Cryptography Reference
In-Depth Information
In 1930 the Japanese Foreign Office put into service
its first rotor machine, which was code-named Red by U.S.
cryptanalysts. In 1935-36 the U.S. Army Signal Intelligence
Service (SIS) team of cryptanalysts, led by William F.
Friedman, succeeded in cryptanalyzing Red ciphers, draw-
ing heavily on its previous experience in cryptanalyzing the
machine ciphers produced by the Hebern rotor machines.
In 1939 the Japanese introduced a new cipher machine,
code-named Purple by U.S. cryptanalysts, in which rotors
were replaced by telephone stepping switches. Because
the replacement of Red machines by Purple machines was
gradual, providing an enormous number of cribs between
the systems to aid cryptanalysts, and because the Japanese
had taken a shortcut to avoid the key distribution prob-
lem by generating keys systematically, U.S. cryptanalysts
were able not only to cryptanalyze the Purple ciphers but
also eventually to anticipate keys several days in advance.
Functionally equivalent analogs to the Purple cipher
machines were constructed by Friedman and his SIS asso-
ciates and used throughout the war to decrypt Japanese
ciphers. Apparently no Purple machine survived the war.
Another Japanese cipher machine, code-named Jade, was
essentially the same as the Purple. It differed from the
latter chiefly in that it typed Japanese kana characters
directly.
The greatest triumphs in the history of cryptanaly-
sis were the Polish and British solution of the German
Enigma ciphers and of two teleprinter ciphers, whose
output was code-named Ultra, and the American crypt-
analysis of the Japanese Red, Orange, and Purple ciphers,
code-named Magic. These developments played a major
role in the Allies' conduct of World War II. Of the two, the
cryptanalysis of the Japanese ciphers is the more impres-
sive, because it was a tour de force of cryptanalysis against
