From Sixteenth-Century Cryptography to the New Millennium — The Last 500 Years - Codes: The Guide to Secrecy from Ancient to Modern Times - page 91

to manufacture and sell his machine, which he called Enigma . In 1934, the
Japanese Navy bought the Enigma for their own use, and developed it into the
Japanese cryptosystem called Purple, which we discussed above. However, the
Japanese version was unlike Enigma in that it used stepper switches, similar to
those used in telephone exchanges. When the SIS built a machine to replicate
Purple, they made the unwitting decision to use exactly the same telephone
stepper switch used by the Japanese designer! This accounts for Friedman's
group at SIS being able to duplicate a machine they had never seen.
In 1932 Hebern designed a machine with five rotors, the HCM. In 1936, a
rotor machine, based on the Hebern machine, called the SIGABA (see page 93),
or M-134-C was developed and used with great success by the U.S. military in
World War II. (It was also called the CSP-889, or ECM Mark II, by the Navy.) It
was so well designed that all the efforts by the Army's cryptanalysts to break it
failed. As it would be learned later, the Germans also could not break the Ameri-
cans' cryptograms enciphered with the ABA s, as they were nicknamed. The fun-
damental idea of electronically controlled rotors was created by William Fried-
man, and he implemented it in the original M-134 device, which had five rotors
that encrypted plaintext, the motion of which was controlled by a paper tape.

Then Frank Rowlett 2.14 (1908-1998) (see Figure
2.17) created the vital concept of the SIGABA,
namely, the idea of using rotors to control the
rotors that enciphered the plaintext. Rowlett
was one of Friedman's earliest assistants, since
1929, and was part of the team that broke Pur-
ple at SIS. The SIGABA had fifteen rotors, ten
of which were conventional 26-contact rotors,
and five of which had smaller rotors with only
ten contacts on each side. Moreover, the rotors
were divided into three sets. Five of the 26-
contact rotors, called cipher rotors, encrypted or
decrypted a message in the same fashion as the
Hebern rotor machine. Another five 26-contact
rotors were called control rotors, and the five 10-
contact rotors were called index rotors. In the
1940s the SIGABA would prove to be the securest of the machines developed
in the West, and it never fell into enemy hands.
When Hitler came to the stage, the cryptographers of the Wehrmacht made
the decision to use the Enigma, upon which they made improvements for their
security purposes. However, the German Enigma cryptosystem was cryptan-
alyzed by British researchers at Bletchley Park, which is a Victorian country
mansion in Buckinghamshire, halfway between Oxford and Cambridge. In Au-
gust of 1939, the Government Code and Cypher School was seconded there.
Perhaps one of the most important among these researchers was Alan Mathison

Figure 2.17: Frank Rowlett.

2.14 Figure 2.17 is courtesy of the National Security Agency Hall of Honor . See:
http://www.nsa.gov/honor/index.html .