PAPER ENIGMA MACHINE, THE

Cryptologia, Jul 2004 by Koss, Mike

ABSTRACT: The Enigma Machine is a complex electromechanical device used by the Germans in World War II to achieve what they thought was complete communications security. While the original machine weighed over 20 lbs, the central mechanics of the machine can be simulated manually by manipulating strips of paper. A Paper Enigma is presented that can be cut out of a single sheet of paper. The resulting simulator is compatible with the electromechanical original in that messages can be encoded on one, and decoded on the other. Copies of The Paper Enigma can be downloaded from http://mckoss.com/crypto/enigma.htm.>KEYWORDS: Enigma Machine, World War II, cryptography, simulation.

INTRODUCTION

Having been fascinated with codes and secret writing since I was young, I had a special fascination for the mechanical cryptographic machines I read about in Martin Gardner's Codes, Ciphers, and secret Writing, and in David Kahn's The Code Breakers. While there are several museums where you can see some of these devices they are usually placed behind a glass case where you cannot see for yourself how they work.

With the advent of the Internet, it became possible for people with obscure hobbies like collecting cryptographic machines to find others with similar interests and a market was born on sites like eBay. I first realized that I could own an Enigma of my own via a Bletchley Park email mailing list. A collector in Washington DC emailed the list indicating he had an Enigma to sell. We agreed on a price and I flew to Washington (from Seattle) to inspect the machine. We also took it to the National Cryptological Museum (http://www.nsa.gov/museum/) to check it for authenticity.

Satisfied, I returned home to Seattle with my first historical cryptographic machine. I found replacement bulbs and wired up a battery so I could put the machine though its paces. Over the years, I've enjoyed demonstrating the workings of the Enigma to friends, and I have taken it to local area schools to lecture on the workings and historical significance of the Enigma.

In 2003, I had the pleasure of lecturing to the MIT Alumni Club of Puget Sound. Knowing my audience would be very technical, I wanted to do more than have a simple show and tell. These folks would want to get their hands "dirty" and actually learn the inner workings of the machine. There are several Enigma simulators available on the Web, but I wanted to be able to create a tangible "kit" that each participant could build and use on their own. Thus was born, The Paper Enigma.

The Paper Enigma requires only a pair of scissors to assemble a working replica of the German Enigma machine; messages encoded on one can be decoded on the other! The Paper Enigma is simplified in two respects. First, it lacks a plug board (stecker) which adds an additional layer of a single transposition cipher to the Enigma. While this innovation, added to the military version of the Enigma, added an extra layer of complexity to the original Enigma, it does not change the fundamental operation of the machine.

Second, there are no "ring settings". These merely allow the inner wiring of each rotor to be rotated with respect to the outer indicator settings. The only cryptographic effect is that the rotors will "roll-over" at a different position. I've left this off for simplicity as it does not change the basic workings of the machine.

The fundamental operation of the Enigma is quite simple. When a keyboard letter is pressed, an electrical contact is made on one of 26 wires entering on the right hand side of the rotor system. That signal is then carried through the three rotors from right to left. The internal wiring of a rotor simply permutes each of the 26 input contacts on the right to one of the 26 output contacts on the left.

After traveling from right to left through all three rotors, the signal is then "reflected" by a fixed permutation (via a "reflecting rotor") and then travels in reverse through all three wheels again (this time from left to right). The 26 contacts are connected to light bulbs so that the ultimate code letter is illuminated on a lamp for as long as the keyboard key is held down.

The right hand rotor rotates to the next letter position when each key is pressed, thus changing the final alphabet permutation for every letter of the message. The other rotors can also rotate as explained below.

An excellent technical description of the workings of the Enigma can be found at Tony Sale's web site: http://www.codesandciphers.org.uk/enigma/.>HOW TO USE THE PAPER ENIGMA

Each part of the Enigma machine is modeled by a corresponding component of the Paper Enigma:

To assemble the Paper Enigma, simply cut out the three rotor strips and place each one over the columns marked "Left Rotor", "Center Rotor", and "Right Rotor".

OPERATION

To use the Paper Enigma, one need know only how to emulate the motion of the rotors after each letter, and be able to trace the path of the letter permutations through the rotor strips. It's probably easiest to follow a specific example (this example is printed on the Paper Enigma in light gray).