Secret diplomatic papers were kept inside the secret drawers of the Bureau du Roi, whose only key the king always carried with him.
For a period of over twenty years, King Louis XV split his diplomacy into official and secret channels. The secret channels became collectively known as the King's Secret (Secret du Roi in French), established in 1745. It actually outlived its creator and some of its agents were involved in bringing France (and its allies) into the American War of Independence, although it was dissolved in 1774.