Rossignols

As early as 1406, the word rossignol has served as the French term for "skeleton key" or for any tool which opens that which is locked.

[1] In 1626, Henri II of Bourbon, Prince de Condé laid siege to the Huguenot city of Réalmont.

He quickly broke the Huguenot cipher, revealing a plea to their allies for ammunition to replenish the city's almost exhausted supplies.

[2] This brought Rossignol to the attention of Louis XIII's chief minister, Cardinal Richelieu,[3] who found secure ciphers and codes of immense use to his diplomatic and intelligence corps.

[9] In the era of Louis XIV of France (reigned 1643–1715), Antoine Rossignol and his son, Bonaventure, worked either at their estate at Juvisy near Paris or in a room next to the King's study at Versailles.

Without the key, and even the base concept, it remained uncrackable until the late 19th century, when Etienne Bazeries deciphered it after three years of work.