Great Cipher

In the history of cryptography, the Great Cipher or Grand Chiffre was a nomenclator cipher developed by the Rossignols, several generations of whom served the French Crown as cryptographers. The Great Cipher was so named because of its excellence and because it was reputed to be unbreakable. Modified forms were in use by the French Peninsular army until the summer of 1811, and after it fell out of current use many documents in the French archives were unreadable.

Antoine Rossignol's cryptographic skills became known when in 1626 an encrypted letter was taken from a messenger leaving the city of Réalmont, controlled by the Huguenots and surrounded by the French army. The letter told that the Huguenots would not be able to hold on to the city for much longer, and by the end of the day Rossignol had successfully deciphered it. The French returned the letter with the deciphered message, forcing the Huguenots to surrender. He and his son, Bonaventure Rossignol, were soon appointed to prominent roles in the court.

Together, the two devised a code so strong it baffled cryptanalysts for centuries. Commandant Étienne Bazeries managed to break the cipher around 1893 over a period of three years, realizing that each number stood for a French syllable rather than single letters as traditional codes did. He guessed that a particular sequence of repeated numbers, 124-22-125-46-345, stood for les ennemis ("the enemies") and from that information was able to unravel the entire cipher.

In one of the encrypted letters between Louis XIV and his marshal Nicolas de Catinat appeared a possible solution to the mystery of the Man in the Iron Mask. The letter concerned a general named Vivien de Bulonde who was to attack the Italian town of Cuneo but instead fled, fearing the arrival of the Austrians, and consequently put in serious danger the success of the entire French campaign in Piedmont. The letter said:

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