How vigenère cipher works
The Vigenere cipher is a polyalphabetic substitution cipher invented in the 16th century and once called 'le chiffre indechiffrable' (the indecipherable cipher). Instead of a single fixed shift like the Caesar cipher, it uses a repeating keyword: each letter of the plaintext is shifted by the value of the corresponding keyword letter (A=0, B=1, ..., Z=25).
To encrypt, the keyword is repeated to match the length of the message, and each plaintext letter is shifted forward by its keyword letter's position. To decrypt, each letter is shifted back by the same amount. Only letters are shifted (case is preserved), and non-letters such as spaces, numbers and punctuation are skipped and do not advance the keyword.
Enter your text, type a keyword using only letters, and pick encrypt or decrypt. The tool repeats the keyword across your message and preserves case and punctuation. The Vigenere cipher resisted casual cracking for roughly 300 years, but it is broken by Kasiski examination and key-length analysis — use it for puzzles, games and learning, never for real security.