Vigenère Cipher

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

Plaintext
Ciphertext
Enter text and a keyword above to encrypt.

How to Use the Vigenère Cipher Tool

  1. Choose a mode — Encrypt converts plaintext to ciphertext, Decrypt reverses it, Brute Force tries all single-letter keys, Reference shows 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 appears instantly on the right.
  4. Copy or download — use the buttons to save your result.

What Is the Vigenère Cipher?

The Vigenère cipher is one of the most famous classical ciphers in cryptography. Invented in the 16th century and popularized by Blaise de Vigenère (though actually created by Giovan Battista Bellaso), it 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.

How Vigenère Encryption Works

The encryption algorithm is straightforward: 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 (A=0, B=1, C=2, ... Z=25). 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

Decryption is the reverse: shift each ciphertext letter backward by the keyword letter's position. The same keyword is required. This tool handles both uppercase and lowercase letters while preserving the original case of the input text.

The Tabula Recta

The tabula recta is a 26×26 grid that serves as a visual lookup table for the Vigenère cipher. Each row is one of the 26 Caesar-shifted alphabets. To encrypt, find the column for your plaintext letter and the row for your keyword letter — the intersection is the ciphertext letter. The Reference mode in this tool displays the full tabula recta for learning and manual verification.

Brute Force and Cryptanalysis

The Brute Force mode tries all 26 possible single-character keys (equivalent to all 26 Caesar shifts) and displays the results. This is useful for cracking short messages encrypted with a single-character Vigenère key. For longer keys, the Kasiski examination identifies repeated sequences in the ciphertext to estimate the key length, and the Index of Coincidence is then used to recover each key character. Despite its historical reputation as "le chiffre indéchiffrable," the Vigenère cipher is completely insecure for modern use.

Modern Context

The Vigenère cipher is purely of historical and educational interest today. Modern encryption standards like AES-256 are computationally infeasible to break and are what you should use for any real security needs. However, studying the Vigenère cipher is an excellent introduction to cryptographic concepts: key management, polyalphabetic substitution, and the relationship between key length and cipher strength. Related tools: Hash Generator for modern one-way hashing and Base64 Encoder for encoding (not encryption).

Frequently Asked Questions

The Vigenère 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 Vigenère 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 Vigenère 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.