Vigenère encode

No. It resists simple brute force better than Caesar, but it has been broken since the nineteenth century by Kasiski examination and index-of-coincidence methods, which recover the key length and then the key from ciphertext alone. Treat it as a puzzle. For anything you need protected, use AES-GCM encrypt.

Letters only, defaulting to the word key. Anything containing a digit, a space or punctuation is refused with "key must contain letters only". Case does not matter, since the key is lowercased before use. Each key letter supplies a shift, a for 0 up to z for 25, applied in turn and repeating for the length of the message.

No. The key position only moves on letters, because non-letters are skipped entirely and passed through unchanged. That matters when you compare against another implementation, since some tools advance the key on every character and produce different output for the same key and the same text.

Use Vigenere decode with the same key. It subtracts each key letter instead of adding it. Nothing else has to match, because there are no other options. Keep the key: without it this site offers no way back, since there is no key-recovery or cryptanalysis mode anywhere here.

Case, spacing, digits and punctuation. Only the 26 English letters are enciphered, and an uppercase letter stays uppercase. The shape of the text survives intact, which is part of why the cipher gives way to analysis on any reasonable amount of material.

No. Your file is read as text and transformed in a Web Worker in this tab. Nothing is uploaded, nothing is stored, and it works with the network off. The privacy comes from where the code runs, not from the cipher, which provides none of its own.