Instructions for making your own machine

This step will show you how to build a simplified Enigma machine using only a Pringles tube.

The wiring in the model is accurate – that is the genuine wiring from one of the rotors used in World War 2, and the wiring on Reflector B is the accurate wiring for that reflector (first introduced on 2nd Nov 1937, and left unchanged throughout the entire War). But this model will only have one rotor, not three, and does not have any plugboard. (If you wish to build a working model that is 100% accurate, at the bottom of this page you can find an additional download that provides the necessary additional rotors and plugboard.)

How to build the model

First print off the attached pdf. It doesn’t need to be printed in colour, although that helps follow the wires. It does need to be printed full size. (In Acrobat Reader, make sure “Actual Size” is ticked, and not “Fit” or “Shrink Oversize pages” as that will mean it won’t perfectly fit around a Pringles tube.)

Now cut out the various strips: Reflector B, Rotor I, and Input/Output.

Take a Pringles tube (or any other cylinder with diameter of about 7.5cm and circumference of 23.5cm) and stick Reflector B around the tube, ensuring the “wires” are pointing inwards. This should be stuck to the tube so it doesn’t rotate.

Next take Rotor I. This needs to also be wrapped around the tube so that its letters touch the ends of the “wires” from Reflector B. However, it’s important this is not stuck to the tube, as it will need to rotate when we use the model. Glue the little tab just beneath the letter A, and then wrap the Rotor I around the tube, sticking the other end (with letter Z) over the glued tab.

Finally, the Input/Output needs to be stuck down onto the tube. Like the Reflector, this doesn’t need to rotate and should be stuck onto the tube. However, it’s very important that the grey line (around letter A) lines up exactly with the grey line on the Reflector.

You have successfully built a working simplified model of an Enigma machine! The completed model should look like this:

How to use the model

Put the machine in its Base Setting, which means with the grey lines all aligned. You may need to rotate the Rotor in order to achieve this.

Next, pretend to press the letter C on the Input/Output. Now move the rotor down by one click, so the letter D on the rotor is directly opposite the letter C on the Input/Output. Now follow the wires:

• in the Rotor, the letter C goes to F.
• in the Reflector that gets sent to the letter R.
• keep following the wires back through the rotor, and it ends up at W on the Input/Output, and this is your encrypted letter.

Here is the path you should have taken:

Continuing with our example, let’s press another key. For the second key press, let’s press the letter A. Move the rotor down by one and follow the wires again. You should go from A through the rotor to M, through the Reflector to P then back through the Rotor to R.

So A is encrypted to R on the second key press.

Finally, on the third key press, let’s press the letter T. As always, move the rotor down by one. This time the wires go from T through the Rotor to B, through the Reflector to D, then back through the rotor to D on Input/Output.

So in conclusion, CAT is encrypted to WRD.

When you wish to decrypt, make sure you put the machine back into its Base Setting, with the three grey lines all aligned. If you do that, and type in WRD, remembering to turn the rotor down by one click each time, you should get back CAT.

Summary

So the key points are:

1. Always start with the machine in the base setting
2. On each key press, always move the rotor on by one
3. Follow the wires from the “input”, through the rotor, through the reflector, through the back of the rotor, and where you end up in the “output” is your encrypted letter.