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. Decrypting enciphered messages involves three stages, defined somewhat differently in that era than in modern cryptography. First, there is the identification of the system in use, in this case Enigma; second, breaking the system by establishing exactly how encryption takes place, and third, solving, which involves finding the way that the machine was set up for an individual message, i.e. the message key. Today, it is often assumed that an attacker knows how the encipherment process works (see Kerckhoffs's principle) and breaking is often used for solving a key. Enigma machines, however, had so many potential internal wiring states that reconstructing the machine, independent of particular settings, was a very difficult task. The Enigma machine The Enigma rotor cipher machine was potentially an excellent system. It generated a polyalphabetic substitution cipher, with a period before repetition of the substitution alphabet that was much longer than any message, or set of messages, sent with the same key. A major weakness of the system, however, was that no letter could be enciphered to itself. This meant that some possible solutions could quickly be eliminated because of the same letter appearing in the same place in both the ciphertext and the putative piece of plaintext. Comparing the possible plaintext Keine besonderen Ereignisse (literally, "no special occurrences"—perhaps better translated as "nothing to report"; a phrase regularly used by one German outpost in North Africa), with a section of ciphertext, might produce the following: Structure The mechanism of the Enigma consisted of a keyboard connected to a battery and a current entry plate or wheel (German: Eintrittswalze), at the right hand end of the scrambler (usually via a plugboard in the military versions). This contained a set of 26 contacts that made electrical connection with the set of 26 spring-loaded pins on