How the Vigenère Cipher Works
The Vigenère cipher pairs each message letter with a keyword letter to pick a different shift. This guide shows how the shifts combine and how decryption reverses them.
What is the Vigenère Cipher?
Vigenère Cipher encrypts text using a keyword, shifting each letter by a different amount as the keyword repeats. Unlike a single fixed shift, the changing shifts hide letter patterns, which made this cipher famously hard to break by hand.
How to use it
- Type your message into the box.
- Enter a keyword and choose Encrypt or Decrypt.
- Click Encrypt and copy the result.
Worked example
For 'hello' with keyword 'key' in Encrypt mode, the output is 'rijvs'.
Why was Vigenère called unbreakable?
For centuries it resisted the frequency analysis that breaks simple substitution ciphers, because the same plaintext letter encrypts to different letters. It was only broken in the 1800s, once analysts learned to find the repeating keyword length.
Frequently asked questions
Do spaces and punctuation get encrypted?
No. They pass through unchanged, and the keyword only advances on letters, so the recipient needs the same rule to decode.
Does a longer keyword make it stronger?
Yes. A longer, less predictable keyword produces more varied shifts and is harder to crack than a short repeated word.