Free The Secrets of Station X by Michael Smith Page A

codebreakers got to work. A description of each message, containing the frequency and callsigns; the number; whether or not it was urgent; and the first two groups, was carefully logged on so-called B-Lists. These became known colloquially as Blists and the female graduates were dubbed the ‘Blisters’.
    At this stage, the Bombes had not yet been built and Enigma was being broken purely by hand, a difficult, if not virtually impossible, task with a machine cypher. The first step in breaking any cypher 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, cyphers replace every individual letter of every word with another letter. 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 encyphered in the same simple cypher 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 formobvious words with letters missing, allowing the codebreaker to fill in the gaps and recover those letters as well.
    Another basic weapon used by the codebreaker, ‘contact analysis ’, 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 ‘this’. So by combining these two weapons, the codebreaker could make a reasonable guess that where a single letter appeared repeatedly after the T which had already been 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 cyphers like Enigma 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 a number of key strokes. By doing this, the Germans hoped to ensure that no original letter was ever represented by the same encyphered 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. This was of great assistance in using cribs, pieces of plain text that were thought likely to appear in an Enigma message. This might be because it was in a common proforma, or because there was an obvious word or phrase it was expected to contain. Sometimes it was even possible to predict that a message passed at a lower level, on a system that had already been broken, would be repeated on a radio link using the Enigma cypher. If the two identicalmessages could be matched up, in what was known as a ‘kiss’, it would provide an easy method of breaking the keys.
    The Germans, with their liking for order, were particularly prone to providing the British with potential cribs. The same words were frequently used at the start of the message to give the address of the recipient, a popular opening being An die Gruppe (To the group). Later in the war, there were a number of lazy operators in underemployed backwaters whose situation