Prince of Condé's Simple Substitution Cipher during the French Revolutionary Wars

Louis Joseph de Bourbon, Prince of Condé, who helped organize a counter-revolutionary army of émigrés in Coblenz in 1791, used a simple substitution cipher during the French Revolutionary War.

Use of this cipher is seen in the following letters in 1792-1800.

1792, 1797

Archives nationales, 34AP/1 ("Correspondance des Princes de Condé pendant l'émigration déposée aux archives par le baron de Surval" (AN)) includes many letters of the Prince of Condé in 1792 and 1797 partly in cipher (deciphered for the most part). Most of them are to his son, Duke of Bourbon (No.171 etc.), while some are to his grandson, Duke of Enghien (No. 102, 120).

1795

Archives nationales, AE/II/2858 (originally 34AP/6bis, p.17), also includes a letter, dated 1 June 1795, to the Prince of Condé from the Count of Provence (Louis XVI's brother, proclaimed Louis XVIII after the death of the ten-year-old Louis XVII in the Temple on 8 June 1795) in his exiled court in Verona ("Lettre du comte de Provence au prince de Conde pour l'engager a participer a l'expedition royaliste qui devait debarquer a Quiberon. Verone" (AN)). This is substantially wholly in cipher but is deciphered interlinearly. The king-to-be expects Condé's participation in the scheduled invasion of France (Wikipedia).

(The Count of Provence was given a cipher by Marie-Antoinette after the Flight to Varennes (1791) but he had difficulty in using it, possibly because it was polyalphabetic (another article).)

1800

Archives nationales, 34AP/4 ("Correspondance des princes de Condé et du duc de Bourbon (1789-1825)" (AN)) includes use of the cipher in the following letters.

Duke of Bourbon to Prince of Condé, London, 9 May 1800 (No.28)

The short ciphertext is undeciphered:

From London, 5 February [1795-1804] (No.84)