Between the Wars: The 1920s
The end of World War I did not mean an end to the Army’s cryptanalytic effort. Using elements of his wartime organization, Herbert O. Yardley set up the Cipher Bureau, a clandestine unit that worked chiefly to break diplomatic codes. Although officially an element of the Military Intelligence Division, it was jointly funded by the State Department. Yardley and his small staff scored a number of intelligence coups, most notably in breaking the Japanese diplomatic code in time for US diplomats to leverage the Japanese bargaining position against them at the Washington Conference of 1921.
In 1921 the Signal Corps hired William F. Friedman to design new codes, such as this one used by the military attachés. Two years later, the Army adopted the M-94 cylindrical cipher device. The M-94 launched the US military cryptographic services into using mechanical ciphers instead of code books to secure military communications.
Yardley Intercepts: The breaking of the Japanese diplomatic code gave American diplomats a key advantage during disarmament conferences of the 1920s. see .pdf version
The Attaché Code: An example of a code used in the 1920s. see .pdf version
M-94 Cipher Device: Developed in the 1920s, the M-94 was virtually identical in principle to a cipher device conceived by Thomas Jefferson. Made of aluminum alloy, it consists of a set of 25 rotatable alphabet disks, which can be arranged in any position. The order of the disks, known only to the encipherer and decipherer, constitutes the key. The M-94 was used extensively between World Wars I and II.