Online Tool Store
Vigenère Cipher Tool
Encrypt or decrypt text using the Vigenère cipher with a custom keyword — a classical educational cipher. Runs entirely in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Educational/classical cipher only — not secure for protecting real sensitive data.
How it works
- Enter text and a keyword.
- Choose encrypt or decrypt.
- Copy the result.
The formula
Encrypt: Cᵢ = (Pᵢ + Kᵢ mod len(K)) mod 26
Decrypt: Pᵢ = (Cᵢ − Kᵢ mod len(K) + 26) mod 26
Where P = plaintext letter, C = ciphertext letter, K = key letter (each as 0–25)
FAQ
How does the Vigenère cipher work?
Each letter of the plaintext is shifted by an amount determined by the corresponding letter of a repeating keyword — "A" shifts by 0, "B" by 1, and so on. It's a polyalphabetic substitution cipher, historically stronger than a simple Caesar shift since the same plaintext letter can encrypt to different ciphertext letters.
Is this secure for real sensitive data?
No — the Vigenère cipher was broken in the 19th century (Kasiski examination, frequency analysis on key-length-divided segments) and offers no real security by modern standards. It's useful for puzzles, education, and historical interest, not for protecting anything you actually need kept secret.
What happens to non-letter characters?
Spaces, punctuation, and numbers pass through unchanged — only letters are shifted, and the key only advances on letters, so word boundaries stay visible in the output.
Is my text uploaded anywhere?
No — encryption and decryption happen entirely in your browser. Nothing is sent to a server.
How we compare
| Feature | Online Tool Store | Manual by-hand cipher table | Programming language snippet |
|---|---|---|---|
| Instant encrypt and decrypt | ✓ | Slow, error-prone by hand | ✓ |
| No install or code editor | ✓ | ✓ | Requires a runtime |
For a classroom cryptography exercise or a puzzle hunt clue, this beats working through a Vigenère square by hand.