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William Frederick Friedman (September 24, 1891 – November 12, 1969) was a US Army cryptographer who ran the research division of the Army's Signals Intelligence Service (SIS) in the 1930s, and parts of its follow-on services into the 1950s. In 1940, subordinates of his led by Frank Rowlett broke Japan's PURPLE cipher, thus disclosing Japanese diplomatic secrets before America's entrance into World War II.
As a child, Friedman 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 personally interesting project, 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.
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 the work as he courted Elizebeth Smith, Mrs. Gallup's assistant and an accomplished cryptographer. 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 cryptography, collectively known as the "Riverbank publications", including the first description of the index of coincidence, an important mathematical tool in cryptanalysis.
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 Government. During this period, the Friedmans broke a code used by German-funded Indian 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 cryptographic technique common at the time. 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.
During the 1920s, several new cipher machines were developed generally based on using typewriter mechanics and basic electrical circuitry. 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.
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 several problems common to most rotor-machine designs. Examples of some dangerous features included having rotors step one position with each keypress, and putting the fastest 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 were 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. At least one patent related to it was finally granted after Friedman had died.
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", was different and much more difficult. The Navy's cryptological unit (OP-20-G) and the SIS thought it might be related to earlier Japanese cipher machines, and agreed that SIS would handle the attack on the system. 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 chosen.
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. The controversy over whether the US had foreknowledge of the Pearl Harbor attack has roiled well into the 21st century.
In 1941, Friedman was hospitalized with a "nervous breakdown", widely 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 establishment at the "Government Code and Cypher School" at 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 Enigma cipher. However Friedman visited Bletchley Park in April 1943 and played a key role in drawing up the 1943 BRUSA Agreement.
Following World War II, Friedman remained in government signals intelligence. In 1949 he became head of the cryptographic 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", which was 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.) During his early years at NSA, he encouraged it to develop what was probably the first super-computers, although he was never convinced a machine could have the "insight" of a human mind.
Friedman spent much of his free time trying to decipher the famous Voynich Manuscript, written sometime between 1403-1437. However, after four decades of study he finally had to admit defeat, contributing no more than an educated guess as to its origins and meaning. Created the first major transcription alphabets to equate the Voynich characters with Latin characters in order to help with cryptanalysis, such as the European Voynich Alphabet in the 1940s, where each line of the manuscript was transcribed to an IBM punch card to make it machine readable.
Friedman retired in 1956 and, with his wife, turned his attention to the problem that had originally brought them together: examining Bacon's supposed codes. Together they wrote a book entitled The Cryptologist Looks at Shakespeare which won a prize from the Folger Library and was published under the title The Shakespearean Ciphers Examined.. The book demonstrated flaws in Gallup's work and in that of others who sought hidden ciphers in Shakespeare's work. (There is a discussion on the Wikipedia talk page to his article.)
At NSA's request Friedman prepared Six Lectures Concerning Cryptography and Cryptanalysis, which he delivered at NSA. But later the Agency, concerned about security, confiscated the reference materials from Friedman's home.
Married to Elizebeth Friedman.