Abstract
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.
Highlights
The document is held in the Dutch Royal Archives, Koninklijk Huisarchief (KHA), under reference Prins Willem III, inv.nr
The digit codes are separated by spaces, the separation is not always clear
It was hypothesized that the two-digit codes might represent single letters, while the threedigit codes represent full words, names, or places
Summary
The document is held in the Dutch Royal Archives, Koninklijk Huisarchief (KHA), under reference Prins Willem III, inv.nr. The document contains a mix of cleartext and encoded parts. The cleartext indicates that it is dated January 9, 1684, and that it was sent by the Comte d’Avaux to Louis XIV (“Votre Majeste”). The digit codes are separated by spaces, the separation is not always clear. There are some punctuation marks, like a comma. On the top of some digits, an accent appears, or a Tilda sign. Their meaning could be identified only after deciphering most parts of Figure 1: First Page of the Encrypted Letter (d’Avaux, 1684b)
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