What is a Caesar cipher?

A code that moves every letter along the alphabet by the same number of places, named after Julius Caesar, who used one for his letters. The number of places is the only secret: if you know it you can read the message, and if you do not you have twenty-five things to try.

Why does the working disappear when I turn the wheel?

Because the letters already worked out were worked out with the old wheel, and a board showing a code from one shift beside a wheel set to another is a board contradicting itself. Turn it back and the letters come from the new setting.

What happens at a shift of nought?

Nothing, and the board says so. Every letter stays where it is and the message is not in code at all. It is worth asking a class about, and so is a shift of twenty-six, which comes all the way round to the same thing.

Can we type our own message?

No. Nothing in these tools is typed, because a teacher at a board has a finger rather than a keyboard, and the words come from a bank instead. The wheel is the part worth having on the board anyway: with it up there a class can code anything they like into their books.

Code Wheel

Computing · Year 3–Year 6

Turn the wheel, put a word into code, read a code back.

About the code wheel

Two rings of letters, one inside the other. Turn the inner ring round by three and every letter has a new letter inside it: A becomes D, B becomes E, and a word turns into something nobody can read. That is a Caesar cipher, and it is about two thousand years old.

The board takes a word and works through it a letter at a time. Find the letter on the outside, read the letter inside it, and press Next letter to check. Then press Out of code and the same wheel reads a secret message back the other way.

It is real computing rather than a puzzle page: a cipher is an algorithm, the shift is its one input, and the same set of steps run backwards undoes it. That last part is worth saying out loud, because it is why anybody can read the message at the other end.

Every letter stays upright as the ring turns. A ring of letters that simply rotated would put half the alphabet upside down at the bottom, which is a picture a class has to tilt their head to read.

A shift of nought is offered, and it says what it does: nothing at all. It is the best question on the board, because a code that leaves every letter alone is not a code.

How to use it on your whiteboard

1Use the plus and minus to turn the inner ring. The shift is the number in the middle.
2Look at the first letter of the word, find it on the outside ring, and read the letter inside it.
3Press Next letter to check, and carry on to the end of the word.
4Press Another for a different word, or Out of code to read a message back.
5Ask the class what happens at a shift of nought, and at a shift of twenty-six.

Questions

What is a Caesar cipher?: A code that moves every letter along the alphabet by the same number of places, named after Julius Caesar, who used one for his letters. The number of places is the only secret: if you know it you can read the message, and if you do not you have twenty-five things to try.
Why does the working disappear when I turn the wheel?: Because the letters already worked out were worked out with the old wheel, and a board showing a code from one shift beside a wheel set to another is a board contradicting itself. Turn it back and the letters come from the new setting.
What happens at a shift of nought?: Nothing, and the board says so. Every letter stays where it is and the message is not in code at all. It is worth asking a class about, and so is a shift of twenty-six, which comes all the way round to the same thing.
Can we type our own message?: No. Nothing in these tools is typed, because a teacher at a board has a finger rather than a keyboard, and the words come from a bank instead. The wheel is the part worth having on the board anyway: with it up there a class can code anything they like into their books.