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Alan Turing and Voice Encryption

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Alan Turing’s Work on Voice Encryption Craig Bauer Prologue: Inverters 1920s analog system put into use by AT&T. Prevented casual eavesdropping, but easily inverted back by determined amateurs. The frequency p of each component is replaced with s − p, where s is the frequency of a carrier wave. This makes the low tones sound high and vice versa. Kahn, p.554 Prologue II: The A-3 Scrambler AT&T and RCA, 1937 Bandsplitting (5 subbands) with inversion. Only 6 “keys” ever used! “Privacy, not Security” Kahn, p.554 Prologue II: The A-3 Scrambler Used by President Roosevelt and Winston Churchill!! Solved by Germans by September 1941. (after only a few months work) Kahn, p. 555-556. June 1942 “The security device has not yet been invented which is of any protection whatever against the skilled engineers who are employed by the enemy to record every word of every conversation made.” – British foreign Office Memorandum FO/371/32346 Hodges, p. 236 October 1943 “In addition, this equipment furnishes a very low degree of security, and we know definitely that the enemy can break the system with almost no effort.” - Colonel Frank McCarthy, Secretary to the Army General Staff to Harry Hopkins, Assistant to President Roosevelt Mehl, p. 5 Too Redundant • Redundancy in speech allows us to comprehend it through music, background noise, bad connections, mumbling, other people speaking, etc. • Some can even understand much inverted speech! • Speech is hard to disguise because of this. • Sound spectrograph attacks can be carried out like jigsaw puzzles. Part I: SIGSALY The Following are Equivalent 1) SIGSALY 2) RC-220-T-1 3) The Green Hornet 4) Project X-61753 5) Project X (Atomic Bomb was Project Y) 6) X-Ray 7) Special Customer Proof – see the literature How We Should Market Texts? Part I: SIGSALY Image from http://www.cryptologicfoundation.org/content/A-Museum-Like-No-Other/COMSEC.shtml Part I: SIGSALY http://homepage.mac.com/oldtownman/WW2Timeline/espionage.html Getting Started Based on earlier Vocoder (voice coder) technology for digitizing speech. 1936 Bell Labs 1977 You tell me! Getting Started “The vocoder was originally invented for speech compression [down to 10%], to reduce bandwidth costs on undersea phone cables.” Tompkins, p. 23 Diverse Applications The Cost of Compression “It made a curious kind of robot voice.” – Henry Stimson, Secretary of War Tompkins p. 63 SIGSALY Nov. ‘42 - Experimental station installed in New York. July ‘43 - Final Version Activated linking Washington D.C. and London. First transmission of digital speech. First practical “Pulse Code Modulation” (refers to digitization process - details to follow) Transmit Terminal SIGGRUV SIGGRUV Add key to encipher Subtract key to decipher Communication ceased if key stopped Speech sampled 50 times per second (about the size of a phoneme) Don’t Play it Again Uncle Sam! Played thermal noise backward 12 minutes per single sided 16” record Over 1,500 key sets made – Tompkins p. 68 Destroyed after use. SIGBUSE Need Air-conditioning Too! Mehl on right → Channels • 10 channel vocoder for speech (from 150 Hz to 2950 Hz) • 11 th and 12 th channel for pitch • Each channel was 25 Hz • Total bandwidth = 12x25 = 300 Hz • Spread spectrum makes jamming harder Sent later at VHF. Discretizing and Enciphering . Logarithmic Companding Log of the signal allows finer discernement at lower amplitudes Good choice, as ear distinguishes lower amplitudes more finely Decibel scale is logarithmic Why not just add without mod? 1) Mod 6 solution was Harry Nyquist’s idea. Mehl, p. 38 2) Must also mod or cipher level 0 ⇒ message level of 0 and cipher level 10 ⇒ message level of 5 3) It would be like hearing the message over the background noise of the key. SIGSALY Originally tried a binary system, but it could not be made understandable on the receiving end. The senary system (6 levels for each channel) worked, but 36 levels were used for pitch. (could count pitch as two channels, as each was enciphered, 6*6=36, but not companded) Hodges, p. 246 SIGSALY: Your Digital Pal Who’s Fun to be With! The Connections National Cryptologic Museum Never Broken! • Germans thought it was just noise or a teletype signal. • Sounded like music played at start of Green Hornet TV show of that era - Tompkins p. 72 • Declassified in 1976 Turing’s Arrival • Americans were ahead of Brits (at Dollis Hill) and Turing was to learn from them. • Entered Bell Labs Jan. 19, 1943 (after much debate probably reaching President Roosevelt – Tompkins, p. 59, Hodges p. 245, Mehl p. 69 – Secretary of War Stimson resolved it.) (in US Nov. 13 1942- Mar. 16, 1943 and made visits elsewhere after starting at Bell Labs) Turing’s Contributions • Most of Turing’s time was spent on ciphony cryptanalysis (beta tester) • Suggested improvements to SIGBUSE key • SIGBUSE key was never used British Concerns • SIGSALY in London would be under U.S. control. • “If the equipment is to be operated solely by U.S. personnel it will be impossible to prevent them listening in if they so desire.” - Turing • Americans were often so focused on their jobs they didn’t know what was actually said. Never Broken! (General Douglas MacArthur – didn’t trust it!) Never Broken! (General Douglas MacArthur – didn’t trust it!) So why replace it? Never Broken! (General Douglas MacArthur – didn’t trust it!) So why replace it? – 55 tons (70 ton shipping weight) Never Broken! (General Douglas MacArthur – didn’t trust it!) So why replace it? – 55 tons (70 ton shipping weight) – Took up 2,500 square ft. Never Broken! (General Douglas MacArthur – didn’t trust it!) So why replace it? – 55 tons (70 ton shipping weight) – Took up 2,500 square ft. – Cost $250,000 – $1,000,000+ (Tompkins p. 61) Never Broken! (General Douglas MacArthur – didn’t trust it!) So why replace it? – 55 tons (70 ton shipping weight) – Took up 2,500 square ft. – Cost $250,000 – $1,000,000+ (Tompkins p. 61) – Converted 30 kilowatts of power into 1 milliwatt of low quality speech. Hodges, p. 247 Never Broken! (General Douglas MacArthur – didn’t trust it!) So why replace it? – 55 tons (70 ton shipping weight) – Took up 2,500 square ft. – Cost $250,000 – $1,000,000+ (Tompkins p. 61) – Converted 30 kilowatts of power into 1 milliwatt of low quality speech. Hodges, p. 247 – Sounded like Donald Duck. Part II - Delilah Inspiration: • Idea hit Turing on trip back to England – March 1943 (Hodges p. 273) • Work not done at BP, but rather Hanslope Park (10 miles to the north) for his own amusement! (Hodges p. 269-270) Delilah Deliliah – the biblical “deceiver of men” The name was suggested by another, in response to a prize offer from Turing. -Hodges p.273 A Response from GCHQ! Protective Marking: UNCLASSIFIED Dear Craig I'm sorry for the delay in replying. I will be able to help (I think) on Feuerstein and on DELILAH - we have already released material on DELILAH to enable a rebuild of the system at Bletchley Park by the team which rebuilt the Bombe. I don't know what - if anything - we have in our Archives about RCS or SIGSALY but I will follow this up in the New Year. (If Ralph can't point you at anything in The National Archives, then nobody can!) Best wishes for Christmas and the New Year Tony XXXXX Departmental Historian Speech System 'Delilah' – Report on Progress Found by Ralph Erskine, Dated 6 June 1944 “The unit uses seven valves and when suitably rearranged will probably occupy a space of about 10" x 8" x 5".”- Turing British National Archives, HW 62/6 Available online at http://www.turing.org.uk/sources/delilah.html Big X, small x Junior X - Mobile version of SIGSALY -Fit in a van -Used an 8 channel vocoder -Never deployed - Tompkins p. 54 Bell Labs also designed a 30 channel vocoder that took up 2 floors. – Tompkins p. 60 Let’s Get Small • No vocoder • Sent an analog signal, in contrast to the digital SIGSALY • Only one keying system Taking the Limit as Δx→0? Delilah sampled speech 4,000 times per second (SIGSALY was only 50) Message amplitudes scaled to not exceed 1 Key added (mod 1) – didn’t work for SIGSALY! Without mod step, cryptanalysis is possible, as with SIGSALY. For text, basic unit is letter (or bit) and OTP is proven unbreakable Not so clear with speech, although too infrequent sampling is obviously insecure. Shannon’s Bandwidth Theorem (Whittaker, 1915) • “If a signal time function is sampled instantaneously at regular intervals and at a rate at least twice the highest significant signal frequency, the samples contain all of the original message.” - Mehl, p. 71 • Turing discussed this with Shannon at Bell Labs. • Delilah’s 2,000 Hz required 4,000 samples/sec Some Modern Comparisons CD Audio DVD Audio Sampling Rate 44.1 kHz 192 kHz Samples per second 44,100 192,000 Sampling Accuracy 16-bit 24-bit Number of Possible Output 65,536 16,777,216 Levels http://electronics.howstuffworks.com/question487.htm Early CD players were not as good at reconstructing analog waveforms from samples and this could be heard, but not now. Fixing a Problem • The result of the encryption was too high frequency for telephone circuits • Solution was to “feed each “spike” into a specially devised electronic circuit with an orthogonal property” Hodges p. 275 Turing’s Orthogonal Circuit Now pretty common in high speed digital communications systems. – intersymbol interference (ISI) is prevented – zero crossings are at sampling points so they don’t interfere Probably Turing or Shannon originated this idea.
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