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Secure voice
Voice scramblers and encryption devices

This section deals with secure speech equipment, such as voice encryption devices, from a variety of manufacturers. Such devices come in many flavours, ranging from simple voice scramblers to digital voice encryptors. Most of the devices shown below, are also featured elsewhere on this website as they fall into multiple categories. Secure telephones are a class of their own, but since they also belong to the group of voice encryption devices, they are linked from this page as well.

Voice encryption units on this website
Frequency Changer No. 6AC/3 - British wartime telephone scrambler as used by Churchill
German WWII phone scrambler
High-end portable telephone encryptor
Racal MA-4204
MA-4014B Audio Encryption Unit
Racal MA-4204
Racal MA-4470 Voice Crypto Unit
Racal MA-4225 Portable voice encryption unit
Racal MA-44741 Secure Phone Adapter
Racal Digital voice encryptor
Tadiran SEC-13
Tadiran SEC-15
Telsy TS-500
Teltron SP-810
Teltron SP-612 voice scrambler
Hagelin CSE-280 voice encryption device with delta modulation
Hagelin CRM-008 (HC-230, HC-235, CRYPTOCOM)
BBC Cryptophon 1100
BBC Vericrypt 1100 voice scrambler
External voice scramblers for mobile radios
Plug-in frequency domain scrambler for Fug-8 (KF-802)
SVZ
Siemens MSC-2001 voice encryption unit
Telemit AN/PSQ-188 voice encryption unit
Wide-band Voice and Data Encryption Unit
Narrow-band Voice and Data Terminal
Philips Spendex-10, Narrow-band Voice and Data Terminal
F/T voice scrambler for telephone and HF radio
Hagelin CVX-396 (SVZ-B) voice encryptor
Telsy TDS-2004M mobile voice encryptor
Telsy TDS-2003 mobile voice encryptor
Telsy TX-900S mobile phone (analogue) scrambler and (digital) encryptor
Telsy TX-1020C narrow-band radio voice scrambler and encryptor
Teltron SP-850 voice scrambler
Mobile secure radio voice system
Two-dimensional voice scrambler for carphones
Motorola Saber II secure portable radio
Motorola STX secure trunking radio
STX
Simple digitially controlled analogue voice scrambler
PFX-PM portable radio with digital encryption
SE-20 secure handheld VHF/UHF radio with scrambler
Ascom Cryptovox SE-160 secure handheld VHF/UHF radio
Ascom SE-660 mobile radio
Skanti DS-6001 digital voice scrambler
The Siemens DSM Voice telephone encryptor
Tele Security Timmann, TST-7595 Voice scrambler for HF radio
Telesecurity Timmann, TST-7698 Digital voice encryptor for HF/VHF/UHF radio
Tele Security Timmann, TST-7700 voice and data encryption system
Gretacoder 101, speech scrambler
Elcrovox 1/3 voice encryptor
Elcrovox 1-4D narrow band voice and data terminal (STU-II compatible)
Tait T-2000/II mobile radio with optional voice scrambler
Tait T-3000/II handheld radio with optional voice scrambler
BID/250 SAVILLE-based voice encryption unit for Clansman DMU
Replacement for the BID/250 and other (obsolete) cryptographic units
Remote Control Unit GL-7171 of the Saudi Arabian Piece Shield system
Voice scrambler handset
DSP-9000 EDT voice scrambler
Wide-band voice encryption unit used by the former Yugoslav Army
Norrow-band voice encryption unit used in the former Yugoslav Republic
HF modem with FEC and (optional) voice and data encryption
Racal PRM-4515 Cougar handheld radio with encryption
Racal Cougar PRM-4735 body-wearable covert radio with voice encryption
Racal Cougar PRM-5120 body-wearable covert radio with voice encryption
Harris RF-5811 secure voice and data unit
Selex/Marconi H-4855 Personal Role Radio (PRR) - not really crypto, but listed because LPI/LPD and use of Spread Spectrum technology
PRR
Selex H4855 ELSA Enhanced Encrypted Personal Role Radio (EZ-PRR)
Motorola MTM-5400 and MTM-5500 TETRA radios
Mobile frequency and time domain voice scrambler
T-219 (Yachta) voice scrambler
SIGSALY secure telephony system
Secure Telephones (Crypto Phones)
French mechanical Cryptophone by Jules Carpentier (1919)
 Crypto telephones
Secure voice families
NESTOR family of digital wideband voice encryptors
VINSON family of digital wideband voice encryptors
ANDVT family of digita; narrowband/wideband voice encryptors
Systems
Secure speech systems are known by various names, such as Voice Privacy Unit, Secure Speech System, Voice Protection Device, Speech Encryptor, Voce Encryption Device, Secure Voice Device, etc. Basically, there are only two systems for voice protection:

  1. Analogue scrambling
  2. Digital encryption
Methods
  • A.1 - Frequency domain voice scrambler
    In this analogue system, the frequency domain of the human speech is mirrored and/or transposed around a given center frequency, so that it becomes unintelligible. Such sys­tems can easily be broken, even if the audio band is split into multiple smaller bands first.

  • A.2 - Time domain voice scrambler
    In this system, tspeech is first stored in memory, after which the individual parts are scrambled in the time domain. It is more secure than a frequency domain scrambler, but can still be broken as the individual sound samples still bear the properties of speech.

  • A.3 - Frequency and Time Domain voice scrambler
    This system, also known as an F/T Scrambler, is a combination of the above methods. It is the most complex one, but can still be broken with the right equipment, no matter how complex the randomizer is, as the individual samples still bear the properties of speech.

  • B - Digital Encryption
    This method uses a digital representation of the analogue voice signal (samples), which is mixed with a digital key stream. This method is much safer than the ones above and is the only one that can really be called encryption.
A — Analogue scrambling
Before digital speech encryption became widely available, analogue techniques were used to protect voice transmissions. This technique is commonly known as voice scrambling and comes in three flavours which are further explained below. Scramblers are inherently insecure and only provide protection against an occasional eavesdropper, such as the telephone exchange operator.


A.1 — Frequency domain scrambling   FD
The oldest method uses frequency inversion and is also known as voice inversion. It is based on mirroring of the audio frequency spectrum around a given center frequency, and can be applied to a discrete number of sub-bands. This principle is best explained using a simplified model:


The audio spectrum of the voice data (1) is mixed with a fixed carrier frequency fc (2). This results in two spectra: one that is the sum of the original sectrum and the carrier (3), and one that is the difference of the two signals (4). A low-pass filter (LPF) is then applied to filter-off the sum and leave only the difference, effectively resulting in a mirrored audio band (5). At the receiving end, this process of mirroring of the spectrum is repeated to make the speech 'legible' again:


The advantage of this technique is that it completely takes place within the audio bandwidth of a channel, whereas digital encryption generally requires more space. This allows scrambling to be used in existing systems. At the time, scramblers were also cheaper than digital encryptors, which is why scramblers were used by the police in many countries from the 1970's well into the 1990's.

The disadvantage of this method is that an evesdropper can easily reverse the mirroring process with a simple electronic circuit. In addition, experienced listeners could sometimes even extract useful information from the seemingly garbled speech directly, without a descrambling circuit.

Although voice inversion is commonly achieved by using an electronic diode-based ring mixer, the French inventor and engineer Jules Carpentier showed in 1919 that the same effect can be obtained mechanically, by using a motor-driven commutator running at the centre frequency.


In a more complex scheme, one could vary the carrier frequency and also split-up the audio band into several (e.g. five) smaller bands that are then mirrored individually. In addition, the individual frequency bands can be swapped as shown in the rightmost diagram above. Continuously varying these parameters by putting them under digital control, can make it harder to decode the signal.

Examples of frequency domain scramblers
British WWII scrambler phone
T-219 (Yachta) voice scrambler
Racal MA-4204
MA-4014B Audio Encryption Unit
Teltron SP-612 voice scrambler
Tele Security Timmann, TST-7595 Voice scrambler for HF radio
External voice scramblers for mobile radios
Plug-in frequency domain scrambler for Fug-8 (KF-802)
SVZ
French mechanical Cryptophone by Jules Carpentier (1919)
A.2 — Time domain scrambling   TD
Another method for speech protection is the so-called time-division or time-domain (TD) speech scrambling. This method is more secure than the simpler frequency-inversion system, but far less secure than modern digital speech encryptors. The simplified diagram below, shows how it works.

Sampled speech data is cut into a number of small fragments which are then scrambled in an ever changing order. The order in which the packets are scrambled is determined by a pseudo random number generator (PRNG) which is seeded (initialised) by the user by means of a secret KEY.


In the diagram above, the top row shows the clear speech (input) in time. The second row shows the speech after it is scrambled. The bottom row finally shows the speech once it is descrambled again (output). The whole process of scrambling and descrambling causes a noticable delay which is typically in the range of 0.3 to 0.6 seconds or even longer. Delays like this are often unaccept­able as they can lead to confusion.

As the time segments are scrambled in an ever changing pattern, it is important that transmitter and receiver are correctly synchronised. To ensure that both ends are kept in sync, a pilot signal is transmitted with the scrambled speech by means of Audio Frequency Shift Keying (AFSK). An example of a speech scrambler that uses Time Domain Scrambling, is the BBC Cryptophon 1100.

Although scramblers of this type are not safe, many police and other law enforcement agencies around the world, used this method for securing their conversations for many years, as it has the advantage that it can be used on existing narrow-band FM radio channels. Despite the fact that the experienced listener can't make any sense of the garbles, the system is prone to cryptanalytic attacks. It is possible to reconstruct the original signal, without knowning the key or the PRNG, by using a computer to analyse the signal to find any discontinuities, and then reorder the frames.

Examples of time domain scramblers
BBC Cryptophon 1100
BBC Vericrypt 1100 voice scrambler
Telsy TS-500
SE-20 secure handheld VHF/UHF radio with scrambler
Tele Security Timmann, TST-7595 Voice scrambler for HF radio
A.3 —  Frequency and Time domain scrambling   F/T
The third and most complex type of voice scrambler, is the so-called Frequency and Time Domain Scrambler, also known as the F/T Scrambler, which is basically a combination of the two methods explained above. This solution is also known as two-dimensional voice scrambling. Although scrambling and descrambling of this method is much more complex, the system is equally prone to cryptanalysis as the previous ones. Any kind of analogue scrambling is inherently insecure.

Example of an input signal in the frequency and time domain.  Crypto Museum 2024.

The diagram above shows roughly how it works. The audio spectrum is divided into a number of discrete sub-bands (here shown in different colours and numbered I to VIII). The sub-bands are then sampled individually, after which they are stored in a memory buffer in a pseudo random pattern that transposes their place in the sub-band order (i.e. the frequency domain) as well as their place in time (i.e. the time domain). The resulting pattern now looks something like this:

Example of an output signal in the frequency and time domain.  Crypto Museum 2024.

The samples for each of the sub-bands are then read from memory, and combined into a new (scrambled) voice signal that still fits the original 3 kHz bandwidth. The black curve in the diagrams above illustrates how the signal has changed from its original shape. In older voice scramblers, band splitting is generally achieved with discrete filters, mixers and other electronic parts, whereas in modern devices this is commonly done with a Digital Signal Processor (DSP).

Audio samples
Below are some examples of scrambled speech. These samples were recorded by Barry Wels [1] from the built-in analogue voice scrambler of the Icom IC-H11 radio. If you listen carefully to the scrambled audio, you may be able to descramble some of it yourself with a little exercise.


Examples of frequency and time domain (F/T) scramblers
Racal MA-4470 Voice Crypto Unit
Hagelin CRM-008 (HC-230, HC-235, CRYPTOCOM)
Telsy TX-900S mobile phone (analogue) scrambler and (digital) encryptor
Telsy TX-1020C narrow-band radio voice scrambler and encryptor
Tele Security Timmann, TST-7595 Voice scrambler for HF radio
Gretacoder 101, speech scrambler
Tait T-2000/II mobile radio with optional voice scrambler
Tait T-3000/II handheld radio with optional voice scrambler
Voice scrambler handset
Teltron SP-850 voice scrambler
Telsy TDS-2004M mobile voice encryptor
Telsy TDS-2003 mobile voice encryptor
F/T voice scrambler for telephone and HF radio
DSP-9000 EDT voice scrambler
B — Digital Encryption
In its simplest form, a digital voice encryption device digitizes the voice signal by means of an Analog-to-Digital Convertor (ADC). The resulting data stream is them 'mixed' by means of an XOR-operation with a key stream that is generated by a Pseudo-Random Number Generator (PRNG). In this context, the latter is also known as a Key Generator (KG). The resulting ciphertext is then converted back to the analogue domain by means of a Digital-to-Analog Converter (DAC).


The Key Generator (KG, PRNG) is seeded by a KEY that is entered manually or by means of a key transfer device or fill gun. Modern systems sometimes use Public Key Encryption (PKE) to pass the key over an insecure channel (e.g. Diffie-Hellman key exchange). Due to the fact that the digital XOR-operation (modulo-2 addition) is used for mixing the plaintext with the key stream, the same opration can be used for decryption. This principle is also known as the Vernam Cipher.

Digitizers
Before speech can be encrypted, it must be converted from the analogue to the digital domain, by means of a sampler, or digitizer, or Analog-to-Digital Converter (ADC). In the 1970s, devices like KY-57 (VINSON) and Spendex 10 used Continuous Variable Slope Delta-modulation (CVSD) to con­vert speech to digital data. This wide-band solution is only suitable for VHF and UHF radio.

Generally speaking, a digital signal needs more bandwidth than its analogue equivalent (typically twice the bandwidth), but methods have been developed to compress the data, allowing it to be send over a narrowband (3 kHz) channel. At the other end, the data must then be decompressed before it can be used. Such a compressor/decompressor is commonly known as a CODEC.

An example of a voice compression algorithm, also known as a VOCODER, is Linear Predictive Coding (LPC-10e), developed in the 1970s by the US Department of Defense. It analyses the voice data and converts it to a set of coefficients, which are then sent as numeric values. At the re­ceiving end, these coefficients are used to reconstruct, or synthesize, the original sound. LPC-10 allows voice data to be sent at 2400 baud, and LPC-10e can even be used at 800 baud. The first vocoder, named VODER, was developed at Bell Labs in 1939. Its principle was first used during WWII on SIGSALY — the transatlantic secure telephone line between Washington and London.

 Different CODECs


Encryption
Once analogue speech has been digitized, it can be encrypted digitally, by means of a variety of encryption algorithms. Some devices use publicly available algorithms such as DES, Triple-DES (3DES) or AES, but others use proprietary encryption algorithms that are kept secret. An example of the latter is SAVILLE that was jointly developed by GCHQ (UK) and NSA (USA), and is still widely used in US/NATO military equipment today.

Audio samples
Below are some sound samples of digitally encrypted speech, recorded from an Icom IC-H10SR radio by Barry Wels [1]. The first file contains the original audio file. The second file plays the encrypted audio. The last file finally contains the resulting audio once it has been decrypted.


Examples of digital voice encryptors
SIGSALY secure telephony system
Racal Digital voice encryptor
Siemens MSC-2001
Wide-band Voice and Data Encryption Unit
Narrow-band Voice and Data Terminal
Philips Spendex-10, Narrow-band Voice and Data Terminal
Hagelin CSE-280 voice encryption device with delta modulation
Hagelin CVX-396 (SVZ-B) voice encryptor
Telsy TX-900S mobile phone (analogue) scrambler and (digital) encryptor
Telsy TX-1020C narrow-band radio voice scrambler and encryptor
Motorola Saber II secure portable radio
Motorola STX secure trunking radio
STX
PFX-PM portable radio with digital encryption
Ascom Cryptovox SE-160 secure handheld VHF/UHF radio
Ascom SE-660 mobile radio
Tele Security Timmann, TST-7700 voice and data encryption system
Elcrovox 1-4D narrow band voice and data terminal (STU-II compatible)
BID/250 SAVILLE-based voice encryption unit for Clansman DMU
Wide-band voice encryption unit used by the former Yugoslav Army
Norrow-band voice encryption unit used in the former Yugoslav Republic
Racal PRM-4515 Cougar handheld radio with encryption
Racal Cougar PRM-4735 body-wearable covert radio with voice encryption
Telesecurity Timmann, TST-7698 Digital voice encryptor for HF/VHF/UHF radio
References
  1. Barry Wels, Audio samples from ICOM radio equipment
    Crypto Museum, March 2011.

  2. George Sugar, Voice Privacy Equipment for Law Enforcement Communication Systems
    US Department of Justice. LESP-RPT-0204.00. May 1974. Page 16.
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Crypto Museum. Created: Tuesday 04 August 2009. Last changed: Friday, 14 June 2024 - 08:49 CET.
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