 | Homepage Crypto Index Glossary Enigma Hagelin Fialka Rotor Pin-wheel TROL Voice Data Hand OTP EMU HSM Mixers Phones Bulk FILL Codebooks Algorithms Chips Cryptanalysis Toys World War II  Countries Czechoslovakia DDR France Germany Netherlands Russia Switzerland UK USA USSR Yugoslavia Manufacturers ANT Aristo Ascom AT&T BBC Bosch Compumatica Crypto AG Datotek Fox-IT GD Gemalto Gretag Harris HELL ITT Lorenz Lugagne MEL Mils Motorola Mykotronx Nokia OMI Omnisec Philips PTT R&S Racal RanData Raytheon RCA Safenet Secunet Siemens STK Tadiran Tait TCC Telefunken Telsy Teltron Thales TNO Transvertex TST Tutus Data AB Ultra Electronics Utimaco Zellweger OTHER Spy radio Burst encoders Intercept Covert Radio PC Telex Telephones People Agencies Manufacturers DONATE Publications Standards For sale Kits Shop News Events Wanted Contact About us Links | | |  | VoiceGretag 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 | | 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. | | | 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. |  |
- Full duplex was possible by using two GC-101 units on a 4-wire line, or split-frequency duplex radio link.
| A × A 1 / 8  A 2 / 8  A 3 / 8  A 4 / 8  A 5 / 8  A 6 / 8  A 7 / 8  A 8 / 8  ❮ ❯ | 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). | | | Two-dimensional scrambling | | | | 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. | | 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. | | 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. | | 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. | | 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) [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]. | | | 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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