In the history of cryptography, the Great Cipher was a nomenclator cipher developed by the Rossignols, several generations of whom served the French Crown as cryptographers. It was excellent of its class and so was given this name; it was reputed to be unbreakable. And, indeed, after it went out of current use, messages in the French archives (mostly diplomatic, apparently) were entirely 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 onto 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 which used 587 different numbers that was so strong it baffled cryptanalysts for centuries until Commandant Étienne Bazeries managed to break the cipher around 1893, 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 letters appeared a possible solution to the mystery of The Man in the Iron Mask. It 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 putting in serious danger the success of the entire French campaign in Piedmont. The letter said:

His Majesty knows better than any other person the consequences of this act, and he is also aware of how deeply our failure to take the place will prejudice our cause, a failure which must be repaired during the winter. His Majesty desires that you immediately arrest General Bulonde and cause him to be conducted to the fortress of Pignerole, where he will be locked in a cell under guard at night, and permitted to walk the battlefield during the day with a mask.

References

• Singh, Simon. The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography. New York: Anchor Books, 1999. ISBN 0385495323