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The Navy was the first military branch to adopt Enigma. This version, named Funkschlüssel C ("Radio cipher C"), had been put into production by 1925 and was introduced into service in 1926.[27]
The keyboard and lampboard contained 29 letters—A-Z, Ä, Ö and Ü—which were arranged alphabetically, as opposed to the QWERTZU ordering.[28] The rotors had 28 contacts, with the letter X wired to bypass the rotors unencrypted.[9]
Three rotors were chosen from a set of five[29] and the reflector could be inserted in one of four different positions, denoted α, β, γ and δ.[30] The machine was revised slightly in July 1933.[31]
By 15 July 1928,[32] the German Army (Reichswehr) had introduced their own version of the Enigma—the Enigma G, revised to the Enigma I by June 1930.[33] Enigma I is also known as the Wehrmacht, or "Services" Enigma, and was used extensively by German military services and other government organisations (such as the railways[34]), before and during World War II.
The major difference between Enigma I and commercial Enigma models was the addition of a plugboard to swap pairs of letters, greatly increasing cryptographic strength. Other differences included the use of a fixed reflector and the relocation of the stepping notches from the rotor body to the movable letter rings. The machine measured 28×34×15 cm (11 in×13.5 in×6 in) and weighed around 12 kg (26 lb).[35]
By 1930, the Army had suggested that the Navy adopt their machine, citing the benefits of increased security (with the plugboard) and easier interservice communications.[36] The Navy eventually agreed and in 1934[37] brought into service the Navy version of the Army Enigma, designated Funkschlüssel ' or M3. While the Army used only three rotors at that time, the Navy specified a choice of three from a possible five.[38]
In December 1938, the Army issued two extra rotors so that the three rotors were chosen from a set of five.[33] In 1938, the Navy added two more rotors, and then another in 1939 to allow a choice of three rotors from a set of eight.[38] In August 1935, the Air Force introduced the Wehrmacht Enigma for their communications.[33]
A four-rotor Enigma was introduced by the Navy for U-boat traffic on 1 February 1942, called M4 (the network was known as Triton, or Shark to the Allies). The extra rotor was fitted in the same space by splitting the reflector into a combination of a thin reflector and a thin fourth rotor.
There was also a large, eight-rotor printing model, the Enigma II. In 1933 the Polish Cipher Bureau detected that it was in use for high-level military communications, but that it was soon withdrawn, as it was unreliable and jammed frequently.[39]
The Abwehr used the Enigma G (the Abwehr Enigma). This Enigma variant was a four-wheel unsteckered machine with multiple notches on the rotors. This model was equipped with a counter which incremented upon each key press, and so is also known as the "counter machine" or the Zählwerk Enigma.
During World War II the Abwehr used these machines to control and report the locations of submarines in the Atlantic and to pass information about bombing raids, the movement of military units, and the location and cargo of military supply ships. Before Enigma Britain was in danger of being starved into submission and after it the roles were virtually reversed. The British moved one step ahead of the Germans and began sinking submarines faster than they could be built.