Vigenre Cipher

Encrypt and decrypt text using the Vigenre polyalphabetic substitution cipher. Enter a keyword to shift each letter.

Last reviewed: April 2026
Reviewed by , software engineer · last updated April 2026

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Plaintext
Ciphertext
Enter text and a keyword above to encrypt.

How to Use the Vigenre Cipher Tool

To use the Vigenre Cipher Tool, follow these steps:

1. Choose a mode: Encrypt to convert plaintext to ciphertext, Decrypt to reverse it, Brute Force to try all single-letter keys, or Reference to show the full tabula recta.

2. Enter your keyword: The keyword should contain only letters (A-Z). Numbers and special characters are ignored.

3. Paste or type your text: The result will appear instantly on the right.

4. Copy or download: Use the buttons to save your result.

  • Encrypt: Converts plaintext to ciphertext.
  • Decrypt: Reverses the encryption process.
  • Brute Force: Tries all single-letter keys.
  • Reference: Displays the full tabula recta for learning and manual verification.

When to Use the Vigenre Cipher Tool

The Vigenre Cipher Tool is ideal for educational purposes and historical decryption. It's not recommended for modern security needs due to its vulnerabilities. Consider using AES-256 for any real security requirements.

How It Works

The Vigenre cipher extends the simple Caesar cipher by using a keyword to apply multiple different shifts throughout the message. This polyalphabetic approach means that the same plaintext letter can map to different ciphertext letters depending on its position, making frequency analysis far more difficult than with a monoalphabetic cipher.

To encrypt, repeat the keyword over the plaintext (aligning only with alphabetic characters), then shift each plaintext letter by the position of the corresponding keyword letter in the alphabet. For example, with key 'KEY' and plaintext 'HELLO': H+K=R, E+E=I, L+Y=J, L+K=V, O+E=S - producing 'RIJVS'. Non-alphabetic characters like spaces, punctuation, and digits pass through unchanged.

Decryption is the reverse: shift each ciphertext letter backward by the keyword letter's position. The same keyword is required.

Frequently Asked Questions

The Vigenre cipher is a polyalphabetic substitution cipher that uses a keyword to shift each letter of the plaintext by a different amount. Each letter of the key determines the Caesar shift applied to the corresponding plaintext letter, making it harder to break than a simple Caesar cipher.
The keyword is repeated to match the length of the plaintext. Each plaintext letter is shifted forward in the alphabet by the position of the corresponding keyword letter (A=0, B=1, ... Z=25). Non-alphabetic characters pass through unchanged. Decryption reverses this by shifting backwards.
No - the Vigenre cipher is not secure by modern standards. It was broken in 1863 by Friedrich Kasiski. The Kasiski test and index of coincidence analysis can recover the key length and then the key itself. Use it only for educational purposes; use AES for real encryption.
A tabula recta is a 26-26 grid of letters used as a reference table for the Vigenre cipher. Each row represents one of the 26 Caesar shifts. To encrypt, find the plaintext column and key row; the intersection is the ciphertext letter.
Brute force mode tries all 26 single-character keys (Caesar shifts) and shows each decrypted output. This is useful when you have a short ciphertext and want to find the most readable decryption. The most human-readable result is likely the correct key.