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Alberti disk

The Caesar wheel with a scrambled inner ring. Turn it mid-message and you have invented a much harder cipher.

about 25 minutes needs scissors makes a working machine

What you need

The printed template
Card
Scissors
A split pin

How to build it

Build it like the wheelGlue to card, cut out both circles, pin them together through the middle.
Notice the inner ringIt is not the alphabet in order. It is scrambled, and that alone makes it stronger than Caesar.
Agree a starting letterLine the inner A up with an agreed outer letter and begin.
Now turn it mid-messageEvery few words, move the inner ring one place on. Write a capital in your message to show where you turned it.
Send the letter and the ruleYour friend needs the starting letter and the rule for turning, and nothing else.
Turning the disk is the whole invention. A wheel you never turn is only a Caesar shift with extra steps.

The template

Print this at 100 per cent, not fit to page, or the sizes will be wrong. The ruler line below tells you whether it printed correctly.

10 cmABCDEFGHIJKLMNOPQRSTUVWXYZQWERTYUIOPASDFGHJKLZXCVBNMthe inner ring is scrambled on purpose
The bar above should measure exactly 10 centimetres. If it does not, your printer has shrunk the page.

Now send something

Write a message and turn the disk after every fifth letter. Then count how many times the letter E came out as something different.

Where it comes from

Italy, the 1460s
The first cipher that moved

Leon Battista Alberti was an architect, a painter and the man who wrote the first book in Europe on breaking codes. His answer to the code-breakers was to stop the cipher standing still: turn the inner disc part-way through the message and every rule a code-breaker had worked out becomes wrong. He is often called the father of Western cryptography for exactly this one idea.

Why it works

Turning the disc mid-message breaks the one thing a code-breaker relies on: that a letter always becomes the same other letter. Now E might be Q here and K there, and the counting attack has nothing to hold on to.

When it goes wrong

It usually does, the first time. Almost always one of these.

Your friend cannot decode the second halfYou turned the disc and did not tell them where. Write a capital letter in the message at the point you turned it.
It is no harder than a Caesar wheelYou are not turning it. A wheel you never turn is a shift cipher with extra steps.

Where to go next

Lewis Carroll’s tableBack to Make