Paper Enigma Machine
by Mike Koss
April, 2005 - FAQ: Detailed Explanation of Rotor Motion
I've had many fine emails from people who have enjoyed the Paper Enigma. Many have been confused about the subtlety of the motion of the rotors. One of the benefits of building and using a model like this is that you can gain a deeper understanding of the details of the Enigma machine.
that the VERY FIRST STEP before encoding (or decoding) each character is to move
the rotors. That is why, even though the starting position of my sample is
"MCK" - the rotors are moved to the "MCL" position while decoding the first
letter. Also note that any rotor motions should be done in one step (the
actual Enigma machine uses a pawl to rotate 1, 2, or all 3 rotors all at once
when a letter key is pressed - and before a lamp is lit with the encoded letter).
The right rotor always moves up by one space; the other rotors move up if
the rotor to their right has the up-arrow symbol in the first row.
No rotor will move more than one position for each letter encoded.
Before encoding each letter, there are only three possibilities for rotor motion:
Right and Middle
Right, Middle, and Left all move
Follow these rules to determine which rotors move:
If your starting position has the middle rotor arrow (notch) in the top position -> all three rotors will move.
If your starting position has the right rotor arrow in the top position (but not the middle rotor arrow) -> the Right and Middle rotors move.
All other times, just the Right rotor moves.
Note that the notch on the left rotor position is irrelevant - it cannot effect the rotor motion at all. Perhaps this table sums it up best:
no no Right only no yes Right and Middle yes no All Three Rotors yes yes All Three Rotors
All rotors move at the same time based on the position of the notches BEFORE any of them are moved. You have to look at where the arrows are, use the rules above to determine which of the three conditions apply, and then move 1, 2, or all 3 rotors accordingly.
January, 2005 - User feedback
Special thanks to René Tajoburg (Vienna, Austria) for his innovation to cut slits in the background to more permanently hold the strips in place. It's optional, but I really like how it holds all the pieces together.
January 21, 2003 - MIT Club Lecture
The Paper Enigma began as a lecture given to the MIT Club of Puget Sound. In addition to showing the actual Enigma (and other coding machines in my collection), I wanted to provide a hands-on experience for this relatively technical audience. So I created the Paper Enigma, and handed out scissors so that the club members could learn by doing (in the MIT tradition).
Photos courtesy John Malley