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ABSTRACT: Only in 1974 did German intelligence and cryptologists admit that the Enigma cipher machine was not, and had not been, a secure system. Throughout World War II, German experts relied on a theoretical statistical security that took neither wartime operational reality nor their opponents' years of attention and attack into account. They ignored the far more important operational weaknesses and human errors that actually provided enemy cryptanalysts with their most valuable entries into the cipher system.
KEYWORDS: World War II, Enigma, Ultra, signal intelligence, signal security, operational security, stastistics, German military intelligence, Wehrmacht, machine ciphers, cryptanalysis, cribs, key repeats, re-encodements.
"[W]e believe that the enigma cannot be solved...Enigma when used according to instructions is unbreakable. It might be broken if a vast Hollerith [punch card machine] complex is used but this is only slightly possible."
Dr. Erich Huttenhain, cryptologist, Cipher Bureau of the German High Command (Chi/OKW), in postwar interrogations.1
German intelligence officers ended World War II certain of one thing: Enigma machine ciphers had kept their communications secure. After the war, Allied officers interrogated German cipher machine operators, cipher experts and military officers in countless conversations and written homework. Over and over German intelligence officers and cryptologic experts repeated their confidence in the Enigma machine's security. They had, after all, conducted numerous investigations of their cipher systems' security. A quarter century later, Heinz Bonatz, wartime head of the Kreigsmarine [German navy] intelligence division, wrote
...the Enigma Cipher Machine M was secure against break-in and the German naval radio signals could not be read.2
Then in 1974, F. W. Winterbotham's book, The Ultra Secret, revealed that the Allies had read Enigma ciphers throughout the war. Bonatz and other intelligence officers had to admit, finally, that Enigma was not, and had not been, a secure system. In the rush of works that followed, the Germans learned that Enigma had been compromised almost from its adoption by the Heer [Army] in 1928 - the Polish mathematician Marian Rejewski had made the first break into the machine in 1932.
Gradually a remarkable - and now familiar - story emerged, revealing the Allied development and use of intelligence based on decrypts of Enigma material. The success of this Allied decryption effort raises questions about German intelligence and...