Cryptography Reference
In-Depth Information
breaking into the daily communications of the German navy's
U-boats. Great care was always exercised to conceal the fact
that Bletchley had deciphered these messages. For instance,
British intelligence leaked false information hinting at revolu-
tionary new developments in long-range radar.
In March 1940, Turing's first Bombe, a code-breaking
machine, was installed at Bletchley Park; improvements sug-
gested by British mathematician Gordon Welchman were
incorporated by August. This complex machine consisted of
approximately 100 rotating drums, 10 miles of wire, and about
1 million soldered connections. The Bombe searched through
different possible positions of Enigma's internal wheels, look-
ing for a pattern of keyboard-to-lamp board connections that
would turn coded letters into plain German. The method
depended on human instinct, though; to initiate the process,
a code breaker had to guess a few words in the message (these
guessed words were called a crib). The Polish Bomba, a simpler
18-drum machine, was a forerunner of the Bombe, but it was
based on Rejewski's method for finding the wheel positions at
the start of the message. Unlike Rejewski's method, the more
powerful crib-based method invented by Turing survived the
May 1940 change. The war on Enigma was transformed by
the high-speed Bombes, and more of them were installed in
Britain and the United States.
and a reverse standard alphabet ZYX….Thus, if no tooth
were present, A would encrypt to Z, B to Y, and so forth,
while one tooth present would cause A to encrypt to Y, B
to Z, etc. This is strictly a Vernam-type encryption—i.e.,
encryption by subtraction modulo 26 of the key symbol
from the plaintext symbol. To decrypt, the ciphertext is
processed with the same pin settings that were used to
encrypt it but with the cyclical shift set to occur in the
opposite direction.
