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Caesar cipher

By David Shaw, 3 March 2015

Learn the Caesar cipher, one of the oldest codes ever devised!

You will need

Push the pin through the centre of the circle.

• Scissors
• Split pin
• Copy of the discs – download the discs in PDF format
• paper and pens to write messages with

Make the disc

1. Cut out the two discs.
2. Push the split pin through the centre of the back of the large disc.
3. Push the split pin through the centre of the back of the small disc. Then open up the legs of the pin so it holds the two discs together.

To use the cipher disc

Open the legs of the split pin to hold your disc together.

1. Line up the ‘A’ with the ‘a’.
2. Choose a code number between 1 and 26.
3. Rotate the inner disc clockwise by this number of letters. For example, if your code number was 3, ‘a’ would line up with ‘D’.

Encode text

1. Look at the first letter in your text.
2. Find that letter on the outside ring of the disc (in capitals).
3. Write down the inside letter that lines up with it.
4. Repeat with the next letter until you’ve encoded each letter.

Decode text

Line up two letters so you can use your disc for encoding and decoding!

1. Look at the first letter in your encoded text.
2. Find that letter on the inside ring of the disc (in lower case).
3. Write down the outside letter that lines up with it.
4. Repeat with the next letter, until you’ve decoded each letter.

What’s happening?

This code is called a Caesar cipher. It works by moving each letter up the alphabet the same number of letters. This way, every original letter has its own new coded letter to replace it. It is named after Julius Caesar, who used it for his communications when he invaded Gaul (France) around 50 BCE. Julius always rotated his disc by three letters to encode his messages.

The Caesar cipher is not a very hard code to crack. If you know someone is using a Caesar cipher, you can try decoding the first word or two with the disc rotated one letter, and then rotated two letters, and so on. If you find something that makes sense, you are almost certainly decoding it properly. It’s not completely certain, though. There are a few words that can be decoded in two different ways. For instance, “vgddk” could be shifted by one place to make ‘wheel’, or shifted 8 places to make ‘dolls’.

Even if you didn’t know that the encoded text was a Caesar cipher, you could still crack this code. Because you are directly substituting one letter for another, although the letters are changed, some patterns are kept. In English, some letters occur a lot more often than others. For instance, ‘e’ and ‘t’ are the most common letters in English, so the letter that appears most often in the encoded text is probably an ‘e’ or a ‘t’ Once you guess some of the letters, you can use them as clues to work out the rest.

Real-life maths

Codes are very important if you want to pass on a message without letting anyone else know what you are saying. They are particularly important during wars, and when spying on people, but they are also used in our everyday lives.

Online email services use codes to check who you are. When you type in your password, your computer puts it into a code and sends it to your email provider. If it wasn’t coded, then hackers could intercept your password, and then they could access your email. If you ever go to a website with an address that starts with ‘https’ instead of ‘http’, then your computer is using a code.

The codes that are used in computers are much harder to crack than the Caesar cipher. Public key codes have a public codeword and a private codeword. Messages coded with the public codeword can only be deciphered with the private codeword, and privately coded messages can only be decoded by the public codeword. This type of code is used for a lot of the coded messages on the internet.

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