Late 18th-Century French Encrypted Diplomatic "Letters of Recommendation"-Or, How to Unwittingly Carry Your Own Warrant
This article presents a cryptographic method that Count Vergennes, Minister of the Interior of Louis XVI of France, used and refined in communication between the diplomatic representations abroad and Paris. After the visa applicants' initial interview (and secret investigation of their backgrounds), they were given an ingeniously encoded "Letter of Recommendation'' in the form of a personalized "Carte de Visite'' (calling card) when they picked up such permits, which were, in turn, decoded by the count himself upon presentation at the Ministry. Vergennes could then keep such visitors under surveillance during their sojourn in France.
count vergennes louis xvi of france encoded calling cards graf von lamberg invasion of privacy johann ferdinand opitz late 18th-century french diplomatic service