SIGSALY
SIGSALY (not an acronym; also known as X System and Green Hornet; the latter from the noise it made, in operation) was a technologically very advanced (for its day) digital encrypted radio speech system, created by Bell Laboratories during World War II. It first went into service in 1943, and was used extensively during the war for direct, live communication among top Allied leaders (e.g. Churchill and Roosevelt).
It first converted speech into a bit stream with a vocoder; that bit stream was then encrypted with the equivalent of a 'one-time pad'; the resulting encrypted bit steam was then transmitted to the other end of the link over an FM radio link.
The first step was done by dividing the incoming speech signal (with frequencies in the 250-3000 Hz range) into 10 frequency bands, and measuring and digitizing the amplitude of the signal in each band, with six amplitude levels (i.e. values of 0-5) for each. In addition to those 10 channels, two additional channels were used; one for pitch information, and one for whether the speech was voiced or un-voiced (i.e. a sibilant, or 'hiss'). All channels were sampled 50 times per second.
The bit steam which was the key, used in the encryption/decryption stage, was quantum thermal noise recorded onto a phonograph record; only two copies of each one were produced, one for each end. Complex electro-mechanical mechanisms were required to start the playback at the exact same time, and keep them synchronized during use.
A galaxy of talent was brought in to help create it; Claude Shannon and Harry Nyquist made important contributions, and Alan Turing advised on the cryptography stage. A SIGSALY terminal was constructed of vacuum tube technology (the only thing available at the time), and as a result was physically massive: 40 racks of equipment weighing over 50 tons. In operation, it used about 30 kW of electricity.
The Germans had easily broken the previous 'scrambler' technique used in an attempt to secure voice communications. Although they recorded a lot of SIGSALY traffic, they never made any inroads into it (in fact, they had wrongly decided that it encoded a telegraphic system of some sort). Indeed, using as it does a completely random one-time key, it would probably be unbreakable even today.
Further reading
- M. D. Fagen (editor), National Service in War and Peace (1925–1975), Volume II of A History of Engineering and Science in the Bell System, Bell Telephone Laboratories, Murray Hill, 1978 - pp. 296–317
- Donald E. Mehl, The Green Hornet: America's Unbreakable Code for Secret Telephony
- Donald E. Mehl, Top Secret Communications of WWII: Unbreakable Encryption for Secret High-level Conferences, U.S. Army Signal Corps, Kansas City, 2002
External links
- J. V. Boone and R. R. Peterson, The Start of the Digital Revolution: SIGSALY - Secure Digital Voice Communications in World War II, Center for Cryptologic History, National Security Agency, Fort George G. Meade, July 2000
