Code breaking (Enigma)
Code breaking, particularly regarding the Enigma machine and the Japanese Purple code, played a pivotal role during World War II in intelligence and military strategy. The Enigma machine, developed in the early 20th century, utilized a complex system of rotors to encode messages, with different branches of the German military using their own variants to enhance security. Notably, significant breakthroughs in deciphering these codes emerged from the efforts of cryptanalysts at Bletchley Park, including Marian Rejewski’s discovery of the machine’s encipherment patterns and Alan Turing’s innovative approach to exploiting these patterns. The British also developed the Bombe, a mechanical device designed to assist in deciphering Enigma messages.
Meanwhile, the U.S. Army's efforts to break the Japanese Purple code were spearheaded by William F. Friedman and his team, who made critical advances in understanding the machine's intricate scrambling mechanisms. Successful code-breaking efforts contributed to key victories, such as the U.S. at the Battle of Midway and the ambush of Admiral Yamamoto. Overall, these cryptographic achievements significantly impacted military operations, highlighting the crucial intersection of technology and intelligence in warfare.
Code breaking (Enigma)
The Event The Allies achieved dramatic successes during World War II by breaking both the Japanese diplomatic Purple code and the German Enigma ciphers. The information gained from Purple became known as Magic and that from Enigma as Ultra
Date 1940 for Magic; 1940-1941 for Ultra
Places Washington, D.C., for Magic; Warsaw, Poland, and Bletchley Park, England, for Ultra
By breaking the Purple code a year before the Pearl Harbor attack, Leo Rosen and Genevieve Grotjan gave the United States a great advantage unknown to the Japanese. The brilliant work of Marian Rejewski, Henryk Zygalski, and other Polish mathematicians in the 1930’s enabled the successes of Alan Mathison Turing and the other British code breakers working at Bletchley Park during World War II.
The attack on the Japanese Purple code was led by William F. Friedman of the Army Signal Intelligence Service and the crew of talented cryptanalysts he assembled at Arlington Hall Junior College across the Potomac from Washington, D.C. The difficult technical problem Friedman’s team faced was figuring out the Purple machine scrambling patterns. Leo Rosen, an electrical engineer at the Massachusetts Institute of Technology (MIT), discovered the way the six most common letters were rotated through twenty-five contacts—a major advance, but of no help with the other twenty letters. Then, one year later, on September 20, 1940, Genevieve Grotjan had the crucial insight into the manner by which the many thousands of patterns were linked in their own pattern of cycles. Whereas a single scrambler mixed up the six common letters, three scramblers interconnected by hundreds of wires transmitted the other twenty letters. Finally, after three weeks, Rosen and Frank Rowlett soldered the last of thousands of connections, flipped a switch, typed in a Purple cipher text, and watched the deciphered message roll out of the printer.
The first Enigma machine was exhibited in Bern, Switzerland, in 1923, and over the next two decades several versions of increasing complexity, both commercial and military, were developed. The first breakthrough in solving Enigma codes came in 1932 when Marian Rejewski discovered the patterns of encipherment programmed into the machine’s three rotors, an accomplishment that stunned the French and British code breakers when they learned of it in 1939.
The Germans created separate Enigma codes for its army, air force, and navy and added more rotors to complicate the work at Bletchley Park. However, the British benefited from several advances of their own. First, they invented what they called a Bombe, an arrangement of multiple Enigma machines connected for use with IBM cards. Another great help was the capture of Enigma materials from several trawlers and from the submarines U 110 and U 559. Finally, in the most dazzling intellectual feat of the war, Alan Mathison Turing perceived that matching strings of plain and cipher text revealed a geometrical relationship, and he intuited that introducing a contradiction into an interconnected loop of Enigma machines would bypass the Germans’ built-in safeguards. Historian Stephen Budiansky has said that this idea was really the crux of Turing’s invention, an idea that went beyond ordinary brilliance.
Impact
Deciphering the Japanese Purple code played an important role in the U.S. defeat of Japan, but it was the decipherment of a crucial Japanese naval code that enabled the stunning U.S. naval victory at Midway Island in early June, 1942, and the ambush and death of Admiral Isoroku Yamamoto on April 18, 1943. Breaking the German Enigma ciphers was extremely helpful in thwarting General Erwin Rommel in Africa in 1942 and in forcing Admiral Karl Dönitz to withdraw his U-boats from the North Atlantic in May, 1943.
Bibliography
Budiansky, Stephen. Battle of Wits. The Complete Story of Codebreaking in World War II. New York: Free Press, 2000.
Kahn, David. The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet. Rev. ed. New York: Scribner, 1996.