Vigenere Cipher Online — Encrypt With a Keyword

The longer the better. A three-letter key repeats dozens of times along a message, and the Kasiski examination recovers that period from repeated ciphertext fragments. A key as long as the message itself, and not a dictionary word, makes manual recovery essentially hopeless. Five to eight letters is plenty for coursework.

Every key letter encodes a numeric shift over the Latin alphabet: A means a shift of zero, B a shift of one, and so on. A digit or a space has no such value, so the Key field is restricted to letters. If your passphrase is several words, simply drop the spaces.

You get a string of the same length that is complete gibberish. There is no error message, because the arithmetic is valid for any key. That is exactly why the key must be remembered or stored separately — without it only cryptanalysis recovers the text. Check for typos and case; the key has to match character for character.

No. Only Latin letters are transformed; Cyrillic characters, digits and punctuation pass through unchanged. For an assignment based on the Russian alphabet, transliterate before enciphering, otherwise a large part of the message stays in the clear.

Caesar offers twenty-five keys and falls to a minute of brute force. The Vigenere key space grows exponentially with keyword length, and the same plaintext letter enciphers differently depending on its position, which defeats simple single-letter frequency analysis. By modern standards it is still weak, but it is far harder to break by hand.

You should not. Vigenere yields to the Kasiski and Friedman methods known since the nineteenth century, and with a short key an automated attack takes minutes. It is a historical teaching cipher. For real protection use modern algorithms and an established end-to-end encrypted messenger.