Frank Rowlett's Bright Idea (MAY 1934)

Photo By Erin Thompson | William Friedman’s sketch of the wiring between Frank Rowlett’s wheel control unit and the cipher unit of the SIGABA from the mid-1930s  

UNITED STATES

05.16.2025

Courtesy Story

U.S. Army Intelligence Center of Excellence

by Michael E. Bigelow, INSCOM Command Historian

FRANK ROWLETT'S BRIGHT IDEA
In May 1934, Frank Rowlett, a junior cryptanalyst with the Army’s Signal Intelligence Service (SIS), devised a means that played a critical role in securing Army communications during World War II. His concept was fundamental to the design of the SIGABA, a cipher machine that served as the secure backbone of Army high-level communications into the 1950s.

Rowlett, a 26-year-old former schoolteacher, had joined William Friedman’s SIS in April 1930. Over the next four years, Rowlett and his colleagues, Solomon Kullback and Abraham Sinkov, underwent a training program in both cryptography and cryptanalysis. Friedman wanted his analysts to participate in the generation of codes and ciphers so they understood cryptographic processes and, consequently, would be better cryptanalysts.

Beginning in 1931, the SIS analysts started working with a variety of foreign and American cipher equipment with the intent of developing an improved machine for the Army. The existing machines relied on rotating wheels to encipher messages; these rotors were set by an internal keying mechanism. Friedman was convinced an internal keying device was an inherent weakness that provided a pathway to eventually break into the cipher.

In April 1932, he introduced his idea of a cipher machine with an external keying system. His machine, called the M-134 Converter, used a separate paper key tape to regulate the movement of the machine’s five rotors. This helped randomize the rotor motion, making it difficult to break into the cipher itself. The new machine went into limited production.

Friedman gave Rowlett the task of creating a quantity of paper tapes for keying the M-134 Converter. This was a laborious task. Also, Rowlett doubted the practicality of tapes, which could be torn or misused in field conditions. Furthermore, he lacked confidence in the feasibility of maintaining large inventories of the paper keying material. For months, the former schoolteacher struggled with what he considered an insurmountable assignment.

In May 1934, Rowlett had a burst of inspiration. Instead of using paper tape to key the movement of the enciphering rotors, he devised a way to use one set of rotors to generate a random stepping movement for the others. Rowlett was convinced he had hit upon the ideal solution. Enthusiastically, he told Friedman of his revelation. Unfortunately, because Rowlett’s keying rotors would be internal to the machine, they were anathema to Friedman. The director steadfastly refused to believe a machine could generate its own keying with sufficient randomness to be secure. Rowlett’s tenaciousness, nevertheless, matched his boss’s stubbornness. Over the next several months, the junior analyst continued to approach his director with his idea. Their working relationship became strained.

In the end, however, Rowlett won over Friedman with a well-prepared presentation. To each of Friedman’s comments and questions, he provided clear, reasoned answers. Finally, Friedman relented and had Rowlett begin drafting specifications for his machine. Rowlett’s persistence paid off. His keying rotors, combined with Friedman’s original design, eventually became the SIGABA, the most successful cipher device of its type. During its active service, no successful cryptanalysis of the machine ever occurred.


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• Date Taken: 05.16.2025
• Date Posted: 05.16.2025 15:17
• Story ID: 498218
• Location: US