William F. Friedman - Signals Intelligence Service

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. To support the program, Friedman wrote a series of technical monographs, completing seven by early 1918. He then enlisted in the Army and went to France to serve as the personal cryptographer 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 cryptography to that time. His texts for Army cryptographic training were well thought of and remained classified for several decades.

In 1921 he became chief cryptanalyst for the War Department and later led the Signals Intelligence Service(SIS) -- a position he kept for a quarter century. 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.

Friedman coined several terms, including "cryptanalysis", and wrote many monographs on cryptography. One of these (written mostly in his spare time) was the first draft of his Elements of cryptanalysis, which later was expanded to four volumes and became the U.S. Army's cryptographic main textbook and reference. Realizing that mathematical and language skills were essential to SIS' work, Friedman managed to get authority to hire three men with both mathematical training and language knowledge. They were Solomon Kullback, Frank Rowlett and Abraham Sinkov, each of whom went on to distinguished service for decades. In addition he also was finally able to hire a man fluent in Japanese, John Hurt.

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.