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, through works such as
Station X: Decoding Nazi Secrets, by Michael Smith, from
which the excerpts below were drawn, 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.
Tricks of the Trade
frequency, contact analysis, cribs: These are just some of the myriad weapons
in the arsenal of Bletchley Park codebreakers.
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.
This fact 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 pro forma, 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 cipher. If the two identical
messages could be matched up, in what was known as a 'kiss,' it would provide
an easy method of breaking the key settings.
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 messages 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 reports regularly
read simply: Keine besondere Ereignisse, literally "no special
occurrences," perhaps better translated as "nothing to report."
Most cribs could appear at any point in the message. Even Keine besondere
Ereignisse was likely to be preceded or followed by some piece of routine
information. But the fact that none of the letters in the crib could ever be
matched up with the same letter in the enciphered message made it much easier
to find out where they fitted.
If we take Keine besondere Ereignisse as our crib and place it above an
enciphered message, divided into the five-letter groups in which they would
normally be sent by the German Army or Air Force operators, it is easy to see
that the number of places it would fit are limited by the fact that no letter
can be enciphered as itself.
Moving the message just one place to the left or right would have one of the
first two Es of Ereignisse enciphered as itself, an obvious impossibility on
the Enigma machine. Moving it even further to the left or right only produces
more duplicated letters. In this particular case, and it was only rarely ever
that easy, this is in fact the only place in which the crib could fit. Mavis
Lever, a member of Knox's team, described the codebreaking process.
If you think of it as a sort of crossword technique of filling in what it might
be. I don't want to give the impression that it was all easy. You did have
inspired guesses. But then you would also have to spend a lot of time,
sometimes you would have to spend the whole night, assuming every position that
there could be on the three different wheels.
You would have to work at it very hard and after you had done it for a few
hours you wondered, you know, whether you would see anything when it was before
your eyes because you were so snarled up in it. But then of course, the magic
moment comes when it really works and there it all is, the Italian, or the
German, or whatever it is. It just feels marvellous, absolutely marvellous. I
don't think that there is anything one could compare to it. There is nothing
like seeing a code broken, that is really the absolute tops.
The Herivel Tip
Herival was a 21-year-old Cambridge mathematician hell-bent on breaking Red,
one of the main Enigma ciphers used by the Germans in World War II. Combining
mathematical skills with what Smith describes as "Alice in Wonderland-type
thought processes," Herivel got his wish late one evening.
[Herivel] had arrived at the Park at the end of January 1940 and was taken to
the mansion where a naval officer made him sign the Official Secrets Act. "Then
he gave me the address of my digs, which were just down the road from the Park
and also he told me where to go, to Hut 6 [one of many units within B.P.],
which of course had been effectively founded by [codebreaker Gordon] Welchman.
I had been recruited by Welchman and I was going to work in his show."
Unlike [codebreaker Dilly] Knox, Welchman believed in giving the new recruits
like John Herivel some training on the Enigma machine.
There were two people that instructed me in the mysteries of Enigma and the
method which they had been using to solve it from time to time. They were Alan
Turing and Tony Kendrick. I don't know how many hours they devoted to us but it
didn't all happen in one day.
I do remember that when I came to Hut 6, we were doing very badly in breaking
into the Red code. Every evening, when I went back to my digs and when I'd had
my supper, I would sit down in front of the fire and put my feet up and think
of some method of breaking into the Red code. I had this very strong feeling:
"We've got to find a way into the Red again." I kept thinking about this every
evening and I was very young and very confident and I said I'm going to find
some way to break into it. But after about two weeks I hadn't made any progress
Then, just like Knox asking which way round does the clock go [when people
answered clockwise, he replied "Not if you're the clock"], Herivel examined the
problem from a totally different perspective.
Up until the middle of February, I had simply been thinking in terms of the
encoded messages which were received daily and which came to Hut 6. Then one
evening, I remember vividly suddenly finding myself thinking about the other
end of the story, the German operators, what they were doing and inevitably
then I thought of them starting off the day.