key is generated by the GC-101 itself at the start of a transmission and is added to the elementary key by means of an XOR operation. Each time the PTT is pressed, a new auxiliary key is generated by a built-in random generator. Furthermore, a new auxiliary key is generated during a transmission every 70 seconds [6].

The auxiliary key is sent three times to the other end. A system known as 'majority voting' is used to determine the correct auxiliary key in case of interference on the channel. Generally speaking, auxiliary key transmission and re-transmission is very robust, even under bad circumstances.

The pseudo-random scrambling generator consists of a number of shift-registers and non-linear logical configurations. A total of 4620 secret structures are possible, whilst there are approx. 16 million possible elementary keys that can be entered by the user (88. The generator has a period of 8.555·109 with an interval of 100ms. Nevertheless, speech scramblers are inherently insecure.

PA-101 Telephone Adapter
The line IN/OUT connector at the front left of the GC-101 allows the device to be connected to different types of (audio) lines. A typical use of the unit is in combination with analogue (PSTN) telephone lines, in which case the separate PA-101 Telephone Adapter shown below is used.

The image on the right shows the PA-101 placed on top of the GC-101 and connected to the IN/OUT socket at the bottom left. The PA-101 has connections to the (analogue) telephone line and to the actual phone and is CCITT compliant.

At the front is a red toggle switch that allows switching between the standard telephone set (clear) and the GC-101 (secure) [10]. When in secure mode, the handset of the GC-101 is used and a secure voice conversation can take place in half-duplex. For full duplex, two complete units and two telephone lines are required [6].
GC-101 with PA-101 telephone adapter

At the center of the PA-101 is a hole with a long bolt, that allows the unit to be mounted to the bottom of an old style telephone set. A conversation is always started in CLEAR mode, with the PA-101 set to the standard telephone set. Once the parties have agreed to 'go secure' the switch is set to GRETACODER and the secure part of the conversation is started in half-duplex.

GC-101 with PA-101 telephone adapter GC-101 with PA-101 telephone adapter Line interface
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GC-101 with PA-101 telephone adapter
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GC-101 with PA-101 telephone adapter
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Line interface

The Gretacoder 101 is a very complex well-built device that consists of four large PCBs and two smaller plug-in units. A common misunderstanding with this device is that it is often thought that the plug-in unit at the bottom left contains the cryptographic hardware. It is often missing from surplus devices that have been offered in recent years on auction sites such as eBay.

This is not the case however. The crypto board is part of the upper section of the device, that can be accessed by loosening the four bolts at the corners. The upper section consists of four large PCBs: the control board, a (crypto) logic board, a dynamic memory board and an analogue (audio) board, all connected via a backplane.

When removing the front panel, the control board is removed from its slot, as it is mounted to the front panel. The image on the right shows the remaining three boards after the front panel and the control board have been removed.
Looking inside the GC-101 after removing the front panel

The board with the short red bar is the actual crypto logic board that is often missing from surplus GC-101 devices. Together with the control board, it forms the digital cipher unit that controls the parameters of the two-dimensional (analogue) scrambler. The lowest PCB is the analogue board, that controls filters, amplifiers and delay lines, whilst a separate board houses the dynamic memory, consisting of eight Intel 1404A 1024-bit dynamic shift register ICs [12].

The lower section of the Gretacoder 101 consists of two plug-in units. The leftmost one is the line interface through which the device is connected to the outside world. The plug-in unit itself consists of two sandwiched PCBs that connect to the rest of the device via the backplane. The image on the right shows the filters close to the connectors of the line interface plug-in unit.

The rightmost plug-in unit is the power input section. As the device was intended for desktop as well as mobile use, Gretag offered two power plug-ins, that could be installed by the user.
Close-up of the line interface unit

The actual Power Supply Unit (PSU) itself, was mounted in the rear part of the GC-101, behind the backplane. The power plug-in unit, simply acts as a selector. For desktop use, an AC mains plug-in unit was available. It contains a mains switch, the fuses and some additional filtering. For battery operation, a separate DC power plug-in with an input range of 10V-32V was used [11].

Removing the front panel with the logic board Front panel with the control board Control board Detail of the cipher board Cipher unit Detail of the analogue memory board Intel 1404A dynamic shift registers Analogue memory board
Analogue board Close-up of the analogue board Analogue board Line interface unit Close-up of the line interface unit DC power board Close-up of the DC power plug-in AC mains power plug-in
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Removing the front panel with the logic board
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Front panel with the control board
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Control board
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Detail of the cipher board
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Cipher unit
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Detail of the analogue memory board
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Intel 1404A dynamic shift registers
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Analogue memory board
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Analogue board
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Close-up of the analogue board
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Analogue board
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Line interface unit
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Close-up of the line interface unit
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DC power board
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Close-up of the DC power plug-in
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AC mains power plug-in

  • PA-101 - Telephone Adapter for Gretacoder 101
  • RC-202 - Remote control unit for Gretacoder 102
Help required
Although we have quite a bit of information about the GC-101, we are still looking for service documentation, such as circuit diagrams and circuit descriptions. If you have any of these available, or if you have other information about this device, please contact us.

  1. Gretag AG, SC-101 Speech Scrambler
    Datasheet (English). May 1973. 2 pages.

  2. Gretag AG, Security in Ciphering, Operation, Transmission
    The Unique Voice Scrambler Gretacoder 101.
    Gretacoder 101 Sales Leaflet (English). Date unknown. 2 pages.

  3. Gretag AG, Sicherheit bei Chiffrierung, Bedienung, Übermittlung
    Das neuartige Sprachverschleierungsgerät "Gretacoder 101".
    Gretacoder 101 Sales Leaflet (German). Date unknown. 2 pages.

  4. Gretag AG, Gretacoder 101 Speech Scrambler
    Datasheet (English). May 1975. 2 pages.

  5. Gretag AG, Gretacoder 101 Sprachverschleierungsgerät
    Datasheet (German). May 1975. 2 pages.

  6. Gretag AG, Gretacoder 101/2 Speech Scrambler Application Note
    Application Note (English). 21 June 1978.

  7. Gretag AG, Gretacoder 101/2 Sprachverschleierungsgerät Anwendungen
    Application Note (German). 19 July 1978.

  8. Gretag AG, Gretacoder 101 Development and pre-production schedule
    Hand written notes (German). 21 May 1975.

  9. Gretag AG, Gretacoder 102 Wartungskonzeption
    Maintenance schedule (German). 23 August 1978. 8 pages.

  10. Gretag AG, Telefon-Adapter PA-101
    Datasheet (German). January 1976. 2 pages.

  11. Immo Hahn, Handset for Gretacoder 101 and DC plug-in - THANKS !
    Handset and plug-in for the GC-101 kindly dontated by German collector Immo Hahn. June 2013.

  12. Intel, 1404A - 1024 bit Dynamic Shift Register
    Datasheet. Date unknown. 4 pages.
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Crypto Museum. Created: Sunday 21 July 2013. Last changed: Saturday, 24 February 2018 - 06:50 CET.
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