Computer Security and Cryptography

4.2 BLAISE DE VIGENÈRE

Blaise de Vigenère was born in 1523 in Saint-Pourçain, France. While serving as a diplomat in Rome, he came into contact with Giovanni Battista della Porta in 1549 and learned from Porta's Traicté des Chiffres (1585) describing various encryption systems. Vigenère's book A Treatise on Secret Writing was published when Vigenère returned to Paris. It contains the basic 20 × 26 Vigenère tableaux.

The Vigenère encipherment of plaintext x (identified by its column position) with the key k (identified by its row number) is the table entry in the kth row and column position x; for example, plaintext x = B is enciphered with the key K = 2 to ciphertext y = d.

Vigenère polyalphabetic encipherment extends a sequence of r letters (k₀, k₁, …, k_r−1) periodically to generate the running key, k = (k₀, k₁, …, k_n−1, …) with k_i = k_{(i(modulo r))} for 0 ≤ i < ∞. For example, the key of length 12

enciphers plaintext of length 20 using the repeated key

Vigenère's original scheme subtracted rather than added the key from the plaintext

It was rediscovered nearly one hundred years later ...