The Great Cipher (French: Grand chiffre) was a nomenclator cipher developed by the Rossignols, several generations of whom served the French monarchs 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.[1] After it fell out of current use, many documents in the French archives were unreadable until it was decoded.