Cryptanalysis of The Enigma - The Enigma Machines

The Enigma Machines

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"), with a section of ciphertext, might produce the following:

Exclusion of possible positions for the possible plaintext Keine besonderen Ereignisse
Ciphertext O H J Y P D O M Q N J C O S G A W H L E I H Y S O P J S M N U
Position 1 K E I N E B E S O N D E R E N E R E I G N I S S E
Position 2 K E I N E B E S O N D E R E N E R E I G N I S S E
Position 3 K E I N E B E S O N D E R E N E R E I G N I S S E
Positions 1 and 3 for the possible plaintext are impossible because of matching letters.

The red cells represent these crashes. Position 2 is a possibility.

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