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.

What is Kasiski analysis?

Kasiski analysis breaks Vigenère ciphers by finding repeated sequences in the ciphertext. The distances between repetitions are often multiples of the keyword length, revealing the key length — which then allows frequency analysis per position.

Is Vigenère cipher secure today?

No. While much stronger than Caesar, Vigenère was cracked in the 19th century. Any keyword shorter than the message is vulnerable to statistical analysis. For real security, use modern authenticated encryption (AES-GCM, ChaCha20-Poly1305).

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.

Runs entirely in your browser — your input is never uploaded, stored, or logged.

Enter text and a keyword to encrypt or decrypt using the Vigenère cipher. For simple single-shift encryption, use the Caesar Cipher.