What Is Vigenère Cipher?

A free online Vigenère cipher encryption and decryption tool. Uses a keyword to apply different Caesar shifts to each character. Features include autokey mode (where plaintext extends the key), character-by-character key visualization showing exactly how each letter is transformed, and index of coincidence (IC) analysis to determine whether the output resembles monoalphabetic or polyalphabetic encryption. The key space grows exponentially with keyword length — a 5-letter key creates 26^5 ≈ 12 million possible keys. All processing is done locally in your browser.

Why Use Vigenère Cipher?

  • Keyword-based polyalphabetic encryption and decryption
  • Autokey mode for stronger encryption
  • Character-by-character key visualization
  • Index of Coincidence (IC) statistical analysis
  • Random secure key generator using crypto.getRandomValues
  • Key space up to 26^n for an n-letter keyword

How to Use Vigenère Cipher — Step by Step

1

Enter your keyword (letters only, case-insensitive).

2

Type your plaintext or ciphertext.

3

Choose Encrypt or Decrypt mode.

4

Toggle Autokey mode to use plaintext as key extension.

5

Enable Key Visualization to see per-character key mapping.

6

Check the IC value to analyze the output statistically.

Who Is Vigenère Cipher Best For?

  • students learning cryptography
  • CTF players
  • puzzle solvers
  • cryptography enthusiasts

Vigenère Cipher — Frequently Asked Questions

What is the advantage of Vigenère over Caesar?

Vigenère uses a keyword to apply different shifts to each letter, making it a polyalphabetic cipher. This resists simple frequency analysis because the same plaintext letter can encrypt to different ciphertext letters depending on position.

What is autokey mode?

Autokey mode uses the plaintext itself to extend the keyword. After the keyword is exhausted, the plaintext characters are used as the key for subsequent positions. This prevents the repeating pattern that makes standard Vigenère vulnerable to Kasiski examination.

How is the Index of Coincidence interpreted?

IC measures how likely two randomly selected letters are identical. For English text, IC ≈ 0.0667. For random text, IC ≈ 0.0385. A Vigenère-encrypted text with a short keyword will have IC closer to 0.0385, while a value above 0.045 suggests the text may be monoalphabetic or the keyword is very long.

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Free online tool — works in your browser, no sign-up required. Start using it right now.

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