FROM THE ESTATE OF DR. DAVID HAMER, NOTED HISTORIAN OF CRYPTOLOGY
While the codebreakers at Bletchley had been reading messages encrypted by the 3-Rotor machines for some time, the introduction of the M4 caught them completely off-guard, shutting them completely out, and it took them a full 9 months to catch up and solve the new code. Few Enigma machines survived the War intact: the Germans destroyed them as they retreated, and for decades after the war governments around the world kept close control over Enigma technology (indeed two of Turing's wartime papers on cryptography remained classified until 2012). So secure was the system believed to be that some governments, unaware of the work of Bletchley Park, continued to use Enigmas after 1945.
The M4 was a variation of the 3-rotor Enigma I machine used by the Wehrmacht (German Army & Airforce). It accepted 3 standard rotors and a narrow fourth rotor in combination with a narrow reflector. The two narrow components allowed all four rotors to fit into a modified three-rotor Enigma chassis. It was issued with eight standard rotors, the first five having wiring identical to the rotors issued with the three-rotor Enigma-I. Two narrow fourth rotors, Beta and Gamma, were also issued with each machine. With the fourth narrow rotor in the "A" position, the M4 became functionally identical to and could communicate with the three-rotor Enigmas used by other branches of the military. With its stricter operating procedures and the ability to select from among 8 standard rotors and two thin fourth rotors gave the M4 a much higher level of security, foiling the Allies' previously successful decryption of the U-boat signals. It would take the codebreakers at Bletchley 10 months to break the new encryption. Compared with the more common 3-rotor Enigma, the M4 had many significant differences beyond the added rotors. It had a different ring-setting mechanism on the rotors, a lockable rotor cover to keep the wheel settings secret, a removable lamp panel which allowed for the inclusion of a printer (Schreibmax), a power socket for an external power source, and a metal handle instead of the more fragile leather handle found on the 3-rotor machines. The M4 also had a removable top, and two metal bracket that allowed the machines to be lifted out of the compartments in the U-boat radio rooms.
M4 Enigma machines were produced in much smaller quantities than the three-rotor Enigma I machines. In addition, multiple M4 Enigmas were deployed with each U-boat and support ship, and the majority of these were lost when their boats were sunk in combat or scuttled by their crew at the end of the war. Furthermore, German Enigma operators were under strict orders not to allow the enemy to capture any Enigmas; this meant that many Enigmas were stripped of their rotors and destroyed, with many thrown into lakes or oceans to hide any remaining parts.
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