Description
The key to the Hebern design was a disk with electrical contacts on either side, known today as a rotor. Linking the contacts on either side of the rotor were wires, with each letter on one side being wired to another on the far side in a random fashion. The wiring encoded a single substitution alphabet.
When the user pressed a key on the typewriter keyboard, a small amount of current from a battery flowed through the key into one of the contacts on the input side of the disk, through the wiring, and back out a different contact. The power then operated the mechanicals of an electric typewriter to type the encrypted letter, or alternately simply lit a bulb or paper tape punch from a teletype machine.
Normally such a system would be no better than the single-alphabet systems of the 16th century. However the rotor in the Hebern machine was geared to the keyboard on the typewriter, so that after every keypress, the rotor turned and the substitution alphabet thus changed slightly. This turns the basic substitution into a polyalphabetic one similar to the well known Vigenère cipher, with the exception that it required no manual lookup of the keys or cyphertext. Operators simply turned the rotor to a pre-chosen starting position and started typing. To decrypt the message, they turned the rotor around in its socket so it was "backwards", thus reversing all the substitutions. They then typed in the ciphertext and out came the plaintext.
Better yet, several rotors can be placed such that the output of the first is connected to the input of the next. In this case the first rotor operates as before, turning once with each keypress. Additional rotors are then spun with a cam on the one beside it, each one being turned one position after the one beside it rotates a full turn. In this way the number of such alphabets increases dramatically. For a rotor with 26 letters in its alphabet, five such rotors "stacked" in this fashion allows for 26 = 11,881,376 different possible substitutions.
William F. Friedman attacked the Hebern machine soon after it came on the market in the 1920s. He quickly "solved" any machine that was built similar to the Hebern, in which the rotors were stacked with the rotor at one end or the other turning with each keypress, the so-called fast rotor. In these cases the resulting ciphertext consisted of a series of single-substitution cyphers, each one 26 letters long. He showed that fairly standard techniques could be used against such systems, given enough effort.
Of course, this fact was itself a great secret. This may explain why the Army and Navy were unwilling to use Hebern's design, much to his surprise.
Read more about this topic: Hebern Rotor Machine
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