From the correspondence of his courtiers to the military plans of his enemies, Louis XIV wanted to know everything about everyone else, and above all that no one knew anything about him. Within the Ferme des Postes, created to manage postal exchanges, the Sun King set up the Bureau d’en dedans, a black cabinet responsible for scrutinising letters. In a permanent state of war, he knew that his enemies were watching him too, so he called on the services of the genius cryptologists Antoine Rossignol and his son Bonaventure.