Vigenère Cipher
Encrypt and decrypt text using the Vigenère polyalphabetic cipher. Enter any keyword and encode or decode your message instantly. Free, private, runs in your browser.
What Is the Vigenère Cipher?
The Vigenère cipher is a polyalphabetic substitution cipher that uses a repeating keyword to shift each letter of the plaintext by a different amount. Unlike the Caesar cipher (single fixed shift), the Vigenère key cycles through multiple shifts, making frequency analysis significantly harder. It was considered unbreakable for centuries and is known as le chiffre indéchiffrable.
How It Works
| Component | Description |
|---|---|
| Plaintext letter | The original character (A–Z) |
| Key letter | Current character of the repeating keyword |
| Shift amount | Key letter position in alphabet (A=0, B=1 … Z=25) |
| Encrypted letter | (plaintext + shift) mod 26 |
| Non-alpha chars | Passed through unchanged; key does not advance |
Tips & Limitations
The Vigenère cipher is a classical cipher useful for learning cryptography concepts. It is not suitable for modern security needs — use AES-256 (see Encryption / Decryption tool) for real data protection. Longer keys relative to the message length provide stronger obfuscation. A key as long as the message itself and never reused becomes a one-time pad.