What Is a Secret Message?

Introduction

Every message you send travels through the world. In 1939 that world was radio: anyone with a receiver could hear a signal meant for someone else.

So armies stopped hiding the signal and started hiding the meaning. The message goes out loud and clear — but scrambled by a rule, and unscrambled by the friend who knows the key.

Key ideaA cypher is an agreed rule for scrambling a message, plus a key that only friends know.