The primary weakness of the Vigenère cipher is the repeating nature of its key. Vernam-Vigenère cipher, type of substitution cipher used for data encryption.The Vernam-Vigenère cipher was devised in 1918 by Gilbert S. Vernam, an engineer for the American Telephone & Telegraph Company (AT&T), who introduced the most important key variant to the Vigenère cipher system, which was invented by the 16th-century French cryptographer Blaise de Vigenère. Essentially, a Vigenère cipher consists of several Caesar ciphers in sequence with different shift values. the substitution is self-inverse). "The primary weakness of the Vigenère cipher is the repeating nature of its key. However, using the Vigenère cipher, E can be enciphered as different ciphertext letters at different points in the message, thus defeating simple frequency analysis. Our example secret has only 13 letters. The strength of the Vigenère Cipher is that it is not susceptible to Frequency Analysis, due to the fact that the cipher rotates through different shifts, so the same plaintext letter will not always be encrypted to the same ciphertext letter.For example, if "P" is the most common letter in the ciphertext, we might assume this is "e". The weakness of the Vigenère cipher is the small size of the secret. Given a sufficiently large ciphertext, it can easily be broken by mapping the frequency of its letters to the know frequencies of, say, English text. The shift value for any given character is based on the keyword. The Vigenere Cipher -- A Polyalphabetic Cipher. So, in our example, “dog” was repeated twice in order to match the number of letters in the word “attack.” ed, ee, ea, th) regularly in the plain text then patterns start to form in the encrypted text that can hint at the length of the key. If a cryptanalyst correctly guesses the key's length, then the cipher text can be treated as interwoven Caesar ciphers, which individually are easily broken." In fact, three of every 13 letters are encoded with secret “O”. This is a weakness of the Vigenère cipher, as if a key is short and plain text letters are repeated (e.g. An interesting weakness is the fact that a digraph in the ciphertext (AB) and it's reverse (BA) will have corresponding plaintexts like UR and RU (and also ciphertext UR and RU will correspond to plaintext AB and BA, i.e. For more information on the Cipher: It's a quite a weak cipher, being better than a simple substitution cipher by only using digraphs instead of monographs. This means that every 13th letter is encoded with the same letter. Though, the main weakness of a Vigenère Cipher is the repeating of the key. A Vigenère cipher shifts each character of a plain text message a number of positions based on a keyword. One of the main problems with simple substitution ciphers is that they are so vulnerable to frequency analysis.