The Vigenère Cipher

The "indecipherable" cipher

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

• Vigenère uses a repeating keyword to vary the shift.
• It flattens letter frequencies, defeating simple frequency analysis.
• Knowing the key length splits it into simple shift ciphers.

Steps

1. Many shifts at once
2. Encrypt a letter
3. How many keys?
4. Repeating patterns
5. Find the key length
6. The crack