Vigenère Cipher online

How to Use vigenere-cipher

1. Enter your plaintext message.
2. Enter a keyword — the key repeats to match the message length.
3. Click Encrypt: each letter is shifted by the corresponding key letter's position in the alphabet.
4. Click Decrypt (or enter ciphertext + same key) to reverse the process.

The Vigenère cipher extends the Caesar cipher by using a repeating keyword instead of a single shift. Each letter in the message is shifted by the position of the corresponding keyword letter (A=0, B=1...Z=25). For centuries it was considered unbreakable — it was cracked in the 1860s by Kasiski and Babbage using coincidence counting and key-period analysis. It is still studied as the foundation of polyalphabetic substitution ciphers.

Cryptanalysis: Breaking Vigenère

The Kasiski test finds repeated ciphertext segments (which indicate the same plaintext encrypted with the same key segment), measures the distances between repetitions, and uses GCD to estimate the key length. Once the key length is known, the cipher reduces to n parallel Caesar ciphers — each broken by frequency analysis. The Index of Coincidence is another tool that distinguishes monoalphabetic from polyalphabetic ciphers and estimates key length.

• Encrypts and decrypts with any alphabetic keyword
• Shows the expanded key (keyword repeated/truncated to message length)
• Only letters are encrypted — spaces, punctuation, and digits pass through
• Tabula recta reference included for manual verification

See also the Caesar Cipher, AES Cipher, and the XOR Encryptor.