How is the Vigenère cipher different from Caesar?

Caesar uses one fixed shift for all letters. Vigenère uses a keyword where each letter defines a different shift. This means the same plaintext letter encrypts differently depending on its position, making frequency analysis much harder.

What keywords make a strong Vigenère cipher?

Longer keywords provide more security. A keyword as long as the message (a one-time pad) is theoretically unbreakable. Short keywords (under 5 characters) are vulnerable to Kasiski analysis.

Vigenère Cipher

Encrypts and decrypts text using the Vigenère cipher — a polyalphabetic substitution cipher that uses a keyword to vary the shift for each letter. Much stronger than the Caesar cipher against frequency analysis. Supports both encryption and decryption modes when the keyword is known.

encodedecode

Key (letters only)

Input

Output

Key: "SECRET" (6 chars) · Keystream repeats every 6 alphabetic characters.
The Vigenère cipher uses a polyalphabetic substitution — each letter of the key determines the Caesar shift for the corresponding plaintext letter.

Enter text and a keyword to encrypt or decrypt using the Vigenère cipher.