The invention of the Louis XIV's great cipher () ()

My Web page about Cryptology

The Louis XIV's codebooks

The Rossignol family

In April 1628, the Prince of Condé led the French royal army besieging the Protestant (Huguenot) town of Réalmont. Condé's soldiers captured a Huguenot messenger carrying an encrypted message. No one in the Prince's entourage could decipher it. Condé then sent the message to Antoine Rossignol d'Albi, known to be interested in cryptography. Young Antoine resolved the message immediately: the besieged desperately needed ammunition. Condé sent the translation to the besieged who immediately capitulated.

Richelieu, who had the role of Prime Minister of King Louis XIII, as soon as he heard of this story, put the young Antoine at his service. From then on, Antoine proved his worth on many occasions by deciphering very important messages, in particular a message captured during the siege of La Rochelle which led (as for Réalmont) to the capitulation of the city.

Mazarin, Richelieu's successor, appointed Rossignol Master of the Chamber of Accounts and Councilor of State. During the reign of Louis XIV, Rossignol often worked in a room close to that of the king. He also worked in his domain of Juvisy 27 km from Versailles, about 2 hours for a courier on horseback.

Rossignol's successes made him rich. For example, in 1653, he received from the King 14,000 écus (one écu equal to 3 pounds); in 1672 he received 150,000 pounds (a pound is roughly equivalent to 10 euros or 10 dollars).

Antoine Rossignol died in December 1682 at the age of 83.

Antoine had two children: a girl, Marie and a boy, Bonaventure. Antoine introduced his son very early to cryptology. Bonaventure began to assist his father and later, logically succeeded him as the King's cryptanalyst.

Cipher clerks and Cryptanalysts

The cipher clerks formed the cipher department. Their role was to encrypt and decipher the messages sent or received by the king, his ministers or by his plenipotentiaries. It was they (probably the most experienced) who designed the nomenclatures. They were full-time employees.

Depending on the country, the era and their intelligence, cipher clerks also played the role of cryptanalyst.

In France, to break the enemy codes, it seems that consultants were called upon, such as the mathematician Viète at the time of King Henri IV or like the Rossignols at the time of Louis XIV. In England, the mathematician John Wallis acted as a cryptanalyst also as a consultant for the English government.

During the reign of Louis XIV, cipher clerks worked directly in liaison with the king and his ministers and therefore were physically close to these people. This was also the case for the Rossignol. It seems logical that exchanges took place between cryptanalysts (the Rossignol) and cipher clerks. Indeed, the Rossignol knew the weak points of the enemy numbers. They could help cipher clerks to design nomenclators that were more resistant to attacks.

Who invented the Great Cipher: is it Antoine or Bonaventure?

Mr Tomokiyo found in the Catinat's memoirs (published at the beginning of the 19th century) the first codebook using two tables. The document dates from 1676. The first authentic encryption/decryption tables found date from 1688. Oldest document I found using two tables are present in the Catinat's memories and it dates from 1685.

Most of the books or articles concerning cryptology give credit to Antoine (the father) creating the Great Cipher. However, if the first codebook with two tables dates from 1676, Antoine was about 77 years old.

I think it was rather his son, Bonaventure, who invented the Great Cipher. In Indeed, it is logical to imagine that an invention of this magnitude is the result of a young person rather than made by an old man. It is possible that Bonaventure has asked his father for advice and then it would be a common discovery. In any case, we have no details on this discovery.

Coexistence of different types of codebooks

As we will see in the examples, the exclusive use of double tables did not happen overnight. Semi-ordered (and therefore using a single table) numerical codebooks were used concurrently from 1676 until 1693 (at least), i.e. for almost twenty years!

These semi-ordered codebooks are different from those existing in previous eras and overall inferior to them. Breaking these codes was very easy. Indeed, each ciphering group was represented by a single numerical group. Another feature concerns letters that are ordered, for example a:20, b:22, c:24, d:26, e:28, ... . The only difference between two versions of these codes, was the starting point and the gap, for example, for a codebook different from the previous one: a:2, b: 5, c:8, d:11, ...

Why did this cohabitation take place? I have no definitive opinion. Here is my hypothese.

As we said before, it is assumed that the Rossignol proposed their methods to the cipher clerks. In the France of the time (and still today :-), an individual belongs to a caste and reluctantly receives advice from a person who is not of his caste. The Rossignol had to convince the cipher clerks that they had to gain by using new methods. So in my opinion, the invention of dual-table codebooks and single-table codebooks without redundancy was concomitant, each with a particular use:

  • A double-table codebook for the most important communications that must be as secure as possible. To simplify the work of the coders, the code will have few groups (at least at the beginning).
  • Single-table codebook without redundancy for non-critical communications. To help encryptors, the very simplified structure of this type of code allows them possibly to learn them by heart and encode / decode very quickly.
Note that there is one thing in common between these two types of codes: they are purely digital. Cipher clerks quickly see the benefit: it is very easy to add codegroups to encode more and more proper nouns and common nouns and ultimately encode faster.

As you can see, everyone is happy. The French government now has a very secure method which is certainly more complex to handle by the cipher clerks but another method (at the beginning probably the most common) is available and which makes the cipher clerks happy.

References

  • THE CODEBREAKERS, The Story of Secret Writing, by David Kahn, edited by Scribner (1967, 1996). This work, one of the most important in the history of the cryptology, describes (among other things) the history of the Rossignol. It gives for each detail references to books or archives (Bnf, etc.).