Vigenère Cipher

You need to encrypt a message with a keyword, or you got ciphertext from a puzzle, an escape room or a CTF and you know (or suspect) the key. Vigenère turns each letter of the key into a different shift, which makes it far tougher than a plain Caesar cipher. Pick Encrypt or Decrypt, type the key, and the result updates as you write.

Each key letter is a shift amount: A shifts nothing, B shifts by 1, and so on, with the key repeating along the text. Details of this implementation: the key keeps only letters (key123 and KEY are the same), the text keeps its upper and lower case, and spaces or punctuation do not consume a key position. Accented letters pass through unencrypted, which leaks fragments of the message.

Decrypting needs the exact key and Decrypt mode; a wrong key never errors, it just yields scrambled nonsense. The classic weakness is the repeating key: given enough ciphertext, frequency analysis (Kasiski's method, published in 1863) breaks it. So treat it as a tool for puzzles and study, not real secrecy. Strip accents from the text first if you want nothing to leak. Everything runs in your browser.