The One-Time Pad

A provably unbreakable code

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• A one-time pad combines the message with a random key of equal length (e.g. by XOR).
• It has perfect secrecy, proved by Shannon.
• Reusing the key destroys its security.

Steps

1. A key as long as the message
2. XOR by hand
3. Why it’s unbreakable
4. Two-time pad
5. Any message fits
6. Key material