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 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.

During this time, Friedman wrote a series of eight papers on cryptography, collectively known as the "Riverbank Publications", including the first description of the index of coincidence, an important mathematical tool in cryptanalysis.

[3][4] : p. 374 ff 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.

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.

In 1929, after the American Black Chamber in New York City was disbanded, its files were entrusted to SIS, and the cryptographic and intelligence services was reorganized to suit its new position at the War Department.

During this period Elizebeth Friedman continued her own work in cryptology, and became famous in a number of trials involving rum-runners and the Coast Guard and FBI during Prohibition.

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.

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.

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.

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.

(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.)

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.

[12] Friedman has been inducted into the Military Intelligence Hall of Fame and there is a building named after William and Elizebeth at the NSA complex at Fort Meade in Maryland.

Schoen shares a significant background and personality traits with Friedman, including being one of the top cryptanalysts of the U.S. Army, breaking Japanese codes prior to Japan's involvement in World War II, and the psychological problems that he suffered from as a result.

[citation needed] In his acknowledgements, Stephenson writes "Among all these great wartime hackers, some kind of special recognition must go to William Friedman, who sacrificed his health to break the Japanese machine cipher called Purple before the war even began.