Vigenère Cipher Tool
Encrypt and decrypt secret messages using the keyword-based Vigenère polyalphabetic substitution cipher.
1 How to Use the Vigenère Cipher Tool
Select mode: 'Encrypt' or 'Decrypt'.
Enter your secret Keyword (letters only, e.g., 'SECRET' or 'KEY').
Type your message into the text field.
View live polyalphabetic substitution with keystream visualization, and click 'Copy'.
2 Technical Specifications
| Specification | Details & Capabilities |
|---|---|
| Algorithm | Polyalphabetic Caesar shift based on keyword letter indices |
| Keystream | Keyword repeats cyclically across message length |
| Case Handling | Preserves original uppercase and lowercase casing |
| Execution | Real-time client-side calculation |
Trust and Privacy
100% Client-Side Privacy Guarantee
All calculations run directly in your local browser memory. Zero data is ever transmitted, logged, or stored on remote servers.
4 Frequently Asked Questions
Q. Why was the Vigenère Cipher called 'le chiffre indéchiffrable'?
For over 300 years it was believed to be unbreakable because it uses multiple Caesar alphabets, flattening simple letter-frequency analysis that easily cracked monoalphabetic ciphers.
Q. Who finally broke the Vigenère cipher?
Charles Babbage and Friedrich Kasiski independently broke it in the 19th century by identifying repeating patterns to deduce the keyword length.