The History and Mathematics of Codes and Code Breaking

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The Great Cipher Was a Really Great Cipher

The 1600’s were a strange time in the history of cryptography. Monoalphabetic ciphers had run their course, with cryptanalysts having the resources and know-how to crack any monoalphabetic cipher quickly. On the other hand, the newly developed polyalphabetic cipher, a cipher that uses multiple cipher alphabets, was effective but too tedious to be embraced by codemakers. People needed a method of encryption that was unbreakable and also not so difficult to use. That’s when the Rossingols, cryptanalysts employed by the French government, developed the Great Cipher of Louis XIV. It is important to wonder why this particular cipher, which is just an enhanced monoalphabetic cipher, took 200 years to decipher.

The first reason why the Great cipher took so long to decipher was its complexity compared to ciphers cryptanalysis had seen in the past. The Great Cipher was a monoalphabetic cipher, meaning each symbol in the “cipher alphabet” mapped to one and only one thing in the plaintext alphabet. Still, it was extremely different from all other monoalphabetic ciphers. First, it used numbers. The use of numbers to map to letters had been a relatively new development in cryptography, and cryptanalysts still didn’t know the best method to decipher numeric codes. More importantly, however, the Great Cipher included 576 numbers: many more letters than there are in the alphabet. This great of a mismatch between the quantity of symbols and the quantity of letters had never been seen before, so there was initially a huge gap between the experience of the cryptallaists and the complexity of this cipher.

The second reason the Great Cipher took so long to decipher was the technologies available at the time. This was a cipher of 576 numbers. If it were 26 numbers, it would be somewhat obvious that each number matches a letter. However, with 576 numbers, each number could mean anything. So, many possibilities needed to be tested out. However, due to the fact that everything had to be written out, testing a possibility was extremely tedious, time consuming, and daunting. Bazeries, the man who eventually deciphered the Great Cipher 200 years later, spent months testing if it was a homophone and then spent months testing whether the numbers represented pairs of words. In short, no one wanted to do the herculean task of testing failing decriptions over and over again with pen and paper, so it took a long time before someone took up the task.

Lastly, the Great Cipher took so long to decipher because of the creativity and effectiveness of the cipher itself. Each cipher in the past had been based off of letters, but the Rossingols based their cipher off of syllables, matching each number to a syllable in the english language. This system made it just as easy to send messages and decode them with a key, but made the cipher extremely difficult to crack, given that there are so many potential syllables. The creativity put into this cipher showed, as no one thought to look at the syllables for 200 years. The Great Cipher took so long to crack because it was everything a great cipher is: complex, daunting, and way before its time. 


The Great Cipher:Cryptography as Man in the Iron Mask:French Lore

The Great Cipher of Louis XIV proved to be one of the most challenging ciphers ever recorded. This was brought together by the father son duo of Antoine and Bonaventure Rossignol. They immersed themselves in studying and perfecting methods of cryptography from the past and eventually determined how to create a form of encryption stronger than anyone had seen before.

Previously, cryptographers focused on discovering the identities of individual letters. When Bazeries first attempted to decipher the text he used these methods mainly believing that it was a homophonic cipher after discovering that there were 587 different numbers being used. This was another big advantage to the Rossignol cipher, instead of using a normal 26 number system they used double and triple digit numbers. Believing to be homophonic, Bazeries attempted to align each number with a letter. Of course this proved to be futile as the cipher was not homophonic. One method that had never been used previously was assigning numbers with syllables which is exactly what the Rossignol’s did. The code may have remained unsolved had Brazeries not deciphered the text “les ennemis”. The use of traps and numbers representing “delete” also delayed the solving of the cipher.

One big form of security for the cipher that can often be forgotten is that the Rossignols died without letting anyone know how to solve their cipher. It remained a complete mystery, along with the Man in the Iron Mask, until Brazeries. Their greatest achievement was going past the typical ciphertext of representing one letter for a number and instead using numbers to represent syllables.

The Great Cipher: An Unbreakable Cipher for 2 Centuries

The father-and-son team of Antoine and Bonaventure Rossignol invented the Great Cipher for the French king Louis XIV to encrypt the empire’s most secret messages, protecting details of his plans, plots and political schemings. While the nature of the Great Cipher was simply an enhanced monoalphabetic cipher with homophones, it seemed implausible that it remained unbreakable for two centuries. However, there were two main factors that led to such a secure cipher.

The most significant one was considered to be the Rossignol’s ingenuity and resourcefulness. Including 587 different numbers, the Great Cipher was obviously not a straightforward substitution cipher. But when Étienne Bazeries, a distinguished cryptanalyst tried to crack it as a homophonic cipher, he failed. He then came up with the idea that each number might represent a digraph, or a pair of letters. Although his efforts to this deciphering approach again yielded nothing, it enlightened him on the possibility that some numbers corresponded to syllables. After a few attempts, he made a breakthrough, with the discovery of “les-en-ne-mi-s” represented by a cluster of numbers (124-22-125-46-345), and thus his idea eventually proved to be right. During that time when cryptography was mainly about encrypting plain alphabets with cipher alphabets, it was creative of the Rossignol to use syllables for the complexity of homophones. More importantly, they had also laid traps for codebreakers, adding numbers which deviously deleted previous numbers instead of representing any meaningful letters or syllables. All their creative encipherment contributed to the strength of its encryption, making it confusing and harder to decipher.

Additionally, after the death of both father and son, the Great Cipher fell into disuse and many details about it were lost; therefore, for those who wanted to break the codes had to start from scratch. Due to its difficulty, only the most prominent cryptanalysts were capable of deciphering it with consistent dedication and patience. As a result, it was no surprise that the Great Cipher was known as one of the strongest ciphers in the history of cryptography.

Be vague and Roman Numerals

After the execution of Mary Queen of Scots but prior to the development of more complex ciphers, like the Vigenère cipher, “anybody sending an encrypted message had to accept that an expert enemy codebreaker might intercept and decipher their most precious secrets.” (Singh, p. 45) Because of this, it is safe to assume people writing the encrypted messages would still be very careful with what they were actually saying, and how the things that they wrote could potentially incriminate them. Messages would have been written in vague enough language that even if the text were to be deciphered, the cryptanalysts would either not be able to tell what the exact plan was or, even if they could figure it out, would not be obvious enough to be used in a court of law. (I don’t know how the rules for this worked in that time but today people can claim the way a message is interpreted is completely wrong.)

Going off in a completely different direction now, as I was reading I was thinking of different types of ciphers. When I the name Louis XIV, I fixated on the Roman Numerals. It made me curious about the use of Roman Numerals in cipher alphabets. I think it would add a layer of complexity because the cryptanalysts would have to try to figure out which combinations of letters would represent a number, and then which letter was represented by that number. One weakness, however, would be that it would be very easily identifiable as Roman Numerals because of the small number of letters used in Roman Numerals. I tried to look up Roman Numeral ciphers but nothing came up on a quick Google search.

Phonetic Way of Mind

The Great Cipher used by Louis XIV remained unbroken for 200 years.  What were the factors that led to such a secure cipher?

The cipher was pioneered by a father and son duo, most of the specifics to how it exactly worked were known best by these two people. With the death of both father and son, the specifics of the cipher were quickly lost. When there are no people around that know how to use and reproduce the cipher, the motive to crack it is lost. There was some important information enciphered with The Great Cipher, however nobody was actively using it, so resources used to crack ciphers would be diverted to cracking ciphers used at that time. The lack of motive is the smallest reason as to why it took so long to crack; the cipher itself is very elegant and complex. This cipher was not one that took a written word then simply changed letters, it was a completely new way to write down the language. Languages operate with distinct sounds that can be represented by letters, putting two letters together will change the sound. Writing a cipher with syllables in mind will make it more difficult to crack, especially to a cryptanalyst who writes with an alphabetic language (like English). On top of that, The Great Cipher had certain traps put into place that would make certain parts look like gibberish causing cryptanalyst to reevaluate the type of cipher.

In my opinion, a syllabary cipher would be most effective today. This is because most of society is literate and thinks in a similar manner to the way we write, letter for letter, not letter for sound. Using syllables, but re-vamping it with more traps, would confuse people because they are not used to naturally thinking in that manner when writing.


(I double checked some facts here and here

Not Your Average Monoalphabetic Cipher

The Great Cipher used by Louis XIV remained unbroken for 200 years.  What were the factors that led to such a secure cipher?

The father-son team of Antoine and Bonaventure Rossignol invented The Great Cipher while working closely with Louis XIV as his cryptanalysts. Initially, they were mainly code breakers, but their skill gave them the idea to create a much stronger way to encrypt messages. This idea turned into the Great Cipher. This cipher was very useful for the French and no enemy cryptanalysts were able to crack it. Unfortunately, the Rossignols’s death also meant that the Great Cipher’s secrets were lost and any archives encoded using it could no longer be read. Although this was inconvenient for the French, the real struggle would be experienced by future generations of code breakers. Eventually letters encrypted by the Great Cipher were passed on to Étienne Bazeries who worked tirelessly to decipher the letters. The high security of the cipher made it nearly impossible to decode.

The first factor that led to this secure cipher was the amount of characters included. 587 different characters immediately made it clear that the it was not a substitution cipher and later, Étienne also discovered it was not a homophonic cipher (a cipher that replaces letters with a proportional number of symbols to how often that letter is used). Later he would also try to decipher it as a digraph (one number represents a pair of letters), but this also was not correct. The grunt work that decoding the Great Cipher must have required is astonishing because the text says each idea could take Étienne multiple months to prove wrong. Eventually, Étienne was struck with the idea that each number represented a whole syllable. After tirelessly working on this idea, he was able to decode 124-22-125-46-345 as meaning “les ennemis”. This crucial breakthrough led to Étienne’s eventual success despite variations in the cipher and traps laid by the Rossignols. This elaborate cipher truly deserved its name as “The Great Cipher”.

The Great Cipher: 200 Years of Security

Louis XIV used the Great Cipher, invented by Antoine and Bonaventure Rossignol, throughout the seventeenth century. Following the death of the Rossignols, The Great Cipher remained an unsolved mystery until the nineteenth century, when new texts encrypted by The Great Cipher were discovered and passed on to a French cryptographer Bazeries. Bazeries struggled with the cipher for years, but eventually was able to successfully decipher several key historical messages and crack The Great Cipher.

The Great Cipher utilized 587 different numbers and was not a homophonic cipher, as Bazeries found after many failed attempts. Bazeries then explored the idea that the Great Cipher was based off of digraphs, or pairs of letters. Although this idea was wrong, it ultimately led him to his discovery that The Great Cipher paired numbers to syllables. The cipher proved to be even more complicated as certain numbers stood for single letters only while others stood for syllables. There were also tricks embedded in the cipher; for example, certain numbers meant that the number before it should be deleted.

The Great Cipher was protected for 200 years due to its great complexity and ingenuity for the time period. The manipulation of syllables as opposed to letters was revolutionary in the cryptography world. The added complexity through the use of single letters and nulls made Bazeries’s task even more difficult. The Great Cipher was a remarkably secure cipher that stumped the finest cryptographers for 200 years.

Bazeries’ Quest Against The Great Cipher

The Great Cipher used by Louis XIV implemented a different method of cryptography and ciphering than ever before. The monoalphabetic substitution cipher was too easy to break while the polyalphabetic cipher created by Vigenère took too long to encipher and decode which was not efficient for military operations. The Great Cipher, created by the Rossingols and later cracked by Bazeries, utilized not only letters, but also numbers in the cipher. And the different numbers did not represent letters; they mostly represented syllables. This cipher also included traps. For example, some numbers initiated the deletion of the previous number. Some of the numbers did not represent syllables but single letters. The sophisticated nature of this cipher contributed to its dormancy for two centuries. Yet the ease of deciphering a message ciphered using the Great Cipher was quick enough to be used for military purposes, if the cipher was known. Another characteristic of the Great Cipher that was impressive was that it almost completely paralyzed the use of frequency analysis. Although frequency analysis actually lead to Bazeries cracking of the cipher when he noticed a repeated sequence of numbers. But he then completely guessed what those numbers could mean and he happened to be spot on.

The “Unbreakable” Code

Louis XIV’s code was one that was not deciphered for over 200 years. It used 587 number codes which represented letters and syllables, among other things. The cypher was created by Antoine and Bonaventure Rossignol as a means to keep military, economic, and other personal information hidden.
The cypher however did not use a standard substitution alphabet, as could be discovered by the fact that there were 587 number codes. Commandant Etienne Bakeries, who was an expert in military code breaking, devoted himself to trying to break Louis XIV’s cypher in the 1890’s. He first discovered that the codes did not represent a homophone alphabet, which would use multiple symbols for the same letter to remove the use for frequency analysis. Next he discovered that the cypher did not use a digraph, or each symbol representing a pair of letters. Bakeries’ first major breakthrough came in recognizing a cluster of numbers that was repeated often. He found this to mean “les ennemis”, with the numbers representing syllables in the French language. Little by little he was able to begin solving for more letters, followed by some words, which lead to eventually solving the entire code.
It may seem that the cypher in fact failed, but one has to remember, a cypher’s effectiveness doesn’t come from if it can be solved, but actually on if it can’t be solved in time to be able to use the knowledge against the author. 200 years later, the cypher had done its job and was already obsolete.

Syllable Substitution

The Great Cipher of Louis XIV was truly a remarkable cipher, and its longevity only attests to its success. Antoine and Bonaventure Rossignol were the masterminds behind its brilliance, and with their death brought the end of its effective use. During the time of its use, the Rossignols lived adjacent to Louis XIV, since they were the only ones who could effectively use it. The fact that it took two centuries to crack is beyond remarkable, and it deserves praise.

The key to its success and difficulty was that the plain text was not explicitly the 26 letters of the alphabet. Most monoalphabetic and polyalphabetic substitutions start with the initial 26 letters (a, b, c, … z) as the original plain text and substitute each letter with another letter (or in the case of polyalphabetic ciphers) or maybe a couple letters. But the fact is that the plain text and the cipher text will be limited to the 26 letters of the alphabet, no matter the method of cipher used. This cipher was unique compared to others because the original plain text consisted of all of the individual sounds or syllable used, hence there were many, many more original plain text “letters” compared to an ordinary substitution ciphers. Therefore numbers had to be used rather than letters because there was no alphabet large enough to contain all of the syllables in the language.

Additionally, the Rossignols added traps within the cipher, such as numbers that adjusted adjacent numbers (like removing them entirely). Numbers also often translated to single letters rather than syllables which threw off people who attempted to decipher it. These key differences made the code inefficient since there were so many different “letters” that a person needed to keep track of, but at the same time made it virtually uncrackable to even the most scholarly people. If it were not for the sheer size of the pool of the total numbers used, people would have undoubtedly continued to use it.

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