Scytale

about 10 minutes easiest one here makes a working machine

What you need

A round rod: a pencil, a broom handle, a rolling pin

A long strip of paper about 2 cm wide

Sticky tape

A pen

How to build it

Cut your stripLong and thin. Two centimetres wide is about right, and as long as you can manage.

Tape one end to the rodThen wind the strip round in a spiral so the edges touch but do not overlap.

Write along the rodNot round it. Write your message down the length, one letter per turn of paper.

Unwind itThe letters are now scattered along a meaningless strip.

Send the strip onlyNever the rod. Your friend needs a rod of exactly the same thickness.

A different thickness of rod gives a different answer. Agree on the object first: both use a pencil, or both use a broom handle.

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.

The bar above should measure exactly 10 centimetres. If it does not, your printer has shrunk the page.

Now send something

Write MEET ME BY THE GATE AT FOUR. Unwind it, look at the nonsense you are holding, then wind it back on and watch it come together.

Where it comes from

Sparta, about 400 BC

The stick that carried orders to a battlefield

Spartan commanders carried a wooden rod called a scytale. A strip of leather wound round it, written along the length, unwound again: what was left was a jumble. Only a rod of the same thickness would line the letters up. Plutarch describes generals receiving strips and reaching for the matching rod. It is the oldest cipher machine anybody has described, and you are about to build one out of a pencil.

Why it works

The letters are never changed, only moved. Count them: exactly the same letters are on the strip as in your message. That makes it a transposition, and it means counting letters will never break it. The rod is the key, and the key is an object rather than a word.

When it goes wrong

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

The letters do not line up when you wind it backThe strip has overlapped somewhere, or slipped along. Wind it again with the edges touching but not on top of each other.

Your friend gets nonsenseDifferent rod. It has to be the same thickness, not just a similar one: a pencil and a pen will not agree.

The strip tearsPaper strips are stronger along the grain. Cut them the long way down the page.

Where to go next

The rail fence Back to Make