Deciphering a Letter to Louis XIV from his Ambassador to the Dutch Republic, le Comte d’Avaux, 1684


  • George Lasry



Comte d'Avaux, Louis XIV, French cryptography, Dutch Black Chamber, Prince of Orange


The Dutch Royal Archives (Koninklijk Huisarchief - KHA) at The Hague holds a number of enciphered letters written by French diplomats in Holland in the 17th and 18th centuries, including a letter, from January 9, 1684, from Jean-Antoine de Mesmes (1640 – 1709), Comte d’Avaux, the French ambassador at The Hague from 1678 to 1689, to Louis XIV, King of France from 1643 to 1715. In this article, we show how we deciphered the letter, and identified the historical plaintext as a letter intercepted and deciphered by the Spanish authorities in the Southern Netherlands. The letter was also published by the Prince of Orange, to expose d’Avaux secret contacts with deputies of the city of Amsterdam. D’Avaux claimed that the decryption was intentionally modified to harm France’s image, but the modern decipherment demonstrates that the historical decryption was in fact fully accurate.