Track: Unplugged · Lesson 10 of 10 · Ages: 7+ · Time: 45 minutes
Spies, explorers and pirates in stories all send secret messages. You can too! In this lesson you will make a paper code wheel and learn the same trick that a Roman leader used over two thousand years ago.
Beep says A secret code is just a rule for swapping letters. If you know the rule, you can scramble a message and unscramble it again. Beep boop!
What you need
- Thin card or stiff paper, such as a cereal box.
- A bowl and a smaller cup to draw round.
- A pencil and a pen.
- Round-ended scissors, used by a grown-up or with a grown-up nearby.
- A paper fastener (a split pin) to join the middle, or just use your finger.
Safety first Use scissors only with a grown-up nearby, and cut slowly away from your fingers. Ask a grown-up to make the hole in the middle (a hole punch is the safest way). No knives!
Grown-ups The wheel makes a Caesar cipher, a simple code that shifts every letter along the alphabet by the same amount. It is a lovely way to practise the alphabet and counting. It is for fun only. This code is easy to break and is not a way to protect real secrets. Remind your child never to use it for passwords or personal information.
Step 1: make the wheel
- Trace the bowl on the card to make a big circle. Trace the cup on the card to make a small circle. Cut both out with a grown-up.
- Fold each circle in half, then open it out again, so there is a crease line across the middle.
- On the big circle, write the letters A to Z around the edge, in order, going clockwise (the way a clock goes). Put A just above the left end of the crease. Spread A to M evenly along the top half and N to Z along the bottom half. Write 13 letters on each half.
- Do the same on the small circle, with the same spacing. Keep the letters near the edge.
- Put the small circle on top of the big one, in the middle. Ask a grown-up to make a hole through the centre of both and fasten them with the paper fastener, so the small wheel turns.
Test the wheel. Line up A on the small wheel with A on the big wheel. Does every letter match up with the same letter? If one is off, rub it out and rewrite it.
Step 2: choose a shift
A cipher is a rule for scrambling a message. The key is the secret number that tells you how to use the rule. Turn the small wheel until the small A sits under the big D. Look at the other letters. Small B sits under big E, and small C under big F. We moved the alphabet along by 3, so our key is 3.
| Real letter | A | B | C | D | E | F | G | H | I | J | K | L | M |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Secret letter | D | E | F | G | H | I | J | K | L | M | N | O | P |
| Real letter | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Secret letter | Q | R | S | T | U | V | W | X | Y | Z | A | B | C |
Notice how at the end the alphabet wraps around. X turns into A, Y into B and Z into C.
Step 3: scramble a message
To encode a message, find each real letter on the small wheel and write down the big-wheel letter next to it. With key 3, let’s encode the word FISH:
- F becomes I.
- I becomes L.
- S becomes V.
- H becomes K.
So FISH becomes ILVK. What about ZOO? Z wraps around to C, and each O becomes R, so ZOO becomes CRR.
Step 4: unscramble a message
To decode a message, do it backwards. Find the secret letter on the big wheel, and read the real letter on the small wheel below it.
Your friend needs to know the key, which is the number 3 (or “A sits under D”). Without the key, the message looks like nonsense!
Pixel says Never send the key in the same note as the message! Tell it to your friend face to face. Meow!
Want to try it on a computer?
This part is optional. Here is a short Python program that does the wheel’s job for us. It encodes FISH with key 3 and prints the secret word:
alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
key = 3
message = "FISH"
secret = ""
for letter in message:
place = alphabet.index(letter)
secret = secret + alphabet[(place + key) % 26]
print(secret)ILVK
The % sign finds the remainder after dividing. Using % 26 makes the position wrap around from Z back to A, just like your wheel!
Why this code is not very secret
There are only 25 different useful keys. A clever friend can just try them all until a real word appears. Real computers use much stronger codes. Still, the wheel shows the big idea: a rule plus a key can hide a message.
Did you know? The Roman writer Suetonius said that Julius Caesar sent secret messages by shifting each letter along by three. That is why this kind of code is called a Caesar cipher.
Challenges
Try it Decode this message, using key 3: FRPSXWHU
Show a hint
Move each letter back by 3. F goes back to C. Now do the rest!
Show the answer
F is C, R is O, P is M, S is P, X is U, W is T, H is E and U is R. The word is COMPUTER!
Try it Set your wheel to key 1 (small A under big B). Encode the word CAT.
Show the answer
C becomes D, A becomes B and T becomes U. The secret word is DBU.
Try it Make up a message for a friend. Choose a key from 1 to 25, encode your message, and give them only the scrambled words. Tell the key to them in person.
What you learned
- A cipher is a rule for scrambling a message. The key is the secret number needed to use it.
- To encode is to scramble. To decode is to unscramble.
- A Caesar cipher shifts every letter along the alphabet by the same amount, wrapping from Z back to A.
- A simple code like this is fun, but it is not safe for real secrets.