September 11, 1940, The U.S. Army and Navy signed an agreement on joint exploitation of Japanese...

Conquering the “sixes” was critical, but there was still the even more difficult challenge of the “twenties.” Luckily, by September 1940, the Purple team knew enough to uncover the general patterns required to crack the remaining 20 letters.

Using the insights culled from the conquering of the sixes and the twenties, Rosen made the additional modifications to his original prototype. Each section of the “Purple analog” connected to the next through 500 wires. Lastly, Rosen added a typewriter to feed the encrypted traffic into the device and an additional typewriter to spit out the deciphered text.

Incredibly, after countless hours of painstaking effort, the work was finally done. It was time to test the system. Historian Stephen Budiansky notes.

“Late one night, Rosen and Rowlett plugged in the power supply and flipped the main switch. Rowlett began to type in the cipher text of a Purple message. The two cryptanalysts watched in awe as deciphered Japanese text began to emerge from the printer.”