Vigenere Cipher
Encrypt and decrypt messages using the Vigenere polyalphabetic substitution cipher — enter your message and keyword and get the result instantly.
What Does This Tool Do?
Encrypt and decrypt text using the Vigenere cipher — a polyalphabetic substitution cipher that uses a keyword to shift each letter by a different amount, making it much harder to crack than a simple Caesar cipher.
Key Features
Polyalphabetic
Keyword drives a different shift per letter.
Both Directions
Encrypt and decrypt with the same key.
Preserves Case
Uppercase stays uppercase, lowercase stays lowercase.
Copy Result
One-click copy of the output.
How to Use
- Type or paste your message.
- Enter a keyword (letters only — spaces and numbers ignored).
- Click Encrypt or Decrypt.
Frequently Asked Questions
How does the Vigenere cipher work?▾
Each letter is shifted by the corresponding letter of the keyword (A=0, B=1, … Z=25). The keyword repeats to match message length. E.g. with key SECRET: S=18, so A→S, B→T, etc.
Is the Vigenere cipher secure?▾
No — it was considered unbreakable for centuries but can be cracked with the Kasiski test or index of coincidence analysis. It is used for educational purposes only.
Does it work with numbers and punctuation?▾
Non-alphabetic characters are passed through unchanged. Only A–Z letters are encrypted.