Make your own Caesar wheel

In this activity we’ll show you how to create a Caesar wheel, which will make it really easy to work with shift ciphers.

It’s worth taking the time to make a wheel at this stage, since you will need it again later this week when we study the Vigenère Cipher.

To make the wheel:

• download and print out the pdf file which is linked to below
• cut around the two circles indicated, and carefully cut out the hatched rectangle on the smaller one, to make a window
• put a split-pin through the two circles, with the smaller one on top of the larger one. (If you don’t have a split pin, then find something else to put through the centre – just make sure that the small wheel is free to turn.)

You should now have something that looks like this:

Now let’s use the wheel to encipher some text. First we need to choose the shift, (d), that we’re going to use. Let’s suppose that we choose (d=5). This means that we’re going to encipher our plaintext by adding 5 (mod 26) to each letter: (Ato F), (Bto G), etc. So we turn the two parts of the Caesar wheel until the number 5 appears in the Shift window, at which point the plaintext letter (A) (on the inner wheel, in black) is aligned with the ciphertext letter (F) (on the outer wheel, in red). It should look like this:

Using the wheel it’s now very simple to encipher and decipher messages: find the plaintext letter in the inner wheel, and read off the corresponding ciphertext letter from the outer wheel. Just make sure that you remember which wheel is plaintext and which is ciphertext!

Your turn!

Once you’ve made your Caesar wheel, why not show it off by uploading a picture to our Padlet? You can also see what others have done (and whether anyone has customised their wheels!).

Then try the following exercises, to check your understanding; use the comments to report how you’re getting on, but be careful not to give away the answers!

1. With the wheel set up as above (i.e. using (d=5)), decipher the following message: RD BMJJQ BTWPX BJQQ
2. Using the shift (d=17), decipher the following message: DRKYJ ZJ XIVRK
3. The next message was encrypted using a shift cipher. Can you work out what value of (d) was used, and then decrypt the message? T XLVP L YZTDP