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Voice Encryption Units
This section deals with secure voice cipher equipment (voice crypto) from a variety of manufacturers. Voice encryption devices come in many flavours, ranging from small military radio add-ons to desktop telephone encryptors. Most of the units shown here, are also available in other categories on this website. 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 crypto units on this website:
Racal MA-4204 MA-4204 MA-4014B Audio Encryption Unit MA-4014B Racal MA-4204 MA-4224 Racal MA-4470 Voice Crypto Unit MA-4470 Racal MA-4225 Portable voice encryption unit MA-4225 Racal MA-44741 Secure Phone Adapter MA-4471 Racal Digital voice encryptor MA-4777 Tadiran SEC-13 SEC-13
Tadiran SEC-15 SEC-15 Telsy TS-500 TS-500 Teltron SP-810 SP-810 Hagelin CRM-008 (HC-235, Cryptocom) CRM-008 BBC Cryptophon 1100 C-1100 Siemens MSC-2001 MSC-2001 Wide-band Voice and Data Encryption Unit KY-57 Narrow-band Voice and Data Terminal KY-99
Philips Spendex-10, Narrow-band Voice and Data Terminal SP-10 Telsy TDS-2004M mobile voice encryptor TDS-2004 Telsy TDS-2003 mobile voice encryptor TDS-2003 Telsy TX-900S mobile phone encryptor TX-900S Telsy TX-1020C narrow-band radio voice encryptor TX-1020 Mobile secure radio voice system Orthros Motorola Saber II secure portable radio Saber PFX-PM portable radio with digital encryption PFX-PM
Skanti DS-6001 digital voice scrambler Skanti The Siemens DSM Voice telephone encryptor DSM Voice Tele Security Timmann, TST-7595 Voice scrambler for HF radio TST-7595 Tele Security Timmann, TST-7700 voice and data encryption system TST-7700 Gretacoder 101, speech scrambler GC-101 Elcrovox 1-4D narrow band voice and data terminal (STU-II compatible) EX 1-4D Tait T-2000/II mobile radio with optional voice scrambler T-2000/II Tait T-3000/II handheld radio with optional voice scrambler T-3000/II
Secure Telephones (Crypto Phones) Phones

 More crypto phones
Digital Encryption
Most - if not all - modern secure voice terminals use digital encryption. Speech is digitized by means of an Analog-to-Digital Convertor (ADC) or a Vocoder. The resulting digital data stream is then 'mixed' by means of an XOR-operation with a data stream from a pseudo-random number generator, that in turn is seeded by a KEY. This results in an encrypted data stream that is then converted back to the analog domain (modem), so that it can be transmitted. This process is shown in the simplified diagram below:

During the 1970s many systems, such as the KY-57 used Continuous Variable Slope Delta modulation (CVSD) to convert speech into digital data. This wide-band solution was only suitable for VHF and UHF radios. In the 1980s narrow-band systems were introduced, such as the KY-99 that used (enhanced) Linear Predictive Coding (LPC), limiting the data-rate to 2400 baud or even 800 baud.

The Pseudo Random Number Generator (PRNG) is seeded by a KEY that is either entered manuall or by means of a key fill device. Modern systems sometimes use asymmetric encryption methods (e.g. AES) to exchange the keys over an insecure channel (public key encryption).
Below are some sound samples of digital voice encryption. They were recorded by Barry Wels [1] on an Icom IC-H10SR. The first file contains the original audio file. The seconds file plays the encryption audio. Finally, the last file produces the audio once it has been decrypted.
Voice scramblers
Before digital speech encryption became widely available, another technique was used to secure voice transmission. This technique was based on frequency inversion and is commonly called voice scrambling. It evolves around mirroring of the audio frequency spectrum around a given center frequency, sometimes divided over multiple frequency bands. This principle is best explained using a simplified model:

The audio spectrum of the voice data is mixed with a carrier frquency (fc). This results in two spectra: one that is the sum of the original sectrum and the carrier, and one that is the difference of the two signals. 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. At the receiving end, the audio spectrum is mirrored once more to make it 'legible' again.

To make things more complex, one could vary the carrier frequency and also split-up the audio band in several (e.g. five) smaller bands that are then mirrored individually. Continuously varying these parameters by putting them under digital control, can make it harder to decode the signal.

The advantage of this technique is that it completely takes place within the audio bandwidth of a channel, whereas digital encryption generally requires a (much) larger bandwidth. This allows voice scrambling to be added to an existing analog radio system. For this reason, the police in many countries used scramblers from the 1970's well into the 1990's.

The disadvantage however is that an evesdropper can easily reverse the process of frequency mirroring with a simple piece of electronics. Furthermore it is sometimes even possible to extract usefull information from the seemingly garbled speech by listening carefully. Even complex digitally-controlled voice scramblers are easily defeated by today's software defined radio solutions, that have become widely available. Voice scrambling is therefore considered inherently insecure.
Time-division speech scrambler
A third method for secure speech is the so-called time-division speech scrambling. This method is more secure than the simpler frequency-inversion system, but far less secure than modern digital speech encryptors. It was introduced in the mid-1970s and served well into the 1990s.

Many police and other law enforcement agencies world-wide, used this system for securing their conversations. The advantage of this system is that it is suitable for narrow-band FM channels, as the output signal consists purely of voice information. The system is prone to cryptographic attacks however, as it is possible to reconstruct the original signal (and hence the cryptographic key) by examining the output signal on an oscilloscope.

The simplified diagram below, shows how it works. Speech is cut into small time segments and is scrambled with other time segments in an ever changing order. The order in which the packets are scrambled is determined by a pseudo random number generated that is seeded by the user.

In this diagram, the top row shows the clear speech (input) in time. The second row shows the speech after it is scrambled. Finally, the bottom row shows the speech once it is descrambled again (output). The whole process of scrambling and descrambling, causes a typical delay of approx. 0.5 seconds.

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 (FSK) is transmitted with the scrambled speech. An example of a time-division speech scrambler is the BBC Cryptophon 1100.
The examples below 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 actually be able to descramble it yourself.
  1. Barry Wels
    Audio samples of ICOM equipment.

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