SIS Makes Breakthrough Against Japanese Code (20 SEP 1940)

Photo By Erin Thompson | On 20 September 1940, Genevieve Grotjan, a 27-year-old statistician, discovered a pattern in several Japanese ciphered messages. The discovery provided an entry point that led to one of the greatest achievements in cryptanalysis: the breaking of Japan’s highest-level diplomatic cipher system, often referred to as PURPLE.  

UNITED STATES

09.20.2023

Courtesy Story

U.S. Army Intelligence Center of Excellence

by Michael E. Bigelow, INSCOM Command Historian

20 SEPTEMBER 1940
On 20 September 1940, Genevieve Grotjan, a 27-year-old statistician, discovered a pattern in several Japanese ciphered messages. The discovery provided an entry point that led to one of the greatest achievements in cryptanalysis: the breaking of Japan’s highest-level diplomatic cipher system, often referred to as PURPLE.

Since February 1939, William F. Friedman’s team of Signals Intelligence Service (SIS) cryptanalysts struggled to solve the PURPLE cipher. Eighteen months earlier, the Japanese had switched to PURPLE from the older RED cipher, which the SIS had largely solved. The result was a virtual blackout in Washington’s access to Tokyo’s diplomatic traffic.

Frank B. Rowlett’s Japanese team had determined the PURPLE cipher included two groups of enciphered letters (one of six and another of twenty). After six months of pencil and paper work, they discovered how PURPLE enciphered the smaller of the two groups of letters. Leo Rosen, a recent graduate of the Massachusetts Institute of Technology, then devised a mechanism to speed the process of revealing that letter group in a particular message. This produced a skeleton of the message that allowed the team to occasionally guess the meaning of individual words.

To make further progress, Rowlett’s team of Robert Ferner, Genevieve Grotjan. Leo Rosen, and Albert Small worked to find a pattern in how the larger group of letters were enciphered. If they could identify the pattern, they might be able to duplicate the process and break into the cipher system.

The pressure for a solution grew. Maj. Gen. Joseph Mauborgne, the Army’s chief signal officer, took a personal interest in the problem and visited the Japanese section almost daily to discuss the work and encourage the staff. Friedman put aside his administrative duties and studied the cryptographic principles underlying the yet unseen PURPLE machine. Rowlett worked fourteen-hour days. For about a year, the team struggled.

Then, on a hot Washington afternoon, the hard work paid off. Ms. Grotjan, who had come to the SIS from the Railroad Retirement Board only a year earlier, quietly approached Rowlett who was reviewing current efforts with Ferner and Small. Politely interrupting the session, she asked if she could show them what she had found. With the three men looking at the sheets that covered her desk, she pointed to a line on one sheet, then a line on another. She stepped back and silently waited.

Working with six messages that had been partially reconstructed with Rosen’s device, Grotjan had noticed the presence of repeated sequences in the cipher texts of the larger letter group. This discovery provided the crucial entry into what had seemed an impenetrable cipher. Al Small saw it immediately. He yelled, “Whoopee!" and ran around the small room, clasping his hands above his head like a victorious prizefighter. Bob Ferner, normally staid and quiet, clapped his hands like an excited child and shouted "Hooray, hooray!" Rowlett jumped up and down and shouted, "That's it! That's it!" Grotjan’s eyes filled with tears.

As people from other sections came to the room to investigate the commotion, Friedman pushed his way to Rowlett. The team chief pointed to Grotjan’s worksheets and noted “Look what Gene has just discovered." Friedman immediately understood the significance. To mark the moment, the team sent out for Cokes.

One week later, on 27 September, Rowlett handed Friedman two complete, deciphered PURPLE messages. In the subsequent months as more of the cipher’s secrets were uncovered, this initial trickle became a steady stream of deciphered messages of one of the U.S. Army’s most important intelligence sources during World War II.

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• Date Taken: 09.20.2023
• Date Posted: 09.22.2023 10:49
• Story ID: 454045
• Location: US