Editing Enigma machine (section)

=== Reflector ===
[[File:Enigma insides.agr.jpg|thumb|left|Internal mechanism of an Enigma machine showing the type B reflector and rotor stack]]
With the exception of models ''A'' and ''B'', the last rotor came before a 'reflector' (German: ''Umkehrwalze'', meaning 'reversal rotor'), a patented feature<ref name="Korn"/> unique to Enigma among the period's various rotor machines. The reflector connected outputs of the last rotor in pairs, redirecting current back through the rotors by a different route. The reflector ensured that Enigma would be [[Involution (mathematics)|self-reciprocal]]; thus, with two identically configured machines, a message could be encrypted on one and decrypted on the other, without the need for a bulky mechanism to switch between encryption and decryption modes. The reflector allowed a more compact design, but it also gave Enigma the property that no letter ever encrypted to itself. This was a severe cryptological flaw that was subsequently exploited by codebreakers.

In Model 'C', the reflector could be inserted in one of two different positions. In Model 'D', the reflector could be set in 26 possible positions, although it did not move during encryption. In the ''Abwehr'' Enigma, the reflector stepped during encryption in a manner similar to the other wheels.

In the German Army and Air Force Enigma, the reflector was fixed and did not rotate; there were four versions. The original version was marked 'A',{{sfn|Marks|Weierud|2000}} and was replaced by ''Umkehrwalze B'' on 1 November 1937. A third version, ''Umkehrwalze C'' was used briefly in 1940, possibly by mistake, and was solved by [[Hut 6]].{{sfn|Marks|2001|pp=101–141}} The fourth version, first observed on 2 January 1944, had a rewireable reflector, called ''Umkehrwalze D'', nick-named Uncle Dick by the British, allowing the Enigma operator to alter the connections as part of the key settings.{{Clear}}