General Principles
The Enigma machines produced a polyalphabetic substitution cipher. During World War I, inventors in several countries realized that a purely random key sequence, containing no repetitive pattern, would, in principle, make a polyalphabetic substitution cipher unbreakable. This led to the development of rotor cipher machines which alter each character in the plaintext to produce the ciphertext, by means of a scrambler comprising a set of rotors that alter the electrical path from character to character, between the input device and the output device. This constant altering of the electrical pathway produces a very long period before the pattern—the key sequence or substitution alphabet—repeats.
Deciphering enciphered messages involves three stages, defined somewhat differently in that era than in modern cryptography. Firstly, there is the identification of the system in use, in this case Enigma; secondly, breaking the system by establishing exactly how encryption takes place, and thirdly, setting, which involves finding the way that the machine was set up for an individual message, i.e. the message key. Today, it's often assumed that an attacker knows how the encipherment process works and breaking specifically refers to finding a way to infer a particular key or message (see Kerckhoffs's principle). Enigma machines, however, had so many potential internal wiring states that reconstructing the machine, independent of particular settings, was a very difficult task.
Read more about this topic: Cryptanalysis Of The Enigma
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