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Voice Gretag GC-104 → GC-103 → ← SC-101
Voice scrambler
Gretacoder 101, commonly abbreviated GC-101, is a
voice scrambler for use on telephone
and radio networks, introduced in 1975 by Gretag AG
in Regensdorf (Switzerland). The device uses a two-dimensional type of
voice scrambling and
is suitable for half-duplex voice communication. 1
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The GC-101 uses two-dimensional scrambling, consisting of time-domain (T)
and frequency-domain (F) 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 regular (analogue) telephone line
(POTS) or a radio set, the separate PA-101 adapter is available.
The military version of the GC-101 is known as Gretacoder 102 or GC-102.
It is housed in a green ruggedized military-grade metal enclosure, but is
otherwise identical, as it is based on the same hardware components.
The design of the GC-101 and GC-102 was based on the earlier
SC-101 speech scrambler — introduced in 1993 [1] —
that had similar specifications but was not modular.
It is currently unknown whether SC-101 and GC-101 are compatible.
Development of the GC-101 started in 1973 and took most of 1974,
with the first protypes becoming available in October 1974.
During the first half of 1975, the manuals,
the maintenance schedules and the training schemes were completed.
The first production units were available at the end of July 1975 [4].
The GC-101 was popular in Switzerland, Austria, Germany
and many other countries. In Switzerland it was used by the Federal Police
(BuPo) and the Regional Police (KaPo), whilst in Austria it was used by
the Army. The GC-102 was mainly sold to the Swiss Army.
Note that voice scramblers, no matter how sophisticated, are inherently
insecure. They offer little to no protection against a professional
interceptor, such as a state actor.
At the time however, it offered sufficient security against scanner listeners
and the press.
Several years later, in the early 1980s, the GC-101 and 102 were
replaced by Gretacoder 103
and Gretacoder 104 respectively.
Contrary to the GC-101, these units were not developed by Gretag,
but by competitor Telsy in Italy with whom Gretag had
a good relationship. They were sold as OEM products.
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Full duplex was possible by using two GC-101 units on a 4-wire
line, or split-frequency duplex radio link.
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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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Gretacoder 101 uses a two-dimensional analogue scrambling technique,
of which the parameters are digitally controlled in a pseudo-random
pattern that is determined by the encryption key.
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The first technique is
known as time-domain scrambling, or T-scrambling, and is illustrated by
the simplified diagram below. Note that the method used here is completely
different from the regular time-domain scrambling techniques, in which
the samples are stored in a buffer and then read out in a different order.
At the left is the scrambler, whilst the descrambler is at the right.
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 (τ), 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 (τ'), after
which the two components are added together again.
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The second technique is frequency-domain
scrambling, or F-scrambling, which consist of two steps, as illustrated
in the diagram below. In the first
step, the frequency spectrum of the plain 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 upper boundary (3 kHz), is
'wrapped around' at the lower boundary (i.e. cyclic).
The frequency shift Δfi
can vary between a finite 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 encryption key.
To get the best possible security, both techniques (T-
and F-scrambling) are used simultaneously.
If the quality of the transmission channel is too low — for example when
the device is used on HF SSB radio links — it is possible to turn
T-scrambling off with the
red selector at 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 key,
an elementary key and an auxiliary key. The structure key 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 [F].
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) [H]. 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 [E][F].
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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 [3].
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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 [2].
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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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Device Voice scrambler Purpose Secure narrowband voice transmission over telephone and radio circuits Model Gretacoder 101, GC-101 Year 1975 Manufacturer Gretag Predecessor SC-101 Successor GC-103, GC-104 Principle Rolling F/T scrambling Mode Half-duplex (full-duplex on request) Line in/out 4-wire 600Ω (balanced and unbalanced) Bandwidth 300-3000 Hz TX level 0dBm RX level 0 to -30dBm Interval 100 ms Sync 200 ms Resync Every 70 sec Key space 88 = 16,777,216 Structures 4620 Period 8.555 · 109 Power 10 to 32V DC (30W) Mains 110/220V AC ±15% (45-65 Hz, 30W) Temperature -10°C to +50°C Storage -25°C to +75°C Dimensions 334 × 232 × 102 mm Weight 4 kg
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- Security in Ciphering, Operation, Transmission
Gretacoder 101 Sales Leaflet (English). 2 pages.
Gretag AG. Undated.
- Sicherheit bei Chiffrierung, Bedienung, Übermittlung
Gretacoder 101 Sales Leaflet (German). 2 pages.
Gretag AG. Undated.
- Gretacoder 101 Speech Scrambler
Datasheet (English). 2 pages.
Gretag AG, May 1975.
- Gretacoder 101 Sprachverschleierungsgerät
Datasheet (German). 2 pages.
Gretag AG, May 1975.
- Gretacoder 101/2 Speech Scrambler Application Note
Application Note (English).
Gretag AG, 21 June 1978.
- Gretacoder 101/2 Sprachverschleierungsgerät Anwendungen
Application Note (German).
Gretag AG, 19 July 1978.
- Gretacoder 102 Wartungskonzeption
Maintenance schedule (German). 8 pages.
MW 3577/1. Gretag AG, 23 August 1978.
- Telefon-Adapter PA-101
Datasheet (German). 2 pages.
Gretag AG, January 1976.
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© Crypto Museum. Created: Sunday 21 July 2013. Last changed: Wednesday, 21 August 2024 - 06:02 CET.
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