Rossignol's Great Cipher, the secret of Louis XIV

In 1628, during the siege of the Huguenot town of Réalmont in southern France, the royal army intercepted a ciphered letter bound for the world outside the walls. According to the traditional account it was handed to a young man from Albi, Antoine Rossignol, a lover of mathematics, who deciphered it in short order: the besieged were asking for help and admitted they were at the end of their strength. The town surrendered. Thus began one of the most extraordinary careers in French cryptography.

In the service of Richelieu and the king

Cardinal Richelieu wanted Rossignol at his side, and the codebreaker went on to serve Louis XIII and Louis XIV. For decades he read intercepted dispatches and prepared the ciphers of French diplomacy, working in close contact with power. His work was so highly valued that the king rewarded him with honours and wealth, and Rossignol became lord of Juvisy, near Paris, where he owned a château.

He was assisted by his son Bonaventure, and later by his grandson Antoine-Bonaventure, in a true dynasty of cipher experts. Their activity is often associated with the so-called cabinet noir, the black chamber, the secret office in which letters were opened, copied, deciphered and resealed before being delivered to their recipients, who suspected nothing.

A cipher of syllables

The Great Cipher, devised by Antoine and Bonaventure Rossignol, was a highly developed nomenclator. Instead of replacing single letters, it assigned numbers to syllables and groups of letters, several hundred values in all, including homophones, that is different numbers for the same sound. Other numbers had no meaning or served as traps, for example indicating that the previous group should be ignored.

This design made frequency analysis applied to letters almost useless. An observer counting the most frequent numbers would have found syllables, not letters, and the traps would have confused any attempt at reconstruction. The cipher was used for the monarchy's most sensitive correspondence, and after the death of its creators no one, it seems, preserved the complete key.

Bazeries and the Iron Mask

For about two centuries several documents in the French archives remained unreadable. In the 1890s Commandant Étienne Bazeries, cryptologist of the French army, tackled the ciphered papers of Louis XIV's reign. He realised that the numbers stood for syllables and, starting from a repeated group that he interpreted as les ennemis, “the enemies”, gradually reconstructed the system after years of work.

Among the deciphered texts Bazeries believed he had found the solution to a famous mystery: a letter from the minister Louvois ordered the arrest of General Vivien de Bulonde, guilty of abandoning the siege of Cuneo in 1691, and mentioned a mask. Bazeries suggested that he was the Man in the Iron Mask. The identification is now rejected by most historians, not least because Bulonde is known to have been alive and free in later years.

A name that entered the language

Such was the fame of the Rossignols that, according to an often repeated tradition, the French word rossignol, meaning nightingale, also came to denote a picklock, the tool for opening locks without a key. The origin of this usage is debated, but the association says a great deal about the image of those codebreakers able to open any secret.

The story of the Great Cipher reveals a paradox of cryptography: a system can be so well built that it outlives its creators, to the point of becoming indecipherable even to those who ought to read it. Only the patience of a nineteenth-century officer gave voice again to those dispatches of the Sun King, silent for almost two centuries.