Chapter C Breaking the unbreakable

Abstract

Esteemed as “le chiffre indèchiffrable”, the Vigenère cipher was considered unbreakable for over three centuries. Its workings and arithmetic nature have been explained in Example A.2 (v) and Chapter B. We now present an attack from 1863 which brings the system to its knees. However, it did not really diminish the system’s popularity, and the cipher was reinvented again and again by people unaware of this attack.