The Family of Code Masters

The story of the Great Cipher is inseparable from the family who created it: the Rossignols. Antoine Rossignol was a gifted mathematician whose talent for cryptography came to light during the siege of the Huguenot city of Réalmont. When a coded message from the city’s defenders was intercepted, Rossignol deciphered it in a matter of hours, revealing their desperate situation and leading to their swift surrender. His feat earned him the favor of Cardinal Richelieu and, eventually, King Louis XIII.

His reputation as the finest “decoder in Europe” grew, and the craft was passed down to his son, Bonaventure Rossignol. Together, father and son became the indispensable architects of France’s secret communications. For Louis XIV, they refined their methods to create their magnum opus: the Great Cipher. So integral were they to the crown that the King granted them a chateau near Versailles and a title. The word rossignol even briefly became a French synonym for “lock-pick”, a testament to their ability to unlock any secret.

Deconstructing a Linguistic Labyrinth

What made the Great Cipher so impervious to attack for so long? The answer lies in its brilliant subversion of the standard cryptographic techniques of the era. It wasn’t just a code; it was a deeply linguistic system designed to obliterate the very patterns that codebreakers relied on.

Most ciphers of the time were simple substitution ciphers, where one letter is swapped for another (A=X, B=T, etc.). These are vulnerable to frequency analysis. In any language, some letters appear more often than others. In English, ‘E’ is the most common letter; in French, it’s also ‘E’. A cryptanalyst could simply count the frequency of symbols in the coded text and guess that the most common symbol represents ‘E’, the next most common ‘A’ or ‘S’, and so on.

The Rossignols designed the Great Cipher to make this technique utterly useless. Here’s how:

It Wasn’t About Letters, It Was About Sounds

The fundamental genius of the Great Cipher was that it didn’t primarily encrypt individual letters. Instead, its 587 different numbers corresponded to syllables, digraphs (two-letter pairs), and trigraphs (three-letter pairs). For example, instead of encrypting the letters P-R-E, the cipher might have a single number, say 488, to represent the entire syllable “pre.” This completely flattened the frequency distribution. The number for the syllable “les” would appear far less often than the letter ‘e’ would in a simple substitution, making it impossible to identify common letters.

Homophones and Nulls: The Art of Misdirection

To further confuse would-be codebreakers, the Rossignols employed several clever tricks:

• Homophones: Extremely common letters and syllables were assigned multiple numbers. The single letter ‘e’, for instance, might be represented by 131 in one instance and 252 in another. This meant that even if a cryptanalyst suspected a number represented ‘e’, they couldn’t find all instances of it, making frequency analysis a dead end.
• Nulls: The cipher was peppered with “null” numbers—placebos that meant absolutely nothing. These were inserted into the message to break up patterns and send analysts on wild goose chases, trying to decipher a symbol that had no meaning.
• Traps: Some numbers had a contextual function. For example, a specific number might act as a signal to ignore the preceding number, further disrupting any attempt to find reliable patterns.

A Nomenclator for Key Terms

Finally, the Great Cipher was also a nomenclator—part cipher, part codebook. In addition to syllables, certain numbers represented entire words, particularly important names, places, and military terms. The name “Louis XIV” or the word “enemy” would each have its own unique number, completely hiding these crucial terms from prying eyes.

The Cipher That Baffled Historians

After the death of Bonaventure Rossignol, the key to the Great Cipher was lost. For more than two centuries, volumes of encrypted correspondence sat in the French archives, their contents a complete mystery. Historians knew these letters held invaluable insights into the military strategies and diplomatic intrigues of the Sun King’s reign, but they were unreadable. The code that had so effectively guarded Louis XIV’s secrets in his lifetime continued to do so long after his death.

One particularly famous mystery that some hoped the cipher would solve was that of the “Man in the Iron Mask”, an unidentified prisoner held for decades. Could the Great Cipher finally reveal his identity?

The Codebreaker’s Triumph

It wasn’t until the late 19th century that the cipher finally met its match. In 1893, a brilliant-but-stubborn French military cryptanalyst named Étienne Bazeries took on the challenge. For three years, he labored over the seemingly random strings of numbers, attempting and failing with every known method.

His breakthrough came from a linguistic, rather than purely mathematical, insight. He theorized that the numbers must represent syllables, not letters. He focused on a recurring sequence: 124-22-125-46-345. Drawing on his knowledge of French military correspondence, he made an educated guess that soldiers would frequently refer to “les ennemis”—the enemies.

He hypothesized:

• 124 = les
• 22 = en
• 125 = ne
• 46 = mis

(Note: the final ‘s’ in ennemis is often silent and dropped in such syllabic representations, but the core ‘mis’ was identified).

Armed with this tentative key, he tested it against other documents. Slowly, painstakingly, syllable by syllable, the secrets of the Sun King began to spill out. Bazeries had cracked the unbreakable code.

One of the first full letters he deciphered revealed not the identity of the Man in the Iron Mask, but a story of military cowardice. The message concerned General Vivien de Bulonde, who, during the Nine Years’ War, lifted his siege of Cuneo and fled in panic upon hearing of approaching enemy troops, leaving behind his wounded and munitions. A furious Louis XIV had written:

His Majesty has learned with a surprise… that you have raised the siege of Cuneo, yielding to the pretext that the enemy was approaching… His Majesty desires that you have General Bulonde arrested and taken to the fortress of Pignerol, where he will be locked in a cell and allowed to walk the ramparts during the day with a 330 309.

Bazeries deduced that “330 309” was a code for “masque.” The general was to be imprisoned and masked. While this wasn’t the famous masked prisoner of legend, it provided a stunning glimpse into the raw, unfiltered commands of the King.

The Enduring Legacy

The Great Cipher of Louis XIV stands as a monument in the history of cryptography. It was a system centuries ahead of its time, blending substitution, polyalphabetic elements, and codebook features into a formidable linguistic defense. Its creation and eventual deciphering highlight the eternal duel between secrecy and revelation—a battle fought not just with mathematics, but with a deep understanding of the structure, rhythm, and patterns of human language.