1. Why keep secrets?

About 5 minutes · You need: a partner, paper

Try it

Write a note to your partner: "Meet me at the swings at 3". Now imagine the note passes through five other hands on its way across the class.

Two things could go wrong:

1. Someone reads it. Now everyone is at the swings.
2. Someone changes it to "at 4". Your partner waits for an hour.

What's happening

Cryptography is the art of protecting messages from both problems:

• Hiding (keeping it secret): only the right person can read it.
• Checking (keeping it true): the reader can tell if anyone changed it, and who really sent it.

Every time you open a website with a padlock, send a chat message or unlock a phone, cryptography does both jobs, millions of times a second, without you noticing.

People have hidden messages for thousands of years. Spartan soldiers wound a strip of leather around a stick and wrote along it; unwound, the letters were jumbled. Only someone with a stick of the same thickness could read it. It was called a scytale.

Which job does a signature on a birthday card do: hiding or checking?

Checking. Anyone can read the card, but the signature tells you who it is from.

Next: 2. The Caesar shift · Going further: Book, chapter 1