Between the Wars: The 1930s
In March 1929, Henry L. Stimson, the new Secretary of State, reportedly remarked that “gentlemen do not read other gentlemen’s mail” and proceeded to withdraw State Department support from Herbert O. Yardley’s Cipher Bureau, forcing it to close. However, the work was continued by the new Signal Intelligence Service (SIS) under the direction of William F. Friedman. Having the double function of making and breaking cryptosystems, the SIS made significant advances in cryptology. For use in the field, the Army wanted an easy method for changing the random alphabets while still using the basic principle of the M-94. In 1933, the Army turned to a flat, strip cipher. In 1938, an improved model, the M-138-A, was adopted. Meanwhile, Friedman was leading the Army towards encryption by sophisticated electromechanical means. Among various machines, he designed a small, portable cryptographic machine for field use. His work culminated in the production of the M-325 Converter, designed for “enciphering messages in the combat or field code and for identifying stations engaging in radio communication.”
Strip Cipher M-138-A: The M-138-A consists of a hinged aluminum board with 30 grooves designed to hold removable paper strips containing disarranged alphabets. The strips are inserted in the grooves in the order designated by the predetermined key. The strip cipher was used both prior to and during World War II.
M-325 Converter: Not actually manufactured for use until World War II, the M-325s were never sent to the field—thus earning the encipher machine the dubious title of “Friedman’s Folly.”