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← Gretag Voice
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.
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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.
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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.
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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.
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.
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).
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Two-dimensional scrambling
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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 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.
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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 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.
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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.
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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].
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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.
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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.
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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.
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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].
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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.
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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].
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- PA-101 - Telephone Adapter for Gretacoder 101
- RC-202 - Remote control unit for Gretacoder 102
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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.
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- Gretag AG, SC-101 Speech Scrambler
Datasheet (English). May 1973. 2 pages.
- Gretag AG, Security in Ciphering, Operation, Transmission
The Unique Voice Scrambler Gretacoder 101.
Gretacoder 101 Sales Leaflet (English). Date unknown. 2 pages.
- Gretag AG, Sicherheit bei Chiffrierung, Bedienung, Übermittlung
Das neuartige Sprachverschleierungsgerät "Gretacoder 101".
Gretacoder 101 Sales Leaflet (German). Date unknown. 2 pages.
- Gretag AG, Gretacoder 101 Speech Scrambler
Datasheet (English). May 1975. 2 pages.
- Gretag AG, Gretacoder 101 Sprachverschleierungsgerät
Datasheet (German). May 1975. 2 pages.
- Gretag AG, Gretacoder 101/2 Speech Scrambler Application Note
Application Note (English). 21 June 1978.
- Gretag AG, Gretacoder 101/2 Sprachverschleierungsgerät Anwendungen
Application Note (German). 19 July 1978.
- Gretag AG, Gretacoder 101 Development and pre-production schedule
Hand written notes (German). 21 May 1975.
- Gretag AG, Gretacoder 102 Wartungskonzeption
Maintenance schedule (German). 23 August 1978. 8 pages.
- Gretag AG, Telefon-Adapter PA-101
Datasheet (German). January 1976. 2 pages.
- 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.
- 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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