|William Frederick Friedman (September 24, 1891 – November 12, 1969) was a US Army
cryptologist. He ran the research division of the Army's Signals
Intelligence Service (SIS) in the 1930s, and its follow-on services
into the 1950s. In the late 1930s, subordinates of his led by Frank
Rowlett broke Japan's PURPLE cipher, thus disclosing Japanese
diplomatic secrets in the World War II era.
Friedman was born Wolfe Frederick Friedman in Kishinev, Bessarabia,
the son of a postal worker who migrated to Pittsburgh in 1892. Three
years later, his first name was changed to William. As a child, he was
introduced to cryptography in the short story "The Gold-Bug" by Edgar
Allan Poe. He studied at the Michigan Agricultural College (known
today as Michigan State University) in East Lansing and received a
scholarship to work on genetics at Cornell University. Meanwhile
George Fabyan, who ran a private research laboratory to study any
project that caught his fancy, decided to set up his own genetics
project and was referred to Friedman. Friedman joined Fabyan's
Riverbank Laboratories outside Chicago in September 1915. As head of
the Department of Genetics, one of the projects he ran studied the
effects of moonlight on crop growth, and so he experimented with the
planting of wheat during various phases of the moon.
Initial work in cryptology
Another of Fabyan's pet projects was research into secret messages
which Sir Francis Bacon had allegedly hidden in various texts during
the reigns of Elizabeth I and James I. The research was carried out by
Elizabeth Wells Gallup. She believed that she had discovered many such
messages in the works of William Shakespeare, and convinced herself
that Bacon had written many, if not all, of Shakespeare's works.
Friedman had become something of an expert photographer while working
on his other projects, and was asked to travel to England on several
occasions to help Gallup photograph historical manuscripts during her
research. He became fascinated with cryptology as he courted Elizebeth
Smith, Mrs. Gallup's assistant and an accomplished cryptologist. They
married, and he soon became director of Riverbank's Department of
Codes and Ciphers as well as its Department of Genetics. During this
time, Friedman wrote a series of 23 papers on cryptology, known as the
"Riverbank publications", included the first description of the index
of coincidence, an important mathematical tool for breaking ciphers.
With the entry of the United States into World War I, Fabyan offered
the services of his Department of Codes and Ciphers to the government.
No Federal department existed for this kind of work (although both the
Army and Navy had had embryonic departments at various times), and
soon Riverbank became the unofficial cryptographic center for the US
Federal Government. During this period the Friedmans cracked a code
used by German-funded Hindu radicals in the US who planned to ship
arms to India to gain independence from Britain. Analysing the format
of the messages, Riverbank realized that the code was based on a
dictionary of some sort, a common cryptographic technique. The
Friedmans soon managed to decrypt most of the messages, but only long
after the case had come to trial did the book itself come to light: a
German-English dictionary published in 1880.
Signals Intelligence Service
The United States government decided to set up its own cryptological
service, and sent Army officers to Riverbank to train under Friedman.
In support of this program, Friedman produced a series of technical
monographs, completing seven by early 1918. He then enlisted in the
Army and travelled to France to serve as the personal cryptologist for
General John J. Pershing. He returned to the US in 1920 and published
an eighth monograph, "The Index of Coincidence and its Applications in
Cryptography", considered by some to be the most important publication
in modern cryptology to that time. His texts for Army cryptologic
training were well thought of and remained classified for several
In 1921 he joined the government's American Black Chamber, where he
was placed in charge of researching new cryptographic systems and ways
to break them, and in 1922 he was promoted to head the Research and
Development Division. After the dissolution of the Black Chamber in
1929, Friedman moved to the Army's Signals Intelligence Service (SIS)
in a similar capacity.
Friedman coined several terms, including "cryptanalysis", meaning the
study and practice of breaking codes and ciphers, and wrote many
monographs on cryptology for use within the SIS (and its descendants).
During this period Elizebeth Friedman continued her own work in
cryptology, and became famous in a number of cases involving the Coast
Guard and FBI during Prohibition.
Solution of cipher machines
During the 1920s a series of new cipher machines gained popularity,
based largely on attaching typewriter mechanics to basic electrical
circuitry — batteries, switches and lights. An early example was the
Hebern Rotor Machine, designed in the US in 1915 by Edward Hebern.
This system offered such security and simplicity of use that Hebern
heavily promoted it to investors, feeling that all companies would
soon be using them. But his company went bankrupt when the war ended,
and Hebern eventually landed in prison, convicted of stock
Friedman realized that the new rotor machines would be important, and
devoted some time to analysing Hebern's design. Over a period of years
he developed principles of analysis and discovered a number of
problems common to most rotor-machine designs. Examples of some
dangerous features included having the rotors step one position with
each keypress, and positioning the fast rotor (the one that turns with
every keypress) at either end of the rotor series. In this case, by
collecting enough ciphertext and applying a standard statistical
method known as the kappa test, he showed that he could, albeit with
great difficulty, crack any cipher generated by such a machine.
Friedman used his understanding of rotor machines to develop several
that remained immune to his own attacks. The best of the lot, the
SIGABA — which was destined to become the US's highest-security cipher
machine in World War II — was co-invented by Frank Rowlett, a young
mathematician whom Friedman had hired.
In 1939 the Japanese introduced a new cipher machine for their most
sensitive diplomatic traffic, replacing an earlier system that SIS
referred to as "RED." The new cipher, which SIS called "PURPLE,"
proved difficult to crack. The Navy's cryptological unit (OP-20-G) and
the SIS thought it might relate to earlier Japanese cipher machines,
and SIS set about attacking it. After several months trying to
discover underlying patterns in PURPLE ciphertexts, an SIS team led by
Friedman and Rowlett, in an extraordinary achievement, figured it out.
PURPLE, unlike the German Enigma or the Hebern design, did not use
rotors but stepper switches like those in automated telephone
exchanges. Leo Rosen of SIS built a machine — as was later discovered,
using the identical model of switch that the Japanese designer had
Thus, by the end of 1940, SIS had constructed an exact analog of the
PURPLE machine without ever having seen one. With the duplicate
machines and an understanding of PURPLE, SIS could decrypt increasing
amounts of Japanese traffic. One such intercept was the message to the
Japanese Embassy in Washington, D.C., ordering an end (on December 7,
1941) to negotiations with the US. The message gave a clear indication
of impending war, and was to have been delivered to the US State
Department only hours prior to the attack on Pearl Harbor.
In 1941 Friedman was hospitalized with a "nervous breakdown",
generally attributed to the mental strain of his work on PURPLE. While
he remained in hospital, a four-man team — Abraham Sinkov and Leo
Rosen from SIS, and Lt. Prescott Currier and Lt. Robert Weeks from the
U.S. Navy's OP-20-G — visited the British cryptological establishment
at the "Government Code and Cypher School" in Bletchley Park. They
gave the British a PURPLE machine, in exchange for details on the
design of the Enigma machine and on how the British decrypted the
However Freidman was to visit Bletchley Park in April 1943 and play a
key role in drawing up the 1943 BRUSA Agreement.
National Security Agency
Following World War II, Friedman remained in government signals
intelligence. In 1949 he became head of the code division of the
newly-formed Armed Forces Security Agency (AFSA) and in 1952 became
chief cryptologist for the National Security Agency (NSA) when it was
formed to take over from AFSA. Friedman produced a classic series of
textbooks, "Military Cryptanalysis", used to train NSA students.
(These were revised and extended, under the title "Military
Cryptanalytics", by Friedman's assistant and successor Lambros D.
Callimahos, and used to train many additional cryptanalysts.)
Friedman retired in 1956 and, with his wife, turned his attention to
the problem that had originally brought them together: examining
Bacon's codes. In 1957 they wrote The Shakespearean Ciphers Examined,
demonstrating flaws in Gallup's work and in that of others who sought
hidden ciphers in Shakespeare's work. Records that Friedman had used
to prepare Six Lectures Concerning Cryptography and Cryptanalysis,
which he delivered at NSA, were confiscated from his home by NSA
security staff. His health began to fail in the late 1960s, and he
died in 1969. Friedman's wife donated his archives to the George C.
Marshall Library, which also was raided by NSA security.
Friedman has been inducted into the Military Intelligence Hall of Fame
and has a building named after him and his wife, Elizebeth, at the NSA
complex at Fort George G. Meade in Maryland.
Friedman has the distinction of having one of the longest known
suppressed patent applications for U.S. Patent 6,097,812 for a
"cryptographic system" (Filed July 25, 1933, issued August 1, 2000).
^ Rosenheim, Shawn James. The Cryptographic Imagination Johns Hopkins
University Press, 1997. p. 11
Ronald W. Clark, The Man Who Broke Purple: the Life of Colonel William
F. Friedman, Who Deciphered the Japanese Code in World War II, Little
Brown & Co, 1977, ISBN 0-316-14595-5.
James Gannon, Stealing Secrets, Telling Lies: How Spies and
Codebreakers Helped Shape the Twentieth Century, Washington, D.C.,
Brassey's, 2001, especially chapter 6: Who Broke Purple? (pp. 94-106).
Frank B. Rowlett, The Story of Magic: Memoirs of an American
Cryptologic Pioneer, with Foreword and Epilogue by David Kahn, Laguna
Hills, CA, Aegean Park Press, 1999.
Reprints of Friedman's publications are available from Aegean Park