SIGSALY

Green Horneta special scramblercomputers of the era
In cryptography, 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.wikipedia
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Secure voice

voice encryptionCiphonysecure telephone
In cryptography, 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.
To implement the system, the army contracted Bell Laboratories and they developed a system called SIGSALY.

Bell Labs

Bell LaboratoriesBell Telephone LaboratoriesAT&T Bell Laboratories
A prototype was developed at Bell Telephone Laboratories, under the direction of A. B. Clark, assisted by British mathematician Alan Turing, and demonstrated to the US Army.
In 1943, Bell developed SIGSALY, the first digital scrambled speech transmission system, used by the Allies in World War II.

Alan Turing

TuringAlan M. TuringAlan Mathison Turing
A prototype was developed at Bell Telephone Laboratories, under the direction of A. B. Clark, assisted by British mathematician Alan Turing, and demonstrated to the US Army.
Turing also consulted with Bell Labs on the development of SIGSALY, a secure voice system that was used in the later years of the war.

Vocoder

vocodedvocodersvocoding
SIGSALY used a random noise mask to encrypt voice conversations which had been encoded by a vocoder.
Dudley's vocoder was used in the SIGSALY system, which was built by Bell Labs engineers in 1943.

Scrambler

scramblingDescramblerscrambled
At the time of its inception, long distance telephone communications were broadcast using the "A-3" voice scrambler developed by Western Electric.
An unrelated device called SIGSALY was used for higher-level voice communications.

One-time pad

one-time tapeone time padVernam cipher
The insecurity of most telephone scrambler schemes led to the development of a more secure scrambler, based on the one-time pad principle.
The World War II voice scrambler SIGSALY was also a form of one-time system.

Companding

compandercompandedcompandor
This scheme, known as "companding" or "compressing-expanding", exploits the fact that the fidelity of voice signals is more sensitive to low amplitudes than to high amplitudes.
In 1942, Clark and his team completed the SIGSALY secure voice transmission system that included the first use of companding in a PCM (digital) system.

Selfridges, Oxford Street

SelfridgesSelfridges Store basement area of Selfridges, Oxford Street
The second was installed 60 m below street level in the basement of Selfridges department store on Oxford Street, London, close to the US Embassy on Grosvenor Square.
Initially used by U.S. General Dwight D. Eisenhower, the commander of SHAEF, it later housed 50 soldiers from the 805th Signal Service Company of the US Army Signal Corps, who installed a SIGSALY code-scrambling device connected to a similar terminal in the Pentagon building.

Pulse-code modulation

PCMLPCMLinear PCM
It pioneered a number of digital communications concepts, including the first transmission of speech using pulse-code modulation.
The first transmission of speech by digital techniques, the SIGSALY encryption equipment, conveyed high-level Allied communications during World War II.

Churchill War Rooms

Cabinet War RoomsCabinet War RoomChurchill Museum and Cabinet War Rooms
The first conference took place on 15 July 1943, and it was used by both General Dwight D. Eisenhower as the commander of SHAEF, and Churchill, before extensions were installed to the Embassy, 10 Downing Street and the Cabinet War Rooms.
From 1943, a SIGSALY code-scrambling encrypted telephone was installed in the basement of Selfridges, Oxford Street connected to a similar terminal in the Pentagon building.

Spread spectrum

spread-spectrumDigital Spread SpectrumSpread-spectrum clocking
It saw limited use by the German military in World War I, was put forward by Polish engineer Leonard Danilewicz in 1929, showed up in a patent in the 1930s by Willem Broertjes (, issued Aug. 2, 1932), and in the top-secret US Army Signal Corps World War II communications system named SIGSALY.

Cryptography

cryptographiccryptographercryptology
In cryptography, 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.

World War II

Second World WarwarWWII
In cryptography, 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.

Allies of World War II

AlliedAlliesAllied forces
In cryptography, 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.

Data transmission

data transferdigital communicationsdigital communication
It pioneered a number of digital communications concepts, including the first transmission of speech using pulse-code modulation.

SIGABA

ECM Mark II
The name SIGSALY was not an acronym, but a cover name that resembled an acronym—the SIG part was common in Army Signal Corps names (e.g., SIGABA).

The Green Hornet (radio series)

The Green HornetGreen Hornetradio program
The prototype was called the "Green Hornet" after the popular radio show The Green Hornet, because it sounded like a buzzing hornet — resembling the show's theme tune — to anyone trying to eavesdrop on the conversation.

Hornet

HornetsVespainsect
The prototype was called the "Green Hornet" after the popular radio show The Green Hornet, because it sounded like a buzzing hornet — resembling the show's theme tune — to anyone trying to eavesdrop on the conversation.

Western Electric

Western Electric CompanyWestrexWestern Electric Manufacturing Company
At the time of its inception, long distance telephone communications were broadcast using the "A-3" voice scrambler developed by Western Electric.

Spectrum analyzer

spectrum analyserspectral analysisspectrum analysis
Inspection of the audio spectrum using a spectrum analyzer often provided significant clues to the scrambling technique.

Randomness

randomchancerandomly
SIGSALY used a random noise mask to encrypt voice conversations which had been encoded by a vocoder.

Noise

acoustic noisenoisyNoise (audio)
SIGSALY used a random noise mask to encrypt voice conversations which had been encoded by a vocoder.

Encryption

encryptedencryptencrypting
SIGSALY used a random noise mask to encrypt voice conversations which had been encoded by a vocoder.

Sampling (signal processing)

sampling ratesamplingsample rate
Next, each signal was sampled for its amplitude once every 20 milliseconds.