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Alan Turing: Life and Legacy of a Great Thinker

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Alan Turing: Life and Legacy of a Great Thinker Alan Turing: Life and Legacy of a Great Thinker Bearbeitet von Christof Teuscher, D Hofstadter 1st ed. 2003. 2nd printing 2005. Buch. xxviii, 542 S. Hardcover ISBN 978 3 540 20020 8 Format (B x L): 15,5 x 23,5 cm Gewicht: 1116 g Weitere Fachgebiete > EDV, Informatik > EDV, Informatik: Allgemeines, Moderne Kommunikation > EDV & Informatik: Geschichte Zu Inhaltsverzeichnis schnell und portofrei erhältlich bei Die Online-Fachbuchhandlung beck-shop.de ist spezialisiert auf Fachbücher, insbesondere Recht, Steuern und Wirtschaft. Im Sortiment finden Sie alle Medien (Bücher, Zeitschriften, CDs, eBooks, etc.) aller Verlage. Ergänzt wird das Programm durch Services wie Neuerscheinungsdienst oder Zusammenstellungen von Büchern zu Sonderpreisen. Der Shop führt mehr als 8 Millionen Produkte. Alan Turing at Bletchley Park in World War II Tony Sale Ex Museums Director, Bletchley Park, UK Summary. “There should be no question in anyone’s mind that Turing’s work was the biggest factor in Hut 8’s success [in breaking the German Naval Enigma]. In the early days he was the only cryptographer who thought the problem worth tackling and not only was he primarily responsible for the main theoretical work within the hut (particularly the developing of a satisfactory scoring technique for dealing with Banburismus) but he also shared with Welchman and Keen the chief credit for the invention of the Bombe. It is always difficult to say that anyone is absolutely indispensable but if anyone was indispensable to Hut 8it was Turing” [1]. 1 Alan Turing and the Enigma Machine The mathematician Alan Turing had been identified, at Cambridge, as a likely candidate for code breaking. He came to the Government Code and Cypher School (GC&CS) in Broadway in London a number of times in early 1938 to be shown what had already been achieved. He was shown some intercepts of German signals enciphered on the German forces Enigma cipher machine. The Enigma machine, Fig. 1, was an electro/mechanical way of achieving a seven, or nine, layer substitution cipher. The individual substitutions were fixed by wiring within wheels which could be rotated by the operator but which also index round, like a car miles indicator, as letters to be enciphered or deciphered were entered. The Enigma was patented in 1918 by Arthur Scherbius in Berlin, devel- oped by him as a commercial product and shown to the public in 1922. Because the machine could be bought by anyone, the security of the cipher depended not on the machine itself but on the vast number of ways in which it could be configured before the start of an encipherment. To increase the complexity of this setting up, each wheel had a tyre, or ring, round the core containing the cross wiring. Letters or numbers on the surface of this ring appeared in the windows above each wheel. The ring could be rotated around the core and set by the operator before encipherment began. It remained set throughout the message input. The action of pressing a key caused the right hand wheel to index one position (one of 26). At some point this rotation was transferred to the next wheel on the left. This was known as a carry and was caused by a slot, 442 T. Sale Fig. 1. A German Army/Airforce Enigma Machine the carry slot, coming into line with the indexing pawls. This carry slot was initially on the wheels; later it was moved to the ring. An electrical current was used to sense the substitutions. When a key was pressed a connection was made from the battery to a point on the fixed entry disc on the right hand side of the wheels, AFTER the right hand wheel had indexed and any carry had caused other wheels to turn ... The electrical current flowed through the internal wiring in the wheels from right to left, was turned round in the reflector and came back through the wheels to exit at a different point on the entry disc which was connected to a lamp on the lamp panel. The lamp that lit was the encipherment of the key just pressed. The clockwise order of connections to the fixed, right hand, entry disc was known as the “entry order.” In the Scherbius commercial machine this was just the order of the keys on the keyboard from left to right across each of the three rows. This was known as the QWERTZUIO order. Thus the variable elements of the Enigma were: the wheels and their order from left to right in the machine, the ring setting for each wheel, the wheel rotational position, the start position before encipherment started. The first Enigma machines, the glow lamp machines of the 1920s, had three wheels which could be removed and replaced in any order. (6 combina- tions). The reflector was rotatable by the operator (26 positions). The wheel start positions thus gave 26 × 26 × 26 positions. The ring settings gave a 26 Alan Turing at Bletchley Park in World War II 443 position rotational translation of each wheel start position. Total number of different configurations, 2, 741, 856. From 1930 the plug board (Stecker) was added to the Enigma used by the German Army and Air Force. The plug board enabled pairs of letters to be completely transposed. Initially 6 pairs were transposed, later this was increased to 10, the nearly optimum number. At the same time the reflector became fixed. This machine was then also adopted by the German Navy. For this Enigma the wheels give 6 × 26 × 26 × 26 = 105, 456 possible combinations. Six plug pairs gives 100, 391, 791, 500 possibilities; total ap- proximately ten thousand, million, million (1017). Despite this seeming invulnerability due to such a vast number of possible configurations, the Enigma machine had some weaknesses. Firstly because of the reflector no letter could encipher to itself. Secondly, each of the first set of wheels, 1 to 5, had a different point at which turnover occurred to the next wheel on the left. This allowed identification of the right hand wheel and sometimes even the centre wheel. (The later Naval Enigma wheels 6, 7 & 8 had two carries, all at the same wheel positions.) In London, in 1938, Alan Turing would have met Alistair Denniston, the head of GC&CS and Edward Travis, Hugh Foss, John Tiltman and Dilly Knox, all eminent code breakers, some from World War I. Edward Travis had purchased a commercial Enigma machine in 1925 and Hugh Foss had devised a geometric way for breaking it in 1927. Later, in 1936, Dilly Knox devised his “rods” method for breaking the unsteckered Enigma used in the Spanish civil war which also had the QWERTZUIO entry order and the same rotor wiring as the commercial Enigma. Meanwhile in Poland the Polish Security Service had purchased a com- mercial Enigma and worked out, in the 1920s, methods for breaking it. When, in 1930, the Germans changed to the steckered Enigma, the Poles recruited some young mathematicians to try to break it. The greatest of these was Mar- ian Rejewski who found ways of exploiting the German procedural mistakes in using Enigma. The problem for the Germans was how to tell the intended recipient of an enciphered message the exact wheel start letters from which the message could be deciphered. They decided to encipher this start posi- tion, known as the message key, on the Enigma machine itself in order to conceal it from any interceptor. But they also enciphered it twice in order to make certain that it was correctly received by the intended operator. It was this double encipherment which Marian Rejewski exploited in his very successful “characteristics” method of attack. He also correctly deduced that the Germans had changed the entry order to ABCDEFG ... and worked out the new wheel wirings which were different from those in the commercial machine. Later he devised a machine called a “Bomba” specifically to attack the double encipherment of the message key. In 1938 Dilly Knox already knew that the German forces Enigma rotors were wired differently to the commercial rotors, but did not know the entry 444 T. Sale rotor order and apparently did not know of the double encipherment of the message key. All the Polish achievements were divulged to the British and the French at the famous meeting in the Pyry Forest near Warsaw in July 1939. In September 1939 Turing came to Bletchley Park and joined Dilly Knox in the cottage in the stable yard. He started to think of ways to break Enigma using probable words, “cribs” and was intrigued by the problems in breaking the German Naval Enigma. The method based on probable words was a far more powerful method than that used in Rejewski’s Bomba and led later to the development of the Turing Bombe. GC&CS already had a few intercepts and at least one plain text/cipher text pair, reputed to have been smuggled to England by a Polish cipher clerk. 2 “Cribs” and Opened Out Enigmas 2.1 Letter Pairs Among the characteristics that Turing found in these messages was that occasionally the same cipher/plain text pair of characters occurred at different places in the same message. JYCQRPWYDEMCJMRSR SPRUCHNUMMERXEINS Remember that because the Enigma machine is reversible, R→Cisthe same as C→RandM→EthesameasE→M. Whether such pairings occur is determined by the rotor order and the core rotor start positions. Turing realized that conversely the actual rotor order and core rotor start position could be arrived at by trying all configurations to see if these pairings were satisfied and more importantly he realized that this was independent of the Steckers.
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