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The Vigenère cipher strengthens the simple Caesar shift by using a repeating keyword, so each letter of the message is shifted by a different amount depending on its position.
This tool encrypts or decrypts text using a Vigenère cipher with a keyword you choose.
How it works
Enter your text and a keyword. Each letter of the keyword supplies a shift amount (A = 0, B = 1, and so on), and the keyword repeats across the length of the text, shifting each letter by the value of the keyword letter lined up with it.
- Enter mode.
- Enter text.
- Enter keyword.
- Click Calculate to see your results.
Examples
Keyword "LEMON"
"ATTACKATDAWN" encrypted with the keyword "LEMON" becomes "LXFOPVEFRNHR" — a classic textbook example.
Who should use it
- Puzzle and escape-room style word games.
- Teaching the history and mechanics of classical cryptography.
Industry applications
- Cryptography education
- Games and puzzles
Advantages
- Significantly harder to break by simple frequency analysis than a Caesar cipher.
- A good introduction to polyalphabetic substitution ciphers.
Limitations
- Breakable with classical techniques like the Kasiski examination, especially for longer ciphertexts.
Common mistakes to avoid
- Using a very short keyword, which weakens the cipher and makes it easier to break with frequency analysis.
- Assuming the Vigenère cipher is secure enough for real-world sensitive data.
Best practices
- Use the Vigenère cipher for puzzles, games, or teaching classical cryptography — not for protecting anything sensitive.
Tips
- A longer, less predictable keyword makes the cipher noticeably harder to break by hand — try comparing a one-letter keyword (equivalent to a Caesar cipher) against a longer one.