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Secure voice (alternatively secure speech or ciphony) is a term in
cryptography
Cryptography, or cryptology (from grc, , translit=kryptós "hidden, secret"; and ''graphein'', "to write", or ''-logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adver ...
for the encryption of
voice communication
Speech is a human vocal communication using language. Each language uses phonetic combinations of vowel and consonant sounds that form the sound of its words (that is, all English words sound different from all French words, even if they are th ...
over a range of communication types such as radio,
telephone
A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
or
IP.
History
The implementation of voice encryption dates back to
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
when secure communication was paramount to the US armed forces. During that time, noise was simply added to a voice signal to prevent enemies from listening to the conversations. Noise was added by playing a record of noise in sync with the voice signal and when the voice signal reached the receiver, the noise signal was subtracted out, leaving the original voice signal. In order to subtract out the noise, the receiver need to have exactly the same noise signal and the noise records were only made in pairs; one for the transmitter and one for the receiver. Having only two copies of records made it impossible for the wrong receiver to decrypt the signal. To implement the system, the army contracted
Bell Laboratories
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mult ...
and they developed a system called
SIGSALY
SIGSALY (also known as the X System, Project X, Ciphony I, and the Green Hornet) was a secure speech system used in World War II for the highest-level Allied communications. It pioneered a number of digital communications concepts, including the ...
. With SIGSALY, ten channels were used to sample the
voice frequency
A voice frequency (VF) or voice band is the range of audio frequencies used for the transmission of speech.
Frequency band
In telephony, the usable voice frequency band ranges from approximately 300 to 3400 Hz. It is for this reason tha ...
spectrum from 250 Hz to 3 kHz and two channels were allocated to sample voice pitch and background hiss. In the time of SIGSALY, the transistor had not been developed and the digital sampling was done by circuits using the model 2051
Thyratron
A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, p ...
vacuum tube. Each SIGSALY terminal used 40 racks of equipment weighing 55 tons and filled a large room. This equipment included radio transmitters and receivers and large phonograph turntables. The voice was keyed to two vinyl phonograph records that contained a
Frequency Shift Keying
Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for communication systems such as telemetry, weather ball ...
(FSK) audio tone. The records were played on large precise turntables in sync with the voice transmission.
From the introduction of voice encryption to today, encryption techniques have evolved drastically. Digital technology has effectively replaced old analog methods of voice encryption and by using complex algorithms, voice encryption has become much more secure and efficient. One relatively modern voice encryption method is
Sub-band coding. With Sub-band Coding, the voice signal is split into multiple frequency bands, using multiple bandpass filters that cover specific frequency ranges of interest. The output signals from the bandpass filters are then lowpass translated to reduce the bandwidth, which reduces the sampling rate. The lowpass signals are then quantized and encoded using special techniques like,
pulse-code modulation
Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the ...
(PCM). After the encoding stage, the signals are multiplexed and sent out along the communication network. When the signal reaches the receiver, the inverse operations are applied to the signal to get it back to its original state. A speech scrambling system was developed at
Bell Laboratories
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mult ...
in the 1970s by
Subhash Kak
Subhash Kak is an Indian-American computer scientist and historical revisionist. He is the Regents Professor of Computer Science Department at Oklahoma State University–Stillwater, an honorary visiting professor of engineering at Jawaharlal N ...
and
Nikil Jayant
Nikil S. Jayant (1945 -- ) is an Indian-American communications engineer. He was a prominent long-term researcher at Bell Laboratories and subsequently a professor at Georgia Institute of Technology. He received his Ph.D. in Electrical Communicatio ...
. In this system permutation matrices were used to scramble coded representations (such as
pulse-code modulation
Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the ...
and variants) of the speech data.
Motorola
Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent public companies, Motorol ...
developed a voice encryption system called
Digital Voice Protection
Digital usually refers to something using discrete digits, often binary digits.
Technology and computing Hardware
*Digital electronics, electronic circuits which operate using digital signals
**Digital camera, which captures and stores digital i ...
(DVP) as part of their first generation of voice encryption techniques. DVP uses a
self-synchronizing encryption technique known as
cipher feedback
In cryptography, a block cipher mode of operation is an algorithm that uses a block cipher to provide information security such as confidentiality or authenticity.
A block cipher by itself is only suitable for the secure cryptographic transform ...
(CFB). The extremely high number of possible keys associated with the early DVP algorithm, makes the algorithm very robust and gives a high level of security. As with other symmetric keyed encryption systems, the encryption key is required to decrypt the signal with a special decryption algorithm.
Digital
A digital secure voice usually includes two components, a
digitizer
DigitizationTech Target. (2011, April). Definition: digitization. ''WhatIs.com''. Retrieved December 15, 2021, from https://whatis.techtarget.com/definition/digitization is the process of converting information into a digital (i.e. computer-r ...
to convert between speech and digital signals and an
encryption
In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can decip ...
system to provide confidentiality. It is difficult in practice to send the encrypted signal over the same
voiceband
A voice frequency (VF) or voice band is the range of audio frequencies used for the transmission of speech.
Frequency band
In telephony, the usable voice frequency band ranges from approximately 300 to 3400 Hz. It is for this reason th ...
communication circuit
A telecommunication circuit is a path in a telecommunications network used to transmit information. Circuits have evolved over time from generally being built on physical connections between individual hardware cables, as in an analog phone swit ...
s used to transmit unencrypted voice, e.g. analog
telephone line
A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or ot ...
s or
mobile radio
Mobile radio or mobiles refer to wireless communications systems and devices which are based on radio frequencies(using commonly UHF or VHF frequencies), and where the path of communications is movable on either end. There are a variety of view ...
s, due to bandwidth expansion.
This has led to the use of Voice Coders (
vocoder
A vocoder (, a portmanteau of ''voice'' and ''encoder'') is a category of speech coding that analyzes and synthesizes the human voice signal for audio data compression, multiplexing, voice encryption or voice transformation.
The vocoder was ...
s) to achieve tight bandwidth compression of the speech signals.
NSA
The National Security Agency (NSA) is a national-level intelligence agency of the United States Department of Defense, under the authority of the Director of National Intelligence (DNI). The NSA is responsible for global monitoring, collecti ...
's
STU-III
STU-III (Secure Telephone Unit - third generation) is a family of secure telephones introduced in 1987 by the NSA for use by the United States government, its contractors, and its allies. STU-III desk units look much like typical office telephone ...
, KY-57 and
SCIP are examples of systems that operate over
existing voice circuits. The
STE system, by contrast, requires wide bandwidth
ISDN
Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Wo ...
lines for its normal mode of operation. For encrypting
GSM
The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation ( 2G) digital cellular networks used by mobile devices such ...
and
VoIP
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...
, which are natively digital, the standard protocol
ZRTP
ZRTP (composed of Z and Real-time Transport Protocol) is a cryptographic key-agreement protocol to negotiate the keys for encryption between two end points in a Voice over IP (VoIP) phone telephony call based on the Real-time Transport Protocol. ...
could be used as an
end-to-end encryption
End-to-end encryption (E2EE) is a system of communication where only the communicating users can read the messages. In principle, it prevents potential eavesdroppers – including telecommunications service providers, telecom providers, Internet ...
technology.
Secure voice's robustness greatly benefits from having the voice data compressed into very low bit-rates by special component called
speech coding
Speech coding is an application of data compression of digital audio signals containing speech. Speech coding uses speech-specific parameter estimation using audio signal processing techniques to model the speech signal, combined with generic d ...
, voice compression or voice coder (also known as
vocoder
A vocoder (, a portmanteau of ''voice'' and ''encoder'') is a category of speech coding that analyzes and synthesizes the human voice signal for audio data compression, multiplexing, voice encryption or voice transformation.
The vocoder was ...
). The old secure voice compression standards include (
CVSD Continuously variable slope delta modulation (CVSD or CVSDM) is a voice coding method. It is a delta modulation with variable step size (i.e., special case of adaptive delta modulation), first proposed by Greefkes and Riemens in 1970.
CVSD encode ...
,
CELP
Code-excited linear prediction (CELP) is a linear predictive speech coding algorithm originally proposed by Manfred R. Schroeder and Bishnu S. Atal in 1985. At the time, it provided significantly better quality than existing low bit-rate algori ...
,
LPC-10e FIPS 137, originally issued as FED-STD-1015, is a secure telephony speech encoding standard for Linear Predictive Coding vocoder developed by the United States Department of Defense and finished on November 28, 1984.
It was based on the earlier STA ...
and
MELP
Mixed-excitation linear prediction (MELP) is a United States Department of Defense speech coding standard used mainly in military applications and satellite communications, secure voice, and secure radio devices. Its standardization and later devel ...
, where the latest standard is the state of the art MELPe algorithm.
Digital methods using voice compression: MELP or MELPe
The
MELPe
Mixed-excitation linear prediction (MELP) is a United States Department of Defense speech coding standard used mainly in military applications and satellite communications, secure voice, and secure radio devices. Its standardization and later deve ...
or enhanced-
MELP
Mixed-excitation linear prediction (MELP) is a United States Department of Defense speech coding standard used mainly in military applications and satellite communications, secure voice, and secure radio devices. Its standardization and later devel ...
(Mixed Excitation Linear Prediction) is a
United States Department of Defense
The United States Department of Defense (DoD, USDOD or DOD) is an executive branch department of the federal government charged with coordinating and supervising all agencies and functions of the government directly related to national secu ...
speech coding standard used mainly in military applications and satellite communications, secure voice, and secure radio devices. Its development was led and supported by
NSA
The National Security Agency (NSA) is a national-level intelligence agency of the United States Department of Defense, under the authority of the Director of National Intelligence (DNI). The NSA is responsible for global monitoring, collecti ...
, and NATO. The US government's MELPe secure voice standard is also known as MIL-STD-3005, and the NATO's
MELPe
Mixed-excitation linear prediction (MELP) is a United States Department of Defense speech coding standard used mainly in military applications and satellite communications, secure voice, and secure radio devices. Its standardization and later deve ...
secure voice standard is also known as
STANAG
In NATO, a standardization agreement (STANAG, redundantly: STANAG agreement) defines processes, procedures, terms, and conditions for common military or technical procedures or equipment between the member countries of the alliance. Each NATO st ...
-4591.
The initial MELP was invented by Alan McCree around 1995. That initial speech coder was standardized in 1997 and was known as MIL-STD-3005. It surpassed other candidate vocoders in the US DoD competition, including: (a)
Frequency Selective Harmonic Coder (FSHC), (b)
Advanced Multi-Band Excitation (AMBE), (c)
Enhanced Multiband Excitation (EMBE), (d)
Sinusoid Transform Coder (STC), and (e)
Subband LPC Coder
In signal processing, sub-band coding (SBC) is any form of transform coding that breaks a signal into a number of different frequency bands, typically by using a fast Fourier transform, and encodes each one independently. This decomposition i ...
(SBC). Due to its lower complexity than Waveform Interpolative (WI) coder, the MELP vocoder won the DoD competition and was selected for
MIL-STD
A United States defense standard, often called a military standard, "MIL-STD", "MIL-SPEC", or (informally) "MilSpecs", is used to help achieve standardization objectives by the U.S. Department of Defense.
Standardization is beneficial in achievin ...
-3005.
Between 1998 and 2001, a new MELP-based vocoder was created at half the rate (i.e. 1200 bit/s) and substantial enhancements were added to the MIL-STD-3005 by
SignalCom (later acquired by
Microsoft
Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washing ...
),
AT&T Corporation
AT&T Corporation, originally the American Telephone and Telegraph Company, is the subsidiary of AT&T Inc. that provides voice, video, data, and Internet telecommunications and professional services to businesses, consumers, and government agen ...
, and
Compandent which included (a) additional new vocoder at half the rate (i.e. 1200 bit/s), (b) substantially improved encoding (analysis), (c) substantially improved decoding (synthesis), (d) Noise-Preprocessing for removing background noise, (e) transcoding between the 2400 bit/s and 1200 bit/s bitstreams, and (f) new postfilter. This fairly significant development was aimed to create a new coder at half the rate and have it interoperable with the old MELP standard. This enhanced-MELP (also known as MELPe) was adopted as the new MIL-STD-3005 in 2001 in form of annexes and supplements made to the original MIL-STD-3005, enabling the same quality as the old 2400 bit/s MELP's at half the rate. One of the greatest advantages of the new 2400 bit/s MELPe is that it shares the same bit format as MELP, and hence can interoperate with legacy MELP systems, but would deliver better quality at both ends. MELPe provides much better quality than all older military standards, especially in noisy environments such as battlefield and vehicles and aircraft.
In 2002, following extensive competition and testing, the 2400 and 1200 bit/s US DoD MELPe was adopted also as
NATO
The North Atlantic Treaty Organization (NATO, ; french: Organisation du traité de l'Atlantique nord, ), also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 member states – 28 European and two No ...
standard, known as
STANAG
In NATO, a standardization agreement (STANAG, redundantly: STANAG agreement) defines processes, procedures, terms, and conditions for common military or technical procedures or equipment between the member countries of the alliance. Each NATO st ...
-4591. As part of NATO testing for new NATO standard, MELPe was tested against other candidates such as
France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
's HSX (Harmonic Stochastic eXcitation) and
Turkey
Turkey ( tr, Türkiye ), officially the Republic of Türkiye ( tr, Türkiye Cumhuriyeti, links=no ), is a list of transcontinental countries, transcontinental country located mainly on the Anatolia, Anatolian Peninsula in Western Asia, with ...
's SB-LPC (Split-Band Linear Predictive Coding), as well as the old secure voice standards such as
FS1015 LPC-10e FIPS 137, originally issued as FED-STD-1015, is a secure telephony speech encoding standard for Linear Predictive Coding vocoder developed by the United States Department of Defense and finished on November 28, 1984.
It was based on the earlier STA ...
(2.4 kbit/s),
FS1016
FS-1016 (also called FED-STD-1016) is a deprecated secure telephony speech encoding standard for Code-excited linear prediction (CELP) developed by the United States Department of Defense and finalized February 14, 1991.
Unlike the vocoder used ...
CELP
Code-excited linear prediction (CELP) is a linear predictive speech coding algorithm originally proposed by Manfred R. Schroeder and Bishnu S. Atal in 1985. At the time, it provided significantly better quality than existing low bit-rate algori ...
(4.8 kbit/s) and
CVSD Continuously variable slope delta modulation (CVSD or CVSDM) is a voice coding method. It is a delta modulation with variable step size (i.e., special case of adaptive delta modulation), first proposed by Greefkes and Riemens in 1970.
CVSD encode ...
(16 kbit/s). Subsequently, the MELPe won also the NATO competition, surpassing the quality of all other candidates as well as the quality of all old secure voice standards (CVSD,
CELP
Code-excited linear prediction (CELP) is a linear predictive speech coding algorithm originally proposed by Manfred R. Schroeder and Bishnu S. Atal in 1985. At the time, it provided significantly better quality than existing low bit-rate algori ...
and
LPC-10e FIPS 137, originally issued as FED-STD-1015, is a secure telephony speech encoding standard for Linear Predictive Coding vocoder developed by the United States Department of Defense and finished on November 28, 1984.
It was based on the earlier STA ...
). The
NATO
The North Atlantic Treaty Organization (NATO, ; french: Organisation du traité de l'Atlantique nord, ), also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 member states – 28 European and two No ...
competition concluded that MELPe substantially improved performance (in terms of speech quality, intelligibility, and noise immunity), while reducing throughput requirements. The NATO testing also included interoperability tests, used over 200 hours of speech data, and was conducted by 3 test laboratories worldwide.
Compandent Inc, as a part of MELPe-based projects performed for
NSA
The National Security Agency (NSA) is a national-level intelligence agency of the United States Department of Defense, under the authority of the Director of National Intelligence (DNI). The NSA is responsible for global monitoring, collecti ...
and
NATO
The North Atlantic Treaty Organization (NATO, ; french: Organisation du traité de l'Atlantique nord, ), also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 member states – 28 European and two No ...
, provided NSA and NATO with special test-bed platform known as
MELCODER device that provided the golden reference for real-time implementation of MELPe. The low-cost FLEXI-232 Data Terminal Equipment (DTE) made by
Compandent, which are based on the
MELCODER golden reference, are very popular and widely used for evaluating and testing MELPe in real-time, various channels & networks, and field conditions.
The
NATO
The North Atlantic Treaty Organization (NATO, ; french: Organisation du traité de l'Atlantique nord, ), also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 member states – 28 European and two No ...
competition concluded that
MELPe
Mixed-excitation linear prediction (MELP) is a United States Department of Defense speech coding standard used mainly in military applications and satellite communications, secure voice, and secure radio devices. Its standardization and later deve ...
substantially improved performance (in terms of speech quality, intelligibility, and noise immunity), while reducing throughput requirements. The NATO testing also included interoperability tests, used over 200 hours of speech data, and was conducted by 3 test laboratories worldwide.
In 2005, a new 600 bit/s rate MELPe variation by
Thales Group
Thales Group () is a French multinational company that designs, develops and manufactures electrical systems as well as devices and equipment for the aerospace, defence, transportation and security sectors. The company is headquartered in Paris' ...
(
France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
) was added (without extensive competition and testing as performed for the 2400/1200 bit/s MELPe) to the NATO standard STANAG-4591, and there are more advanced efforts to lower the bitrates to 300 bit/s and even 150 bit/s.
In 2010 Lincoln Labs.,
Compandent, BBN, and General Dynamics also developed for DARPA a 300 bit/s MELP device .
[Alan McCree, “A scalable phonetic vocoder framework using joint predictive vector quantization of MELP parameters,” in Proc. IEEE Int. Conf. Acoust., Speech, Signal Processing, 2006, pp. I 705–708, Toulouse, France] Its quality was better than the 600 bit/s MELPe, but its delay was longer.
Other
In
Aleksandr Solzhenitsyn
Aleksandr Isayevich Solzhenitsyn. (11 December 1918 – 3 August 2008) was a Russian novelist. One of the most famous Soviet dissidents, Solzhenitsyn was an outspoken critic of communism and helped to raise global awareness of political repres ...
's novel
First Circle the character Volodin's recorded phone call is traced to him since it is not properly encrypted. Its decipherment makes use of spectral analysis.
See also
*
Scrambler
In telecommunications, a scrambler is a device that transposes or inverts signals or otherwise encodes a message at the sender's side to make the message unintelligible at a receiver not equipped with an appropriately set descrambling device. Wher ...
*
MELPe
Mixed-excitation linear prediction (MELP) is a United States Department of Defense speech coding standard used mainly in military applications and satellite communications, secure voice, and secure radio devices. Its standardization and later deve ...
*
MELP
Mixed-excitation linear prediction (MELP) is a United States Department of Defense speech coding standard used mainly in military applications and satellite communications, secure voice, and secure radio devices. Its standardization and later devel ...
*
Cryptography
Cryptography, or cryptology (from grc, , translit=kryptós "hidden, secret"; and ''graphein'', "to write", or ''-logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adver ...
*
Pseudorandom noise
In cryptography, pseudorandom noise (PRN) is a signal similar to noise which satisfies one or more of the standard tests for statistical randomness. Although it seems to lack any definite pattern, pseudorandom noise consists of a deterministic s ...
*
SIGSALY
SIGSALY (also known as the X System, Project X, Ciphony I, and the Green Hornet) was a secure speech system used in World War II for the highest-level Allied communications. It pioneered a number of digital communications concepts, including the ...
*
SCIP
*
Secure telephone
*
Secure Terminal Equipment
Secure Terminal Equipment (STE) is the U.S. government's current (), encrypted telephone communications system for wired or "landline" communications. STE is designed to use ISDN telephone lines which offer higher speeds of up to 128 kbit/s ...
*
VINSON
VINSON is a family of voice encryption devices used by U.S. and allied military and law enforcement, based on the NSA's classified Suite A SAVILLE encryption algorithm and 16 kbit/s CVSD audio compression. It replaces the Vietnam War-era N ...
*
VoIP VPN
A VoIP VPN combines voice over IP and virtual private network technologies to offer a method for delivering secure voice. Because VoIP transmits digitized voice as a stream of data, the VoIP VPN solution accomplishes voice encryption quite simply, ...
*
NSA encryption systems
The National Security Agency took over responsibility for all U.S. Government encryption systems when it was formed in 1952. The technical details of most NSA-approved systems are still Classified information in the United States, classified, but ...
*
ZRTP
ZRTP (composed of Z and Real-time Transport Protocol) is a cryptographic key-agreement protocol to negotiate the keys for encryption between two end points in a Voice over IP (VoIP) phone telephony call based on the Real-time Transport Protocol. ...
*
Fishbowl (secure phone)
References
{{Cryptography navbox , machines
Cryptography
Secure communication
Speech codecs