Herbert Yardley - Codebreaking

Codebreaking

His career in cryptology began with his work in the code room, as he broke the U.S. government codes that crossed his desk. At that time, American codes were very weak and Yardley solved them easily. He was shocked to learn that President Wilson was using a code that had been in use for over ten years. The weakness of American codes worried Yardley, especially considering the war in Europe, so in May 1916 he began writing a hundred-page “Solution of American Diplomatic Codes”, which he gave to his boss.

Breaking American codes got Yardley wondering about the codes of other countries. American participation in the war gave Yardley an opportunity to convince Major Ralph Van Deman of the need to set up a section to break other countries' codes. In June 1917, Yardley became a 2nd lieutenant in the Signal Corps and head of the newly created eighth section of military intelligence, MI-8. One early case was the cryptogram discovered in the clothing of German spy Lothar Witzke after he was arrested at the Mexican border in 1918. The evidence linked Witzke to significant sabotage activity in the U.S.

Yardley proved to be a very good administrator and during the war the people of MI-8 performed well even if they did not have any spectacular successes. After the war, the American Army and the State department decided to jointly fund MI-8 and Yardley continued as head of the "Cipher Bureau". They located their operations in New York City for legal reasons.

Cracking Japanese codes was a priority. David Kahn states:

The most important target was Japan. Its belligerence toward China jeopardized America's Open Door policy. Its emigrants exacerbated American racism. Its naval growth menaced American power in the western Pacific. Its commercial expansion threatened American dominance of Far Eastern markets.

After almost a year, Yardley and his staff finally managed to break the Japanese codes and were still reading Japanese diplomatic traffic when Washington hosted the Washington Naval Conference in 1921. The information the Cipher Bureau provided the American delegation regarding the Japanese government's absolute minimum acceptable battleship requirements was instrumental in getting the Japanese side to agree to a 5:3 ratio instead of the 10:7 ratio the Japanese Navy really wanted. This allowed Japan only 18 battleships to 30 for the U.S. and 30 for Great Britain instead of the 21 battleships Japan desired. This was the height of Yardley's cryptanalytic career.

Unfortunately, Yardley spent much of his time in New York involved in unrelated activities. Also, the flow of diplomatic telegrams dried up as companies became less willing to break the law to help the government. In Washington, William Friedman was actively exploring cryptographic frontiers for the Army. The Cipher Bureau was becoming irrelevant. However, it was moral indignation that finally doomed the bureau. When Henry L. Stimson, Secretary of State under President Herbert Hoover, found out about Yardley and the Cipher Bureau, he was furious and withdrew funding, summing up his argument with "Gentlemen do not read each other's mail". Some believe that Stimson was most offended when Yardley bragged that he could read all traffic of the Vatican, for it was after this remark Stimson turned and left the room.