Vigenere Cipher

Vigenere Cipher is a method of encrypting alphabetic text. It uses a simple form of polyalphabetic substitution. A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets.

The cipher was invented in 1553 by the Italian cryptographer Giovan Battista Bellaso but for centuries was attributed to the 16th-century French cryptographer Blaise de Vigenère, who devised a similar cipher in 1586.

For many years this type of cipher was thought to be impregnable and was known as le chiffre indéchiffrable, literally “the unbreakable cipher.”

The encryption of the original text is done using the Vigenère square or Vigenère table:

➛ The table consists of the alphabets written out 26 times in different rows, each alphabet shifted cyclically to the left compared to the previous alphabet, corresponding to the 26 possible Caesar Ciphers;

➛ At different points in the encryption process, the cipher uses a different alphabet from one of the rows;

➛ The alphabet used at each point depends on a repeating keyword.

The keyword Vigenère cipher is much more secure then the autokey method, but it is still vulnerable. The longer the keyword, the more secure the cipher. If the keyword is as long as the plaintext, for example, a previously agreed upon sample of text, the cipher is unbreakable if a new key is used for every message.

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