The History and Mathematics of Codes and Code Breaking

Tag: Rossignol

When Cryptographers Die

There were many strong ciphers that seemed impossible to decipher, but only one has the name "Great Cipher." The Great Cipher stood undecipherable for 200 years. Created by Antoine and Bonaventure Rossignol, it was used by King Louis XIV as a way to keep his secrets hidden, "protect details of his plans, plots, and political schemes." He was impressed by the cipher and the Rossignols' so much he gave the father-son duo offices near his apartments.

What made the Great Cipher so great was the combination of its use of syllables as cipher text in the form of numbers, and the death of both Antoine and Bonaventure. The Great Cipher was secure because it turned basic french syllables into cipher text into numbers, specifically 587 of them. As mentioned before, 200 years went by before it was deciphered. Many people tried their hand at the cipher and ultimately failed, died, or gave up before they could solve it. Along with the death of the Rossignols, there was no one to read the messages. This lead to messages being unreadable for years, thus securing the cipher for years until Etienne Bazeries deciphered the Great Cipher. This still took him a total of 3 years of work of using various techniques. Some of these techniques led to gibberish and complete restarts of his journey. He finally considered the numbers could be syllables, then he found a single word, "les ennemis," from a cluster of numbers that appeared several times. From here he could examine the other parts of cipher texts and decipher them.

The Great Cipher is remembered as one of the most secure ciphers in all of history. The techniques used to decipher it are still used in other deciphering techniques, and it is one of the "forefathers" of today's unsolved ciphers.

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.

A Great Deal of Creativity

As cryptographers attempted to improve the security of ciphers, while maintaining their practicality, more complex ciphers were being created.  The monoalphabetic substitution cipher was becoming less secure, leading to the advent of the polyalphabetic cipher and the homophonic cipher.  Yet, these ciphers required much more time to encipher, and were too complex for everyday use.  Cryptographers were on a mission to develop a cipher that was less complex than a polyalphabetic cipher and just as secure.  By the 17th century Antoine and Bonaventure Rossignol met that goal by creating the Great Cipher of Louis XIV.  The Great Cipher was simply an enhanced version of a monoalphabetic cipher, yet it remained unbroken for over two hundred years.  How was the Great Cipher so secure?

The Rossignol's were both excellent cryptographers and cryptanalysts.  As cryptanalysts, they had much more insight when creating the Great Cipher.  The Rossignol’s knew that this new cipher had to be very different from ciphers in the past.  This would ensure the security of Louis XIV’s messages and French secrets.  By acknowledging this idea, and using their past experiences as cryptanalysts, the Rossignol’s created a cipher that used numbers to encode syllables.  In the past, no cryptographer attempted to encipher a plaintext according to anything but letters.  By using syllables, it would take years for any cryptanalysts to decipher their codes.  Cryptanalysts rely on past information in order to solve a cipher.  Because the Great Cipher utilized a new method, cryptanalysts found it very difficult to solve.  Another factor that led to such a secure cipher was that the probability of solving the Great Cipher was so low.  The Great Cipher utilized 578 numbers, whereas typical monoalphabetic substitution ciphers featured 26 letters.  The Rossignol’s didn’t rely on just the use of syllables as their only method of security.  They also included traps in their ciphers to confuse cryptanalysts.  Sometimes numbers represented a single letter instead of a syllable, while other times a number represented nothing at all.  Ultimately, the Great Cipher represented a significant change in cryptography.  It utilized creativity and several lines of defense to keep the French secrets safe.

Deciphering the Great Cipher

For an impressive two-hundred years, the Great Cipher of Louis XIV thwarted several generations of accomplished cryptanalysts – a surprising feat, given that it did so through the manipulation of a substitution cipher. The cipher was created by the son-and-father pair of Antoine and Bonaventure Rossignal, who were recognized by King Louis XIV for their cryptological prowess. Their cipher was so secure that upon their deaths, decipherment of the French archives became impossible for the following two centuries. In 1890, however, Commandant  Etienne Bazeries, a distinguished expert of the French Army’s Cryptographic Department, began a successful three year endeavor of cracking the 17th-century code.

Despite Commandant Bazeries’ success in deciphering the Great Cipher of Louis XIV, the cipher can be termed “secure,” for it served its purpose well over its intended lifespan. Its success can be attributed to several ingenious cryptographic techniques that the Rossignal’s implemented into the cipher. The superficial level of complexity in the cipher is found in its range of representative numbers, of which there were 587, altogether representing only 26 letters. The wide range of numbers thus circumvented the technique of frequency analysis in its most basic application, for each letter would be represented by more than a single number. Realizing this, Bazeries applied frequency analysis in search of French diagraphs, with which he had no success. Frequency analysis proved effective only in the search of syllabic combinations, meaning that the cipher was constructed entirely from syllables. This characteristic probably grants the cipher most of its security. Because syllables exist in such variety, can be composed of one, two, or three letters of the English alphabet, and have less obvious patterns, it is considerably difficult to identify an applicable permutation of the assumed cipher. Moreover, the Rossignal’s integrated traps within the cipher to mislead a cryptanalyst from deducing the cipher-text. One trap, for example, included numbers that would essentially remove the number prior to it.

The use of syllabic substitution as well as the traps employed by the Rossignal’s certainly attributed to the considerable success of the Great Cipher of Louis XIV. However, as history has demonstrated time and time again, decipherment is only a matter of time.

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.

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.

The Durability of the Rossignols' Great Cipher

The fact that the Rossignols' Great Cipher remained invincible to decryption for over 200 years can be linked to both the complexity of the cipher and its novelty. The 587 different numbers used in the cipher creates thousands of possibilities; with hundreds of substitutions, any combination of multiple letters can be represented by a variety of numbers, and multiple letters or combinations of letters can have more than one number assigned to them. In Simon Singh’s The Code Book, he says that Bazeries spent months testing theories, only to find that they were incorrect (56). Immense time and effort were required to test simple possible theories, and traps were laid by the Rossignol to derail decryption efforts.

Another important factor in the Great Cipher was its ability to render frequency analysis obsolete. The cryptanalysts’ most useful tool was useless against this cipher. In order to decrypt the cipher, cryptanalysts needed to develop a completely new method, not just adapt an old one. In addition, the use of the cipher slowly faded after the death of the Rossignols, so no new messages could be created and examined. The urgency to decrypt the cipher also lessened after the cipher was no longer being used; the value of the messages became purely historical and held no political, military, or strategic value. The industrialization of cryptanalysis occurred after the Great Cipher and focused on monoalphabetic ciphers and messages in circulations, so the Great Cipher remained relevant to historians, but not to those with power and resources.

The Complexity of the Great Cipher

Antoine and Bonaventure Rossignol created the Great Cipher of Louis XIV and made it so complex that it took over 200 years to decipher. Generation after generation attempted to crack the cipher, yet no progress was made. The Rossignol’s both died, which terminated the ciphers use, as well as cutting off any potential collaboration with the creators and knowledge of exact details that could have been useful to the hundreds of codebreakers that tried to uncover the mystery. It was not until Commandant Etienne Bazeries came along and spent three years of his life working on deciphering letters of Louis XIV that the code was finally solved. Bazeries knew that it was not a substitution cipher, as there were 587 different numbers instead of the usual 26 different numbers. To Bazeries’ dismay, the cipher was also not a homophonic cipher, which was a possibility he entertained for months.

His final attempt proved to be worth it after all. The main factor that made this cipher so secure was the fact that each number represented a whole syllable, not a pair of letters. Bazeries finally got on a roll, guessing the remaining letters of an unfinished word, which enabled him to recognize other syllables. Another major deceiving factor in the Great Cipher was the traps that the Rossignols inserted; some numbers occasionally deleted previous numbers instead of standing for another syllable. The combination of the traps, the vague numbers, and the inability to collaborate with the Rossignols created an extremely secure and virtually unbreakable cipher.



The Great Decipherment

The Great Cipher, invented by the Rossignol family, was such a difficult cipher to crack because of two important factors. First the details and use of it was lost one the Rossignols died, which meant hat cryptanalyst had to start from scratch without knowing anything about how the cipher was created besides what they assume based on what other ciphers looked like during the same time period as Louis XIV.  The other significant factor that made the Great Cipher so powerful is the fact that it was not a simple single letter substitution cipher but actually a double letter substitution with a twist: instead of substituting letters, it instead substituted syllables. This greatly complicated the cipher because there an incredibly large number of ways in which to rearrange and cipher syllables, and if there is now starting point at which to begin deciphering the Great Cipher, then it becomes infinitely harder to crack the Great Cipher as opposed to a simple single letter substitution cipher. While these things combine to make it difficult to crack the Great Cipher; the inclusion of traps, such as numbers to delete the previous syllables, made it difficult to tell whether or not a postulated key to the cipher is correct until the traps are detected and accounted for. The combination of having no information about the formation of the Great Cipher as well as the unique substitution it uses as well as traps to trick cryptanalysts all combined to make it unbroken for 200 years after it was created despite the effort of many intelligent cryptologists.

Cipher Complexity of the Great Cipher

The Great Cipher, created by Antoine and Bonaventure Rossignol was, at its time, one of the greatest ciphers ever made. Cryptanalysists for 200 years worked to break the secrets hidden in the Rossignol's cipher, but it was only until Étienne Bazeries, a French expert in cryptography, spent three years tirelessly working on this cipher that it was finally broken. At that time, cryptanalysists had discovered the secret of frequency analysis to crack monoalphabetic substitution ciphers and had even discovered how to decipher homophonic substitution ciphers by looking at the unique character each letter in the English alphabet has.

The Rossignols' cipher built on these techniques but made cryptanalysis exponentially harder by making a few adjustments to the old techniques. The Rossignols, instead of assigning multiple numbers to the most common letters as in homophonic substitution, assigned multiple three digit numbers to the most common syllables. This made frequency analysis, though not impossible, much harder and more complex.

Bazeries, who finally cracked the cipher, was only able to discover their methods after three years of many trials and errors. Bazeries first thought the cipher might be a regular homophonic cipher, then perhaps a similar cipher with pairs of letters represented by numbers. After many months of educated guesses, Bazeries finally was able to discover one word, "les enemis," by using advanced frequency analysis but with French syllables instead of words.

This cipher was much harder to crack than its predecessors because of the use of syllables instead of letters. The Rossignols complicated their system even more by adding some numbers as syllables and some as single letters. They also made their cipher harder to crack by adding "traps" such as a number that represented no syllable or letter at all, but rather the deletion of the previous number. By increasing the layering of their cipher, the Rossignols were able to create a very complex cipher that effectively kept secret the information for over 200 years. Though even the Great Cipher was not impervious to the scrutiny of cryptanalysists, and all ciphers will eventually be figured out, the strength of a cipher is not measured by if it is able to be broken but how long it keeps the information safe.

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