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Enigma variation: The Lorenz machine at Bletchley Park 

Ian Fleming, the author of the James Bond books, was the liaison between naval intelligence and the Government Code and Cypher School (GC&CS) during the Second World War. The GC&CS was a secret organisation, and if the Germans had known what it could do and where it was located they'd have bombed the living daylights out of the place. The employees, if that's the right term, chess masters, mathematicians, classicists, crossword puzzle enthusiasts and other assorted oddballs, worked in Nissen huts, after overflowing the original mansion. They were spread around a large estate and didn't necessarily know one another, so at a reunion 46 years after the war they were surprised to see who else had been working there. No one spoke about it to outsiders, not a word, not a peep, and when one woman found out that her husband had an invitation to the reunion she asked how on earth he'd got it. She'd worked there but never told him — he likewise. 

Now we all know. It was at Bletchley Park — just outside Milton Keynes, Buckinghamshire — in a place one can visit and be taken round to see the captured Enigma machines with their plug boards and three coding wheels, the naval and Abwehr ones with four wheels and the Lorenz machine with its 12 wheels, used by the German High Command. A method for cracking the Enigma code was originally achieved by the Poles, but figuring out the latest settings was an ongoing process, using huts full of brain-power. The standard Enigma used by the German army and air force was one thing, but getting hold of the settings for a naval machine was an important priority and Fleming had an idea. Crash a captured German aeroplane into the English Channel with British Special Forces on board. When picked up by the German navy they would overpower the crew and capture the week's settings. It never happened, but something else did. In late 1942, when food supplies in Britain were down to about six weeks' holdings, three men — one a mere 16-year-old — entered a U-boat in the North Atlantic and brought out the monthly key settings for the Kriegsmarine Enigma. The 16-year-old survived; the other two died in the operation.

Hut Eight, where the mathematician Alan Turing worked, dealt with German naval codes. This brilliantly original thinker designed an electro-mechanical decipherment machine to replace the original Polish one. His work helped reduce the losses in the Atlantic from 500,000 to 50,000 tons a month. Some mathematicians have been faintly asexual people working in the quiet seclusion of English vicarages and Oxbridge colleges, but Turing was a man of physical and sexual energy who occasionally ran the 40 miles to London to attend important meetings. After the war, he was awarded an OBE, but later was prosecuted for his sexuality — he was gay. In 1954, aged 41, he committed suicide, and only in September last year did the then Prime Minister apologise for his prosecution, describing his treatment as "appalling". It was.

While German naval codes were a tough nut to crack, the German high command ciphers (called Tunny) on the 12-wheel Lorenz machine were all but impossible, particularly since Bletchley did not have a captured Lorenz machine — they acquired one only after the war. But there was a lucky break on August 30, 1941. A German operator in Athens transmitted a 4,500-character message to Vienna. It wasn't received correctly, so the operator requested retransmission — but without encrypting the request. This alerted the codebreakers and when Athens sent the message a second time, using abbreviations but the same settings, they betrayed invaluable information. A mathematician named Bill Tutte then reverse-engineered the Lorenz machine, and the Post Office Research Centre in Dollis Hill, north London, started to design a huge computer called Colossus to decipher messages from the German high command. This intellectual feat dwarfed Enigma, but it was a race against time. By June 1 1944, Colossus Mark 2 was in operation at Bletchley, just in time for the Normandy D-Day landings. Cracking the high command ciphers showed that Hitler had been convincingly misled about the location of the landings and German troops were being kept in the wrong position.

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