  The Enigma machine was a device used by the Germans to cipher and decipher
messages during World War II.  The breaking of its code by the British was a
major contribution to the war effort.
  The machine, which had no printing or transmitting capabilities, measured
13.5" by 11" by 6" and weighed about 26 pounds.  It consisted of a keyboard
connected to a plugboard, that was connected to three rotating wheels, called
rotors.  There were five prewired wheels (I, II, ..., V) available to be used
as rotors. Each wheel had the letters A through Z on one side, wired to A
through Z on the other side.  For example, A might be wired to D, B to L, and
so on.
  Before ciphering or deciphering a message, a code book was used to select
the three rotors from these five prewired wheels based on the date.  The code
book also specified the initial setting for each rotor.
  The plugboard connected the keyboard to the three rotors and it too could
be wired.  The plugboard connections were also specified in the code book.
  The Enigma machine was used to encode a message and if the same rotors and
settings were used, the coded message was automatically decoded.  Thus, if
the first letter of the uncoded message was A and it was coded as D, then the
first letter of the encoded message, D, was decoded as A.
  The machine worked in the following way.  A letter was entered on the
keyboard which the plugboard could change to another letter, depending on its
settings.  This letter went into rotor 1 on one side and came out as another
letter, depending on the wiring of rotor one.  The letter from rotor 1 went
to the letter on rotor 2 across from it, and out the other side, and so on.
The letter was then bounced back by a fourth fixed wheel, through rotors 3,
2, 1, and the plugboard, lighting up the ciphered letter.  At this stage
rotor 1 would rotate one letter, and the process would continue.  After rotor
1 rotated 26 times rotor 2 would rotate once.  After rotor 2 rotated 26 times
rotor 3 would rotate once.
  For more details on the Enigma machine, see "Code Breakers: The Inside
Story of Bletchley Park" by F.H. Hinsley and A. Stripp, Oxford University
Press, 1993, and the references cited there.
  This software package simulates an Enigma machine.
