How to Invent Your Own Secret Code

Learn how to create a custom cipher to send secret messages to your friends while exploring the basics of cryptography.

1. Create your alphabet map. Write the letters A through Z in a straight line across the top of your paper. Directly underneath each letter, write a different letter or draw a unique symbol. This is your 'key'—it tells you which letter replaces the real one.
2. Write a practice message. Think of a simple sentence, like 'Meet me at lunch.' Look at your key, find the letter 'M' and write down the symbol you chose for it. Repeat this for every letter in your sentence until it looks like a jumble of secret characters.
3. Share the code. To let a friend read your message, you must give them a copy of your key. Never share your key with anyone you don't want to be able to read your secrets. Ask a grown-up if you are unsure about who to share your code with.