First patented in 1918, the Enigma was the pre-eminent German military cyphering machine, used for all cyphered signals traffic by the Luftwaffe, Army, Kreigsmarine, railroad administration, Abwehr and the Sicherheitsdienst (Nazi party intelligence) from 1935 onwards. The Germans believed the cyphers created by Enigma to be so complicated as to be impregnable.
Using a number of hard-wired wheels ("rotors") and a manually-set plugboard, the typewriter-like Enigma would scramble the letters typed into it one at a time, lighting a bulb behind a completely different letter on a display on the top of the machine which the operator would write down. As each letter was typed the wheels would advance in turn, in effect forming a new cypher for each letter typed. The rotors could be placed in any order, and set to any starting position. The operator would have a book of 'keys' giving each day's rotors and plug-board connections, and had the choice of rotor starting positions, which would also be the first three characters of the message. Since the action of Enigma was reciprocal (that is, if P coded to L, then L would code to P), the message could then be decoded by the recipient simply by typing the encyphered message and writing down the displayed letters.
To make the process even more complicated, by the start of the war there was a choice of five rotors for the Enigma. Before the war, Polish Intelligence (Biuro Szyfrów), working from information which had been shared with them by French Intelligence, had reverse-engineered Enigma and succeeded in working out the wiring of the three rotors then in use. They built 6 devices they called bomby (singular bombe) which duplicated the wiring of the Enigma rotors, testing all possible combinations of rotor order and settings on a message, to show which ones gave a sensible text, thereby giving the key for that particular day.
In December of 1938, the Germans added two new rotors to the Enigma set, and expanded the number of plugboard connections from six to ten. This meant that, even after the wiring of the new rotors was worked out, sixty bomby would be necessary to process them, at a total cost of fifteen times Biuro Szyfrów's annual equipment budget. In any case, recovering the new rotors' wiring by analysis would take months, and the Poles were running out of time.
In July 1939, as tensions increased between Poland and Germany, the Biuro invited French and British cryptographers to Warsaw, giving them each a duplicate Enigma to be smuggled home in diplomatic bags. On September 5th, five days after the Germans invaded, the Polish cryptanalysts burned their records and destroyed their equipment, then evacuated to France where they continued their work. When France was in turn invaded, some succeeded in evacuating to Britain, others remained behind in Vichy France, where they were later captured. Despite being held in an SS camp, none of these five ever revealed the extent of the Allies' knowledge of Enigma, instead satisfying the SS with lesser Polish secrets.
Although most of the German military believed Enigma to be unbreakable, Admiral Dönitz was a little more sceptical. Kreigsmarine Enigma rotor sets therefore had an extra three wheels, which enormously increased the complexity of decyphering. By 1941, while 'Red' (Luftwaffe) keys could usually be broken by Bletchley Park cryptanalysts by breakfast of the same day, naval keys could take two or three days. Although British Intelligence worked very hard to disguise the source of ULTRA (the Enigma decrypts), Dönitz became suspicious enough in 1941 to order four-rotor Enigmas for the Kreigsmarine.
The complexity of decrypting the new naval Enigma keys ('Shark') was so high that from February to December 1942 naval Enigma was effectively 'blacked out' to Bletchley Park; using all available British bombes, it could take over two weeks to break a single days' key, if it could be broken at all. This had dire consequences for the Battle of the Atlantic, since a U-boat could detect a convoy and call together a pack to intercept it within a day or so. The blackout only ended with the capture of naval codebooks from a sinking U-boat, for which feat two George Crosses were awarded, posthumously.
Although there were later blackouts when naval code books were changed, for example, new American-built high-speed bombes supplied to Bletchley Park ensured that none lasted for more than a couple of weeks.
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This page updated Mon, September 04, 2006, around about 24:00 ish (BST).