VigenèreChiffre

VigenèreChiffre, commonly known in English as the Vigenère cipher, is a method for encrypting alphabetic text by using a keyword to determine a sequence of Caesar ciphers. It is a polyalphabetic substitution cipher, designed to overcome the frequency analysis weaknesses of a single shift. The system is named after Blaise de Vigenère, though earlier versions and related ideas were discussed by Giovan Battista Bellaso and others in the 16th century.

Encryption is performed with a tabula recta, a 26-by-26 grid of shifted alphabets. For each position in the plaintext, the corresponding letter of the keyword selects a row; the plaintext letter is found in that row’s column to yield the ciphertext letter. The keyword is repeated as needed to cover the entire message. Decryption uses the inverse process, subtracting the key shift to recover the plaintext.

Security of the VigenèreChiffre relies on the length and secrecy of the key. If the key repeats, the cipher is vulnerable to frequency analysis and methods such as Kasiski examination that can reveal the key length and eventually the plaintext. A key as long as the message and chosen at random provides theoretical perfect secrecy (the one-time pad), but practical use often falls short. In modern cryptography, it is mainly of historical and educational interest.

Variants include the Beaufort cipher, which uses opposite shifts, and autokey versions that append plaintext to the key. The Vigenère square and related polyalphabetic ideas remain foundational in teaching substitution techniques and the distinction between monoalphabetic and polyalphabetic ciphers.