<P> 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, setting, 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 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 . </P> <P> 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 . </P> <P> 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"), with a section of ciphertext, might produce the following: </P> <Table> Exclusion of some positions for the possible plaintext Keine besonderen Ereignisse <Tr> <Th> Ciphertext </Th> <Td> O </Td> <Td> </Td> <Td> J </Td> <Td> Y </Td> <Td> </Td> <Td> </Td> <Td> O </Td> <Td> </Td> <Td> Q </Td> <Td> </Td> <Td> J </Td> <Td> </Td> <Td> O </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> Y </Td> <Td> </Td> <Td> O </Td> <Td> </Td> <Td> J </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> U </Td> </Tr> <Tr> <Th> Position 1 </Th> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> O </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Th> Position 2 </Th> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> O </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Th> Position 3 </Th> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> O </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Th> </Th> <Td_colspan="31"> Positions 1 and 3 for the possible plaintext are impossible because of matching letters . <P> The red cells represent these crashes . Position 2 is a possibility . </P> </Td> </Tr> </Table>

What was the main weakness of the enigma machine