Breaking the
Code
On average more
than 3,000 coded messages arrived at Bletchley Park each
day from the 'Y' Stations.
Messages
were taken to different 'Huts', depending on whether they had
come from the German army, air force, navy or another source.
A message from a U-boat would go to Hut 8.
There the code-breakers
would study it. The only way they could break the code was to
compare the message with others to see if they could work out
exactly how the U-boat commanders had set their Enigma rotor wheels
and plugboard settings that day.
Imagine their
task - to find the RIGHT ANSWER out of 150,000,000,000,000,000,000
POSSIBILITIES - and start all over again every 24 hours.
Sometimes messages
began with the same words, such as a weather report. This gave
clues (called a CRIB) about how the rest of the message had been
encoded.
When
the code-breakers eventually worked out what the CRIB letters
might be, they tested them on the machine shown on the left, called
a BOMBE. Bombes were huge, noisy electro-mechanical machines
which could check through combinations of letters far quicker
than a human being could. When the Bombe stopped, this meant that
the code-breakers' guesses were right. All that days U-boat messages
could then be decoded.
The decoded messages
were in German in blocks of five letters. They had to be carefully
translated into English.
Sometimes a code
could be cracked in less than an hour. But when the U-boats started
to use 4-rotor Enigma machines in February 1942 it took TEN
MONTHS to break the code.
