Jacksonville, Florida - For nearly 250 years The Great Cipher remained uncrackable, even today much of its ciphertext remains undecrypted… It’s time to take another look back at some historical encryption, today we’re going to talk about The Great Cipher. Invented sometime around the middle of the 17th century, under King Louis XIV, The Great Cipher remained unsolved until 1893. In fact, even to this day many of its enciphered documents in the French Archives are unreadable.

So, who invented The Great Cipher? Who cracked it? And what made it so formidable?

Let’s hash it out.

Rossignol is an Aptronym

At the heart of today’s historical tale is a family with the surname Rossignol. Rossignol is French for “nightingale,” but perhaps more appropriately, since about 1400 the word had been used as the French term for “skeleton key.” A skeleton key is, in reality, a type of master key that has been filed down to its essential parts, which can open most locks—though the term takes on near-mythic qualities in popular lore.

Antoine Rossignol first appears in the historical record in 1626. Rossignol was under the employ of a local prince, assisting with what seemed like it would be a months-long siege of a city called Realmont. When a courier carrying an enciphered letter from the besieged city was intercepted, Rossignol decrypted it within an hour. The letter showed that the Huguenot (a French Protestant minority) forces’ hold on the city was tenuous—they were under-supplied and out of ammunition. The prince’s forces returned the letter along with the deciphered copy and the Huguenots surrendered immediately.

This caught the attention of Louis XIII’s chief minister, who was quite fond of using secure ciphers and cryptography in both his diplomatic and intelligence gathering functions. Rossignol acquitted himself so well in his years of service that famous poems were even written about him and his cryptanalytic abilities.

By the time Louis XIV’s reign began Rossignol was working with his son, Bonaventure, splitting time between his own estate and a room in the Palace of Versailles adjacent to the King’s study. There they developed The Great Cipher that would remain uncracked for the next 250 years.

What is The Great Cipher?

The Great Cipher was a nomenclator cipher that the Rossignols continued using for several generations in the service of the French crown. A nomenclator cipher is a kind of substitution cipher. In addition to substitution tables (nomenclators) that were used to replace each letter of a text, there was also a codebook. Originally the code words or symbols only replaced the names of people (hence the prefix nomen-) but eventually it evolved to also replace important places and other common words. This was done to avoid a specific cryptanalytic technique called frequency analysis, where an attacker would look for commonly occurring words to help break the encryption.

What made The Great Cipher so special? Well – and this definitely adds to the lore – a lot of the specifics of the cipher died with the Rossignol family. What we do know is this: the cipher had thousands of symbols, but only 587 of them were different (others were redundant, with multiple symbols for the same thing). Now, this is where it gets unique. Most substitution ciphers are going to be limited to the 26 letters in the alphabet and maybe ten numerals. Regardless, you are working with a knowable, finite number of characters.

The Great Cipher didn’t work that way because the Rossignols were starting with a plaintext that was basically written phonetically. They were substituting on a syllabic basis—literally one syllable at a time. This basically meant they were encrypting vocal sounds. Of which there are many, many more than there are characters in the alphabet. 587, by their count.

Nobody ever accused The Great Cipher of being efficient. To counter the frequency analysis technique we discussed earlier some number sets were Nulls, meant to be ignored by the decryptor, some dictated adjustments that needed to be made to other parts of the ciphertext and some were straight up traps including one code group that told the decryptor to ignore the previous one. In one nomenclator, 131 of the 711 code numbers stood for the sound made by the letter “E.”

All of this made The Great Cipher nearly unbreakable. In fact, at the time it was thought to be completely unbreakable. It wasn’t until 250 years later that anyone would get close.

Étienne Bazeries cracks The Great Cipher… kind of

Étienne Bazeries was a French cryptanalyst that served from 1890 until the first World War. His contribution to cryptography is worth its own article (a topic for another day), but he began as little more than a hobbyist—solving cryptograms in local newspapers until he began applying his cryptanalytic skills to his role in the military.

After his decryption forced the French military to switch to a new cipher, word began to spread about his abilities. Much like Antoine Rossignol centuries earlier, he became one of the foremost cryptographers in all of France, working at the Ministry of Foreign Affairs starting in 1891.

It was there, after he had begun work on it three years earlier, that in 1893 Bazeries cracked The Great Cipher. This was truly a strenuous undertaking, where Bazeries would work out various theories over the course of months before ultimately proving them to be false. He finally arrived at the hypothesis that Rossignol had been encrypting syllables after exhausting nearly every other reasonable idea he had.

The string of numbers that proved to be the strand that unraveled everything was 124-22-125-46-345, which he surmised stood for les ennemis (the enemies). From there, he began to reverse engineer the cipher until he was finally able to begin decrypting some of the enciphered documents.

The legacy of The Great Cipher

Part of what made The Great Cipher so robust was the experience Rossignol had accumulated in his own time as a cryptanalyst. Rossignol was a skilled mathematician. He also ran the Cabinet Noir, an early intelligence agency that intercepted communication headed to and from criminal suspects and political enemies so that it might be decrypted and the information contained there within used by Louis XIV and his government. The man knew what he was doing.

Bazeries for his part was self-taught. The son of a mounted police officer, he enlisted in the military in 1863 and continued to serve until 1924 when he retired at 78. He wrote a book on cracking ciphers in 1901 that is still revered in cryptography circles to this day.

Unfortunately, even with what Bazeries discovered on how to break The Great Cipher, few have the skills or the wherewithal to do it today, which means huge sections of the French Archives are unreadable. The cipher was used from the mid 1600s until 1811 when the last of the Rossignols died off and it fell out of use.

That means critical documents and unknown perspectives from entire chapters – over 150 years – of French history are enciphered away.