Decoding Nazi Secrets

Most historians agree that by enabling Allied commanders to eavesdrop on German plans, Station X shortened the war by 2 or 3 years. Its decoded messages played a vital role in defeating the U-boat menace, cutting off Rommel's supplies in North Africa, and launching the D-Day landings. 

With 159,000,000,000,000,000,000 possible settings and an ingenious way of scrambling messages, the Enigma was indeed a powerful enciphering machine.

Led by the brilliant Alan Turing, inventor of the computer, the codebreakers of England's cipher-cracking organization, Bletchley Park, were mathematicians, crossword-puzzle fanatics, and other super-brains. For decades what these men and women did at "B.P.," and how they managed to break the Germans' seemingly impregnable Enigma encoding machine, was classified. Now, one can get a tantalizing glimpse of what went on inside the minds of these codebreakers, who were fully aware of how vital their mission was to the Allied effort.

The first step in breaking any cipher is to try to find features which correspond to the original plain text. Whereas codes substitute groups of letters or figures for words, phrases or even complete concepts, ciphers replace every individual letter of every word. They therefore tend to reflect the characteristics of the language of the original text. This makes them vulnerable to studies of letter frequency; for example, the most common letters in English are E, T, A, O, and N. If a reasonable amount, or 'depth,' of English text enciphered in the same simple cipher were studied for "letter frequency," the letter that came up most often would represent E. The second most common letter would be T and so on. By working this out and filling in the letters, some will form obvious words with letters missing, allowing the codebreaker to fill in the gaps and recover those letters as well.

Contact analysis, another basic weapon used by the codebreaker, takes this principle a step further. Some letters will appear frequently alongside each other. The most obvious example in the English language is TH as in 'the' or 'that'. By combining these two weapons the codebreaker could make a reasonable guess that where a single letter appeared repeatedly after the T which he had already recovered from letter frequency, the unknown letter was probably H, particularly if the next letter had already been recovered as E. In that case, he might conclude that the letter after the E was probably the start of a new word and so the process of building up the message would go on.

Machine ciphers were developed to try to protect against these tell-tale frequencies and letter pairings, which is why the wheels of the Enigma machine were designed to move around one step after each 26 key strokes. By doing this, the Germans hoped to ensure that no original letter was ever represented by the same enciphered letter often enough to allow the codebreakers to build up sufficient depth to break the keys.

But it still left open a few chinks of light that would permit the British codebreakers to attack it. They made the assumption, correct far more often than not, that in the part of the message being studied the right-hand wheel would not have had the opportunity to move the middle wheel on a notch. This reduced the odds to a more manageable proportion. They were shortened still further by the Enigma machine's great drawback. No letter could ever be represented by itself.