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Turing's Nazi Enigma Code-Breaking Secrets Have Been Declassified

Illustration for article titled Turings Nazi Enigma Code-Breaking Secrets Have Been Declassified

Over 70 years ago, father of computer science Alan Turing developed the techniques which enabled quick and efficient decryption of the Nazis' Enigma-scrambled messages. Now, the secrets behind his techniques—hidden away in research documents since the Second World War—have been declassified.

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Two papers were donated by the UK Government Communications Headquarters (GCHQ) to the British National Archive last week. Speaking to the BBC, a GCHQ mathematician, referred to only by the single name Richard, explained that the organization had "squeezed the juice" out of the two papers and was "happy for them to be released into the public domain".

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They won't, however, be freely available online; anyone wanting to see the documents will have to make a trip to the National Archive at Kew, in the UK.

The documents themselves—produced on a typewriter and augmented with hand-written notes and algebra— are titled On Statistics of Repetitions and The Applications of Probability to Cryptography. The statistics paper deals with how repeated characters in two encrypted messages can be used to prove that both passages use the same encipherment key, while the cryptography essay is much longer, and covers in detail the methods of using probability theory to break codes. [BBC via The Register]

Image from The National Archives

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DISCUSSION

Decrypting Enigma cyphers was neither quick nor efficient. Enigma had two flaws which made it possible to break it. First, no letter was ever encrypted as itself. This sounds minor but was in fact an important feature which the codebreakers exploited. Second, the Germans implemented Enigma badly. Sloppy operating procedures and repetitive message formats provided "cribs", hints as to the content of the message; this made decryption in part a "known plaintext" attack. Turing exploited these two factors to reduce the number of possibilities to something they could examine exhaustively with their "Bombe" electromechanical decoders, invented by Polish cryptanalyst Marian Rejewski. It took hundreds of people working around the clock. Some days cyphers took longer to break as the cribs could differ day to day; analyzing cribs was more art than science. A few messages were never broken. Here's one; have at it:

YKRB MGVA TMKF NWZX FFII YXUT IHWM DHXI FZEQ VKDV MQSW BQND YOZF TIWM

JHXH YRPA CZUG RREM VPAN WXGT KTHN RLVH KZPG MNMV SECV CKHO INPL HHPV

PXKM BHOK CCPD PEVX VVHO ZZQB IYIE OUSE ZNHJ KWHY DAGT XDJD JKJP KCSD

SUZT QCXJ DVLP AMGQ KKSH PHVK SVPC BUWZ FIZP FUUP YKRB MGVA VA