The Code Book Part 3

A h.o.m.ophonic cipher might seem similar to a polyalphabetic cipher inasmuch as each plaintext letter can be enciphered in many ways, but there is a crucial difference, and the h.o.m.ophonic cipher is in fact a type of monoalphabetic cipher. In the table of h.o.m.ophones shown above, the letter a can be represented by eight numbers. Significantly, these eight numbers represent only the letter a. In other words, a plaintext letter can be represented by several symbols, but each symbol can only represent one letter. In a polyalphabetic cipher, a plaintext letter will also be represented by different symbols, but, even more confusingly, these symbols will represent different letters during the course of an encipherment.

Perhaps the fundamental reason why the h.o.m.ophonic cipher is considered monoalphabetic is that once the cipher alphabet has been established, it remains constant throughout the process of encryption. The fact that the cipher alphabet contains several options for encrypting each letter is irrelevant. However, a cryptographer who is using a polyalphabetic cipher must continually switch between distinctly different cipher alphabets during the process of encryption. cipher must continually switch between distinctly different cipher alphabets during the process of encryption.

By tweaking the basic monoalphabetic cipher in various ways, such as adding h.o.m.ophones, it became possible to encrypt messages securely, without having to resort to the complexities of the polyalphabetic cipher. One of the strongest examples of an enhanced monoalphabetic cipher was the Great Cipher of Louis XIV. The Great Cipher was used to encrypt the king's most secret messages, protecting details of his plans, plots and political schemings. One of these messages mentioned one of the most enigmatic characters in French history, the Man in the Iron Mask, but the strength of the Great Cipher meant that the message and its remarkable contents would remain undeciphered and unread for two centuries.

The Great Cipher was invented by the father-and-son team of Antoine and Bonaventure Rossignol. Antoine had first come to prominence in 1626 when he was given a coded letter captured from a messenger leaving the besieged city of Realmont. Before the end of the day he had deciphered the letter, revealing that the Huguenot army which held the city was on the verge of collapse. The French, who had previously been unaware of the Huguenots' desperate plight, returned the letter accompanied by a decipherment. The Huguenots, who now knew that their enemy would not back down, promptly surrendered. The decipherment had resulted in a painless French victory.

The power of codebreaking became obvious, and the Rossignols were appointed to senior positions in the court. After serving Louis XIII, they then acted as crypta.n.a.lysts for Louis XIV, who was so impressed that he moved their offices next to his own apartments so that Rossignol pere et fils pere et fils could play a central role in shaping French diplomatic policy. One of the greatest tributes to their abilities is that the word could play a central role in shaping French diplomatic policy. One of the greatest tributes to their abilities is that the word rossignol rossignol became French slang for a device that picks locks, a reflection of their ability to unlock ciphers. became French slang for a device that picks locks, a reflection of their ability to unlock ciphers.

The Rossignols' prowess at cracking ciphers gave them an insight into how to create a stronger form of encryption, and they invented the so-called Great Cipher. The Great Cipher was so secure that it defied the efforts of all enemy crypta.n.a.lysts attempting to steal French secrets. Unfortunately, after the death of both father and son, the Great Cipher fell into disuse and its exact details were rapidly lost, which meant that enciphered papers in the French archives could no longer be read. The Great Cipher was so strong that it even defied the efforts of subsequent generations of codebreakers. enciphered papers in the French archives could no longer be read. The Great Cipher was so strong that it even defied the efforts of subsequent generations of codebreakers.

Historians knew that the papers encrypted by the Great Cipher would offer a unique insight into the intrigues of seventeenth-century France, but even by the end of the nineteenth century they were still unable to decipher them. Then, in 1890, Victor Gendron, a military historian researching the campaigns of Louis XIV, unearthed a new series of letters enciphered with the Great Cipher. Unable to make sense of them, he pa.s.sed them on to Commandant etienne Bazeries, a distinguished expert in the French Army's Cryptographic Department. Bazeries viewed the letters as the ultimate challenge, and he spent the next three years of his life attempting to decipher them.

The encrypted pages contained thousands of numbers, but only 587 different ones. It was clear that the Great Cipher was more complicated than a straightforward subst.i.tution cipher, because this would require just 26 different numbers, one for each letter. Initially, Bazeries thought that the surplus of numbers represented h.o.m.ophones, and that several numbers represented the same letter. Exploring this avenue took months of painstaking effort, all to no avail. The Great Cipher was not a h.o.m.ophonic cipher.

Next, he hit upon the idea that each number might represent a pair of letters, or a digraph digraph. There are only 26 individual letters, but there are 676 possible pairs of letters, and this is roughly equal to the variety of numbers in the ciphertexts. Bazeries attempted a decipherment by looking for the most frequent numbers in the ciphertexts (22, 42, 124, 125 and 341), a.s.suming that these probably stood for the commonest French digraphs (es, en, ou, de, nt). In effect, he was applying frequency a.n.a.lysis at the level of pairs of letters. Unfortunately, again after months of work, this theory also failed to yield any meaningful decipherments.

Bazeries must have been on the point of abandoning his obsession, when a new line of attack occurred to him. Perhaps the digraph idea was not so far from the truth. He began to consider the possibility that each number represented not a pair of letters, but rather a whole syllable. He attempted to match each number to a syllable, the most frequently occurring numbers presumably representing the commonest French syllables. He tried various tentative permutations, but they all resulted in gibberish-until he succeeded in identifying one particular word. A cl.u.s.ter of numbers (124-22-125-46-345) appeared several times on each page, and Bazeries postulated that they represented les-en-ne-mi-s, that is, "les ennemis." This proved to be a crucial breakthrough. occurring numbers presumably representing the commonest French syllables. He tried various tentative permutations, but they all resulted in gibberish-until he succeeded in identifying one particular word. A cl.u.s.ter of numbers (124-22-125-46-345) appeared several times on each page, and Bazeries postulated that they represented les-en-ne-mi-s, that is, "les ennemis." This proved to be a crucial breakthrough.

Bazeries was then able to continue by examining other parts of the ciphertexts where these numbers appeared within different words. He then inserted the syllabic values derived from "les ennemis," which revealed parts of other words. As crossword addicts know, when a word is partly completed it is often possible to guess the remainder of the word. As Bazeries completed new words, he also identified further syllables, which in turn led to other words, and so on. Frequently he would be stumped, partly because the syllabic values were never obvious, partly because some of the numbers represented single letters rather than syllables, and partly because the Rossignols had laid traps within the cipher. For example, one number represented neither a syllable nor a letter, but instead deviously deleted the previous number.

When the decipherment was eventually completed, Bazeries became the first person for two hundred years to witness the secrets of Louis XIV. The newly deciphered material fascinated historians, who focused on one tantalizing letter in particular. It seemed to solve one of the great mysteries of the seventeenth century: the true ident.i.ty of the Man in the Iron Mask.

The Man in the Iron Mask has been the subject of much speculation ever since he was first imprisoned at the French fortress of Pignerole in Savoy. When he was transferred to the Bastille in 1698, peasants tried to catch a glimpse of him, and variously reported him as being short or tall, fair or dark, young or old. Some even claimed that he was a she. With so few facts, everyone from Voltaire to Benjamin Franklin concocted their own theory to explain the case of the Man in the Iron Mask. The most popular conspiracy theory relating to the Mask (as he is sometimes called) suggests that he was the twin of Louis XIV, condemned to imprisonment in order to avoid any controversy over who was the rightful heir to the throne. One version of this theory argues that there existed descendants of the Mask and an a.s.sociated hidden royal bloodline. A pamphlet published in 1801 said that Napoleon himself was a descendant of the Mask, a rumor which, since it enhanced his position, the emperor did not deny. Mask, a rumor which, since it enhanced his position, the emperor did not deny.

The myth of the Mask even inspired poetry, prose and drama. In 1848 Victor Hugo had begun writing a play ent.i.tled Twins Twins, but when he found that Alexandre Dumas had already plumped for the same plot, he abandoned the two acts he had written. Ever since, it has been Dumas's name that we a.s.sociate with the story of the Man in the Iron Mask. The success of his novel reinforced the idea that the Mask was related to the king, and this theory has persisted despite the evidence revealed in one of Bazeries's decipherments.

Bazeries had deciphered a letter written by Francois de Louvois, Louis XIV's Minister of War, which began by recounting the crimes of Vivien de Bulonde, the commander responsible for leading an attack on the town of Cuneo, on the French-Italian border. Although he was ordered to stand his ground, Bulonde became concerned about the arrival of enemy troops from Austria and fled, leaving behind his munitions and abandoning many of his wounded soldiers. According to the Minister of War, these actions jeopardized the whole Piedmont campaign, and the letter made it clear that the king viewed Bulonde's actions as an act of extreme cowardice: His Majesty knows better than any other person the consequences of this act, and he is also aware of how deeply our failure to take the place will prejudice our cause, a failure which must be repaired during the winter. His Majesty desires that you immediately arrest General Bulonde and cause him to be conducted to the fortress of Pignerole, where he will be locked in a cell under guard at night, and permitted to walk the battlements during the day with a mask.

This was an explicit reference to a masked prisoner at Pignerole, and a sufficiently serious crime, with dates that seem to fit the myth of the Man in the Iron Mask. Does this solve the mystery? Not surprisingly, those favoring more conspiratorial solutions have found flaws in Bulonde as a candidate. For example, there is the argument that if Louis XIV was actually attempting to secretly imprison his unacknowledged twin, then he would have left a series of false trails. Perhaps the encrypted letter was meant to be deciphered. Perhaps the nineteenth-century codebreaker Bazeries had fallen into a seventeenth-century trap.

The Black Chambers Reinforcing the monoalphabetic cipher by applying it to syllables or adding h.o.m.ophones might have been sufficient during the 1600s, but by the 1700s crypta.n.a.lysis was becoming industrialized, with teams of government crypta.n.a.lysts working together to crack many of the most complex monoalphabetic ciphers. Each European power had its own so-called Black Chamber, a nerve center for deciphering messages and gathering intelligence. The most celebrated, disciplined and efficient Black Chamber was the Geheime Kabinets-Kanzlei in Vienna.

It operated according to a rigorous timetable, because it was vital that its nefarious activities should not interrupt the smooth running of the postal service. Letters which were supposed to be delivered to emba.s.sies in Vienna were first routed via the Black Chamber, arriving at 7 A.M. A.M. Secretaries melted seals, and a team of stenographers worked in parallel to make copies of the letters. If necessary, a language specialist would take responsibility for duplicating unusual scripts. Within three hours the letters had been resealed in their envelopes and returned to the central post office, so that they could be delivered to their intended destination. Mail merely in transit through Austria would arrive at the Black Chamber at 10 Secretaries melted seals, and a team of stenographers worked in parallel to make copies of the letters. If necessary, a language specialist would take responsibility for duplicating unusual scripts. Within three hours the letters had been resealed in their envelopes and returned to the central post office, so that they could be delivered to their intended destination. Mail merely in transit through Austria would arrive at the Black Chamber at 10 A.M. A.M., and mail leaving Viennese emba.s.sies for destinations outside Austria would arrive at 4 P.M P.M. All these letters would also be copied before being allowed to continue on their journey. Each day a hundred letters would filter through the Viennese Black Chamber.

The copies were pa.s.sed to the crypta.n.a.lysts, who sat in little kiosks, ready to tease out the meanings of the messages. As well as supplying the emperors of Austria with invaluable intelligence, the Viennese Black Chamber sold the information it harvested to other powers in Europe. In 1774 an arrangement was made with Abbot Georgel, the secretary at the French Emba.s.sy, which gave him access to a twice-weekly package of information in exchange for 1,000 ducats. He then sent these letters, which contained the supposedly secret plans of various monarchs, straight to Louis XV in Paris.

The Black Chambers were effectively making all forms of monoalphabetic cipher insecure. Confronted with such professional crypta.n.a.lytic opposition, cryptographers were at last forced to adopt the more complex but more secure Vigenere cipher. Gradually, cipher secretaries began to switch to using polyalphabetic ciphers. In addition to more effective crypta.n.a.lysis, there was another pressure that was encouraging the move toward securer forms of encryption: the development of the telegraph, and the need to protect telegrams from interception and decipherment. secure Vigenere cipher. Gradually, cipher secretaries began to switch to using polyalphabetic ciphers. In addition to more effective crypta.n.a.lysis, there was another pressure that was encouraging the move toward securer forms of encryption: the development of the telegraph, and the need to protect telegrams from interception and decipherment.

Although the telegraph, together with the ensuing telecommunications revolution, came in the nineteenth century, its origins can be traced all the way back to 1753. An anonymous letter in a Scottish magazine described how a message could be sent across large distances by connecting the sender and receiver with 26 cables, one for each letter of the alphabet. The sender could then spell out the message by sending pulses of electricity along each wire. For example, to spell out h.e.l.lo, the sender would begin by sending a signal down the h wire, then down the e wire, and so on. The receiver would somehow sense the electrical current emerging from each wire and read the message. However, this "expeditious method of conveying intelligence," as the inventor called it, was never constructed, because there were several technical obstacles that had to be overcome.

For example, engineers needed a sufficiently sensitive system for detecting electrical signals. In England, Sir Charles Wheatstone and William Fothergill Cooke built detectors from magnetized needles, which would be deflected in the presence of an incoming electric current. By 1839, the Wheatstone-Cooke system was being used to send messages between railway stations in West Drayton and Paddington, a distance of 29 km. The reputation of the telegraph and its remarkable speed of communication soon spread, and nothing did more to popularize its power than the birth of Queen Victoria's second son, Prince Alfred, at Windsor on August 6, 1844. News of the birth was telegraphed to London, and within the hour The Times The Times was on the streets announcing the news. It credited the technology that had enabled this feat, mentioning that it was "indebted to the extraordinary power of the Electro-Magnetic Telegraph." The following year, the telegraph gained further fame when it helped capture John Tawell, who had murdered his mistress in Slough, and who had attempted to escape by jumping on to a London-bound train. The local police telegraphed Tawell's description to London, and he was arrested as soon as he arrived at Paddington. was on the streets announcing the news. It credited the technology that had enabled this feat, mentioning that it was "indebted to the extraordinary power of the Electro-Magnetic Telegraph." The following year, the telegraph gained further fame when it helped capture John Tawell, who had murdered his mistress in Slough, and who had attempted to escape by jumping on to a London-bound train. The local police telegraphed Tawell's description to London, and he was arrested as soon as he arrived at Paddington.

Meanwhile, in America, Samuel Morse had just built his first telegraph line, a system spanning the 60 km between Baltimore and Was.h.i.+ngton. Morse used an electromagnet to enhance the signal, so that upon arriving at the receiver's end it was strong enough to make a series of short and long marks, dots and dashes, on a piece of paper. He also developed the now familiar Morse code for translating each letter of the alphabet into a series of dots and dashes, as given in line, a system spanning the 60 km between Baltimore and Was.h.i.+ngton. Morse used an electromagnet to enhance the signal, so that upon arriving at the receiver's end it was strong enough to make a series of short and long marks, dots and dashes, on a piece of paper. He also developed the now familiar Morse code for translating each letter of the alphabet into a series of dots and dashes, as given in Table 6 Table 6. To complete his system he designed a sounder, so that the receiver would hear each letter as a series of audible dots and dashes.

Back in Europe, Morse's approach gradually overtook the Wheatstone-Cooke system in popularity, and in 1851 a European form of Morse code, which included accented letters, was adopted throughout the Continent. As each year pa.s.sed, Morse code and the telegraph had an increasing influence on the world, enabling the police to capture more criminals, helping newspapers to bring the very latest news, providing valuable information for businesses, and allowing distant companies to make instantaneous deals.

However, guarding these often sensitive communications was a major concern. The Morse code itself is not a form of cryptography, because there is no concealment of the message. The dots and dashes are merely a convenient way to represent letters for the telegraphic medium; Morse code is effectively nothing more than an alternative alphabet. The problem of security arose primarily because anyone wanting to send a message would have to deliver it to a Morse code operator, who would then have to read it in order to transmit it. The telegraph operators had access to every message, and hence there was a risk that one company might bribe an operator in order to gain access to a rival's communications. This problem was outlined in an article on telegraphy published in 1853 in England's Quarterly Review: Quarterly Review: Means should also be taken to obviate one great objection, at present felt with respect to sending private communications by telegraph-the violation of all secrecy-for in any case half-a-dozen people must be cognizant of every word addressed by one person to another. The clerks of the English Telegraph Company are sworn to secrecy, but we often write things that it would be intolerable to see strangers read before our eyes. This is a grievous fault in the telegraph, and it must be remedied by some means or other.

The solution was to encipher a message before handing it to the telegraph operator. The operator would then turn the ciphertext into Morse code before transmitting it. As well as preventing the operators from seeing sensitive material, encryption also stymied the efforts of any spy who might be tapping the telegraph wire. The polyalphabetic Vigenere cipher was clearly the best way to ensure secrecy for important business communications. It was considered unbreakable, and became known as communications. It was considered unbreakable, and became known as le chiffre indechiffrable le chiffre indechiffrable. Cryptographers had, for the time being at least, a clear lead over the crypta.n.a.lysts.

Table 6 International Morse Code symbols. International Morse Code symbols.

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Mr. Babbage Versus the Vigenere Cipher The most intriguing figure in nineteenth-century crypta.n.a.lysis is Charles Babbage, the eccentric British genius best known for developing the blueprint for the modern computer. He was born in 1791, the son of Benjamin Babbage, a wealthy London banker. When Charles married without his father's permission, he no longer had access to the Babbage fortune, but he still had enough money to be financially secure, and he pursued the life of a roving scholar, applying his mind to whatever problem tickled his fancy. His inventions include the speedometer and the cowcatcher, a device that could be fixed to the front of steam locomotives to clear cattle from railway tracks. In terms of scientific breakthroughs, he was the first to realize that the width of a tree ring depended on that year's weather, and he deduced that it was possible to determine past climates by studying ancient trees. He was also intrigued by statistics, and as a diversion he drew up a set of mortality tables, a basic tool for today's insurance industry.

Babbage did not restrict himself to tackling scientific and engineering problems. The cost of sending a letter used to depend on the distance the letter had to travel, but Babbage pointed out that the cost of the labor required to calculate the price for each letter was more than the cost of the postage. Instead, he proposed the system we still use today-a single price for all letters, regardless of where in the country the addressee lives. He was also interested in politics and social issues, and toward the end of his life he began a campaign to get rid of the organ grinders and street musicians who roamed London. He complained that the music "not infrequently gives rise to a dance by little ragged urchins, and sometimes half-intoxicated men, who occasionally accompany the noise with their own discordant voices. Another cla.s.s who are great supporters of street music consists of ladies of elastic virtue and cosmopolitan tendencies, to whom it affords a decent excuse for displaying their fascinations at their open windows." Unfortunately for Babbage, the musicians fought back by gathering in large groups around his house and playing as loud as possible. Babbage, the musicians fought back by gathering in large groups around his house and playing as loud as possible.

The turning point in Babbage's scientific career came in 1821, when he and the astronomer John Herschel were examining a set of mathematical tables, the sort used as the basis for astronomical, engineering and navigational calculations. The two men were disgusted by the number of errors in the tables, which in turn would generate flaws in important calculations. One set of tables, the Nautical Ephemeris for Finding Lat.i.tude and Longitude at Sea Nautical Ephemeris for Finding Lat.i.tude and Longitude at Sea, contained over a thousand errors. Indeed, many s.h.i.+pwrecks and engineering disasters were blamed on faulty tables.

These mathematical tables were calculated by hand, and the mistakes were simply the result of human error. This caused Babbage to exclaim, "I wish to G.o.d these calculations had been executed by steam!" This marked the beginning of an extraordinary endeavor to build a machine capable of faultlessly calculating the tables to a high degree of accuracy. In 1823 Babbage designed "Difference Engine No. 1," a magnificent calculator consisting of 25,000 precision parts, to be built with government funding. Although Babbage was a brilliant innovator, he was not a great implementer. After ten years of toil, he abandoned "Difference Engine No. 1," cooked up an entirely new design, and set to work building "Difference Engine No. 2."

When Babbage abandoned his first machine, the government lost confidence in him and decided to cut its losses by withdrawing from the project-it had already spent 17,470, enough to build a pair of battles.h.i.+ps. It was probably this withdrawal of support that later prompted Babbage to make the following complaint: "Propose to an Englishman any principle, or any instrument, however admirable, and you will observe that the whole effort of the English mind is directed to find a difficulty, a defect, or an impossibility in it. If you speak to him of a machine for peeling a potato, he will p.r.o.nounce it impossible: if you peel a potato with it before his eyes, he will declare it useless, because it will not slice a pineapple."

Lack of government funding meant that Babbage never completed Difference Engine No. 2. The scientific tragedy was that Babbage's machine would have been a stepping-stone to the a.n.a.lytical Engine, which would have been programmable. Rather than merely calculating a specific set of tables, the a.n.a.lytical Engine would have been able to solve a variety of mathematical problems depending on the instructions that it was given. In fact, the a.n.a.lytical Engine provided the template for modern computers. The design included a "store" (memory) and a "mill" (processor), which would allow it to make decisions and repeat instructions, which are equivalent to the variety of mathematical problems depending on the instructions that it was given. In fact, the a.n.a.lytical Engine provided the template for modern computers. The design included a "store" (memory) and a "mill" (processor), which would allow it to make decisions and repeat instructions, which are equivalent to the "IF "IF...THEN..." and "LOOP" "LOOP" commands in modern programming. commands in modern programming.

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Figure 12 Charles Babbage. ( Charles Babbage. (photo credit 2.2) A century later, during the course of the Second World War, the first electronic incarnations of Babbage's machine would have a profound effect on crypta.n.a.lysis, but, in his own lifetime, Babbage made an equally important contribution to codebreaking: he succeeded in breaking the Vigenere cipher, and in so doing he made the greatest breakthrough in crypta.n.a.lysis since the Arab scholars of the ninth century broke the monoalphabetic cipher by inventing frequency a.n.a.lysis. Babbage's work required no mechanical calculations or complex computations. Instead, he employed nothing more than sheer cunning.

Babbage had become interested in ciphers at a very young age. In later life, he recalled how his childhood hobby occasionally got him into trouble: "The bigger boys made ciphers, but if I got hold of a few words, I usually found out the key. The consequence of this ingenuity was occasionally painful: the owners of the detected ciphers sometimes thrashed me, though the fault lay in their own stupidity." These beatings did not discourage him, and he continued to be enchanted by crypta.n.a.lysis. He wrote in his autobiography that "deciphering is, in my opinion, one of the most fascinating of arts."

He soon gained a reputation within London society as a crypta.n.a.lyst prepared to tackle any encrypted message, and strangers would approach him with all sorts of problems. For example, Babbage helped a desperate biographer attempting to decipher the shorthand notes of John Flamsteed, England's first Astronomer Royal. He also came to the rescue of a historian, solving a cipher of Henrietta Maria, wife of Charles I. In 1854, he collaborated with a barrister and used crypta.n.a.lysis to reveal crucial evidence in a legal case. Over the years, he acc.u.mulated a thick file of encrypted messages, which he planned to use as the basis for an authoritative book on crypta.n.a.lysis, ent.i.tled The Philosophy of Decyphering The Philosophy of Decyphering. The book would contain two examples of every kind of cipher, one that would be broken as a demonstration and one that would be left as an exercise for the reader. Unfortunately, as with many other of his grand plans, the book was never completed. exercise for the reader. Unfortunately, as with many other of his grand plans, the book was never completed.

While most crypta.n.a.lysts had given up all hope of ever breaking the Vigenere cipher, Babbage was inspired to attempt a decipherment by an exchange of letters with John Hall Brock Thwaites, a dentist from Bristol with a rather innocent view of ciphers. In 1854, Thwaites claimed to have invented a new cipher, which, in fact, was equivalent to the Vigenere cipher. He wrote to the Journal of the Society of Arts Journal of the Society of Arts with the intention of patenting his idea, apparently unaware that he was several centuries too late. Babbage wrote to the Society, pointing out that "the cypher...is a very old one, and to be found in most books." Thwaite was unapologetic and challenged Babbage to break his cipher. Whether or not it was breakable was irrelevant to whether or not it was new, but Babbage's curiosity was sufficiently aroused for him to embark on a search for a weakness in the Vigenere cipher. with the intention of patenting his idea, apparently unaware that he was several centuries too late. Babbage wrote to the Society, pointing out that "the cypher...is a very old one, and to be found in most books." Thwaite was unapologetic and challenged Babbage to break his cipher. Whether or not it was breakable was irrelevant to whether or not it was new, but Babbage's curiosity was sufficiently aroused for him to embark on a search for a weakness in the Vigenere cipher.

Cracking a difficult cipher is akin to climbing a sheer cliff face. The crypta.n.a.lyst is seeking any nook or cranny which could provide the slightest purchase. In a monoalphabetic cipher the crypta.n.a.lyst will latch on to the frequency of the letters, because the commonest letters, such as e, t and a, will stand out no matter how they have been disguised. In the polyalphabetic Vigenere cipher the frequencies are much more balanced, because the keyword is used to switch between cipher alphabets. Hence, at first sight, the rock face seems perfectly smooth.

Remember, the great strength of the Vigenere cipher is that the same letter will be enciphered in different ways. For example, if the keyword is KING, then every letter in the plaintext can potentially be enciphered in four different ways, because the keyword contains four letters. Each letter of the keyword defines a different cipher alphabet in the Vigenere square, as shown in Table 7 Table 7. The e column of the square has been highlighted to show how it is enciphered differently, depending on which letter of the keyword is defining the encipherment: If the K of KING is used to encipher e, then the resulting ciphertext letter is O.

If the I of KING is used to encipher e, then the resulting ciphertext letter is M.

If the N of KING is used to encipher e, then the resulting ciphertext letter is R.

If the G of KING is used to encipher e, then the resulting ciphertext letter is K.

Table 7 A Vigenere square used in combination with the keyword KING. The keyword defines four separate cipher alphabets, so that the letter e may be encrypted as O, M, R or K. A Vigenere square used in combination with the keyword KING. The keyword defines four separate cipher alphabets, so that the letter e may be encrypted as O, M, R or K.

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Similarly, whole words will be deciphered in different ways: the word the, for example, could be enciphered as DPR, BUK, GNO or ZRM, depending on its position relative to the keyword. Although this makes crypta.n.a.lysis difficult, it is not impossible. The important point to note is that if there are only four ways to encipher the word the, and the original message contains several instances of the word the, then it is highly likely that some of the four possible encipherments will be repeated in the ciphertext. This is demonstrated in the following example, in which the line The Sun and the Man in the Moon has been enciphered using the Vigenere cipher and the keyword KING.

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The word the is enciphered as DPR in the first instance, and then as BUK on the second and third occasions. The reason for the repet.i.tion of BUK is that the second the is displaced by eight letters with respect to the third the, and eight is a multiple of the length of the keyword, which is four letters long. In other words, the second the was enciphered according to its relations.h.i.+p to the keyword (the is directly below ING), and by the time we reach the third the, the keyword has cycled around exactly twice, to repeat the relations.h.i.+p, and hence repeat the encipherment.

Babbage realized that this sort of repet.i.tion provided him with exactly the foothold he needed in order to conquer the Vigenere cipher. He was able to define a series of relatively simple steps which could be followed by any crypta.n.a.lyst to crack the hitherto chiffre indechiffrable chiffre indechiffrable. To demonstrate his brilliant technique, let us imagine that we have intercepted the ciphertext shown in Figure 13 Figure 13. We know that it was enciphered using the Vigenere cipher, but we know nothing about the original message, and the keyword is a mystery.

The first stage in Babbage's crypta.n.a.lysis is to look for sequences of letters that appear more than once in the ciphertext. There are two ways that such repet.i.tions could arise. The most likely is that the same sequence of letters in the plaintext has been enciphered using the same part of the key. Alternatively, there is a slight possibility that two different sequences of letters in the plaintext have been enciphered using different parts of the key, coincidentally leading to the identical sequence in the ciphertext. If we restrict ourselves to long sequences, then we largely discount the second possibility, and, in this case, we shall consider repeated sequences only if they are of four letters or more. parts of the key, coincidentally leading to the identical sequence in the ciphertext. If we restrict ourselves to long sequences, then we largely discount the second possibility, and, in this case, we shall consider repeated sequences only if they are of four letters or more. Table 8 Table 8 is a log of such repet.i.tions, along with the s.p.a.cing between the repet.i.tion. For example, the sequence E-F-I-Q appears in the first line of the ciphertext and then in the fifth line, s.h.i.+fted forward by 95 letters. is a log of such repet.i.tions, along with the s.p.a.cing between the repet.i.tion. For example, the sequence E-F-I-Q appears in the first line of the ciphertext and then in the fifth line, s.h.i.+fted forward by 95 letters.

As well as being used to encipher the plaintext into ciphertext, the keyword is also used by the receiver to decipher the ciphertext back into plaintext. Hence, if we could identify the keyword, deciphering the text would be easy. At this stage we do not have enough information to work out the keyword, but Table 8 Table 8 does provide some very good clues as to its length. Having listed which sequences repeat themselves and the s.p.a.cing between these repet.i.tions, the rest of the table is given over to identifying the does provide some very good clues as to its length. Having listed which sequences repeat themselves and the s.p.a.cing between these repet.i.tions, the rest of the table is given over to identifying the factors factors of the s.p.a.cing-the numbers that will divide into the s.p.a.cing. of the s.p.a.cing-the numbers that will divide into the s.p.a.cing.

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Figure 13 The ciphertext, enciphered using the Vigenere cipher. The ciphertext, enciphered using the Vigenere cipher.

For example, the sequence W-C-X-Y-M repeats itself after 20 letters, and the numbers 1, 2, 4, 5, 10 and 20 are factors, because they divide perfectly into 20 without leaving a remainder. These factors suggest six possibilities: (1) The key is 1 letter long and is recycled 20 times between encryptions.

(2) The key is 2 letters long and is recycled 10 times between encryptions.

(3) The key is 4 letters long and is recycled 5 times between encryptions.

(4) The key is 5 letters long and is recycled 4 times between encryptions.

(5) The key is 10 letters long and is recycled 2 times between encryptions.

(6) The key is 20 letters long and is recycled 1 time between encryptions.

The first possibility can be excluded, because a key that is only 1 letter long gives rise to a monoalphabetic cipher-only one row of the Vigenere square would be used for the entire encryption, and the cipher alphabet would remain unchanged; it is unlikely that a cryptographer would do this. To indicate each of the other possibilities, a is placed in the appropriate column of Table 8 Table 8. Each indicates a potential key length.

To identify whether the key is 2, 4, 5, 10 or 20 letters long, we need to look at the factors of all the other s.p.a.cings. Because the keyword seems to be 20 letters or smaller, Table 8 Table 8 lists those factors that are 20 or smaller for each of the other s.p.a.cings. There is a clear propensity for a s.p.a.cing divisible by 5. In fact, every s.p.a.cing is divisible by 5. The first repeated sequence, E-F-I-Q, can be explained by a keyword of length 5 recycled nineteen times between the first and second encryptions. The second repeated sequence, P-S-D-L-P, can be explained by a keyword of length 5 recycled just once between the first and second encryptions. The third lists those factors that are 20 or smaller for each of the other s.p.a.cings. There is a clear propensity for a s.p.a.cing divisible by 5. In fact, every s.p.a.cing is divisible by 5. The first repeated sequence, E-F-I-Q, can be explained by a keyword of length 5 recycled nineteen times between the first and second encryptions. The second repeated sequence, P-S-D-L-P, can be explained by a keyword of length 5 recycled just once between the first and second encryptions. The third repeated sequence, W-C-X-Y-M, can be explained by a keyword of length 5 recycled four times between the first and second encryptions. The fourth repeated sequence, E-T-R-L, can be explained by a keyword of length 5 recycled twenty-four times between the first and second encryptions. In short, everything is consistent with a five-letter keyword. repeated sequence, W-C-X-Y-M, can be explained by a keyword of length 5 recycled four times between the first and second encryptions. The fourth repeated sequence, E-T-R-L, can be explained by a keyword of length 5 recycled twenty-four times between the first and second encryptions. In short, everything is consistent with a five-letter keyword.

Table 8 Repet.i.tions and s.p.a.cings in the ciphertext. Repet.i.tions and s.p.a.cings in the ciphertext.

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a.s.suming that the keyword is indeed 5 letters long, the next step is to work out the actual letters of the keyword. For the time being, let us call the keyword L1-L2-L3-L4-L5, such that L1 represents the first letter of the keyword, and so on. The process of encipherment would have begun with enciphering the first letter of the plaintext according to the first letter of the keyword, L represents the first letter of the keyword, and so on. The process of encipherment would have begun with enciphering the first letter of the plaintext according to the first letter of the keyword, L1. The letter L1 defines one row of the Vigenere square, and effectively provides a monoalphabetic subst.i.tution cipher alphabet for the first letter of the plaintext. However, when it comes to encrypting the second letter of the plaintext, the cryptographer would have used L defines one row of the Vigenere square, and effectively provides a monoalphabetic subst.i.tution cipher alphabet for the first letter of the plaintext. However, when it comes to encrypting the second letter of the plaintext, the cryptographer would have used L2 to define a different row of the Vigenere square, effectively providing a different monoalphabetic subst.i.tution cipher alphabet. The third letter of plaintext would be encrypted according to L to define a different row of the Vigenere square, effectively providing a different monoalphabetic subst.i.tution cipher alphabet. The third letter of plaintext would be encrypted according to L3, the fourth according to L4, and the fifth according to L5. Each letter of the keyword is providing a different cipher alphabet for encryption. However, the sixth letter of the plaintext would once again be encrypted according to L1, the seventh letter of the plaintext would once again be encrypted according to L2, and the cycle repeats itself thereafter. In other words, the polyalphabetic cipher consists of five monoalphabetic ciphers, each monoalphabetic cipher is responsible for encrypting one-fifth of the entire message, and, most importantly, we already know how to crypta.n.a.lyze monoalphabetic ciphers.

We proceed as follows. We know that one of the rows of the Vigenere square, defined by L1, provided the cipher alphabet to encrypt the 1st, 6th, 11th, 16th,...letters of the message. Hence, if we look at the 1st, 6th, 11th, 16th,...letters of the ciphertext, we should be able to use old-fas.h.i.+oned frequency a.n.a.lysis to work out the cipher alphabet in question. Figure 14 Figure 14 shows the frequency distribution of the letters that appear in the 1st, 6th, 11th, 16th,...positions of the ciphertext, which are W, I, R, E,.... At this point, remember that each cipher alphabet in the Vigenere square is simply a standard alphabet s.h.i.+fted by a value between 1 and 26. Hence, the frequency distribution in shows the frequency distribution of the letters that appear in the 1st, 6th, 11th, 16th,...positions of the ciphertext, which are W, I, R, E,.... At this point, remember that each cipher alphabet in the Vigenere square is simply a standard alphabet s.h.i.+fted by a value between 1 and 26. Hence, the frequency distribution in Figure 14 Figure 14 should have similar features to the frequency distribution of a standard alphabet, except that should have similar features to the frequency distribution of a standard alphabet, except that it will have been s.h.i.+fted by some distance. By comparing the L it will have been s.h.i.+fted by some distance. By comparing the L1 distribution with the standard distribution, it should be possible to work out the s.h.i.+ft. distribution with the standard distribution, it should be possible to work out the s.h.i.+ft. Figure 15 Figure 15 shows the standard frequency distribution for a piece of English plaintext. shows the standard frequency distribution for a piece of English plaintext.

The standard distribution has peaks, plateaus and valleys, and to match it with the L1 cipher distribution we look for the most outstanding combination of features. For example, the three spikes at R-S-T in the cipher distribution we look for the most outstanding combination of features. For example, the three spikes at R-S-T in the standard distribution ( standard distribution (Figure 15) and the long depression to its right that stretches across six letters from U to Z together form a very distinctive pair of features. The only similar features in the L1 distribution ( distribution (Figure 14) are the three spikes at V-W-X, followed by the depression stretching six letters from Y to D. This would suggest that all the letters encrypted according to L1 have been s.h.i.+fted four places, or that L have been s.h.i.+fted four places, or that L1 defines a cipher alphabet which begins E, F, G, H,.... In turn, this means that the first letter of the keyword, L defines a cipher alphabet which begins E, F, G, H,.... In turn, this means that the first letter of the keyword, L1, is probably E. This hypothesis can be tested by s.h.i.+fting the L1 distribution back four letters and comparing it with the standard distribution. distribution back four letters and comparing it with the standard distribution. Figure 16 Figure 16 shows both distributions for comparison. The match between the major peaks is very strong, implying that it is safe to a.s.sume that the keyword does indeed begin with E. shows both distributions for comparison. The match between the major peaks is very strong, implying that it is safe to a.s.sume that the keyword does indeed begin with E.

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Figure 14 Frequency distribution for letters in the ciphertext encrypted using the L Frequency distribution for letters in the ciphertext encrypted using the L1 cipher alphabet (number of occurrences). cipher alphabet (number of occurrences).

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Figure 15 Standard frequency distribution (number of occurrences based on a piece of plaintext containing the same number of letters as in the ciphertext). Standard frequency distribution (number of occurrences based on a piece of plaintext containing the same number of letters as in the ciphertext).

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Figure 16 The L The L1 distribution s.h.i.+fted back four letters (top), compared with the standard frequency distribution (bottom). All major peaks and troughs match. distribution s.h.i.+fted back four letters (top), compared with the standard frequency distribution (bottom). All major peaks and troughs match.

To summarize, searching for repet.i.tions in the ciphertext has allowed us to identify the length of the keyword, which turned out to be five letters long. This allowed us to split the ciphertext into five parts, each one enciphered according to a monoalphabetic subst.i.tution as defined by one letter of the keyword. By a.n.a.lyzing the fraction of the ciphertext that was enciphered according to the first letter of the keyword, we have been able to show that this letter, L1, is probably E. This process is repeated in order to identify the second letter of the keyword. A frequency distribution is established for the 2nd, 7th, 12th, 17th,...letters in the ciphertext. Again, the resulting distribution, shown in Figure 17 Figure 17, is compared with the standard distribution in order to deduce the s.h.i.+ft.

This distribution is harder to a.n.a.lyze. There are no obvious candidates for the three neighboring peaks that correspond to R-S -T. However, the depression that stretches from G to L is very distinct, and probably corresponds to the depression we expect to see stretching from U to Z in the standard distribution. If this were the case, we would expect the three R-S-T peaks to appear at D, E and F, but the peak at E is missing. For the time being, we shall dismiss the missing peak as a statistical glitch, and go with our initial reaction, which is that the depression from G to L is a recognizably s.h.i.+fted feature. This would suggest that all the letters encrypted according to L with our initial reaction, which is that the depression from G to L is a recognizably s.h.i.+fted feature. This would suggest that all the letters encrypted according to L2 have been s.h.i.+fted twelve places, or that L have been s.h.i.+fted twelve places, or that L2 defines a cipher alphabet which begins M, N, O, P,... and that the second letter of the keyword, L defines a cipher alphabet which begins M, N, O, P,... and that the second letter of the keyword, L2, is M. Once again, this hypothesis can be tested by s.h.i.+fting the L2 distribution back twelve letters and comparing it with the standard distribution. distribution back twelve letters and comparing it with the standard distribution. Figure 18 Figure 18 shows both distributions, and the shows both distributions, and the match between the major peaks is very strong, implying that it is safe to a.s.sume that the second letter of the keyword is indeed M. match between the major peaks is very strong, implying that it is safe to a.s.sume that the second letter of the keyword is indeed M.

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Figure 17 Frequency distribution for letters in the ciphertext encrypted using the L Frequency distribution for letters in the ciphertext encrypted using the L2 cipher alphabet (number of occurrences). cipher alphabet (number of occurrences).

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Figure 18 The L The L2 distribution s.h.i.+fted back twelve letters (top), compared with the standard frequency distribution (bottom). Most major peaks and troughs match. distribution s.h.i.+fted back twelve letters (top), compared with the standard frequency distribution (bottom). Most major peaks and troughs match.

I shall not continue the a.n.a.lysis; suffice to say that a.n.a.lyzing the 3rd, 8th, 13th,...letters implies that the third letter of the keyword is I, a.n.a.lyzing the 4th, 9th, 14th,...letters implies that the fourth letter is L, and a.n.a.lyzing the 5th, 10th, 15th,...letters implies that the fifth letter is Y. The keyword is EMILY. It is now possible to reverse the Vigenere cipher and complete the crypta.n.a.lysis. The first letter of the ciphertext is W, and it was encrypted according to the first letter of the keyword, E. Working backward, we look at the Vigenere square, and find W in the row beginning with E, and then we find which letter is at the top of that column. The letter is s, which must make it the first letter of the plaintext. By repeating this process, we see that the plaintext begins sittheedownandhavenoshamecheekbyjowl.... By inserting suitable word-breaks and punctuation, we eventually get: Sit thee down, and have no shame, Cheek by jowl, and knee by knee: What care I for any name?

What for order or degree?

Let me screw thee up a peg: Let me loose thy tongue with wine: Callest thou that thing a leg?

Which is thinnest? thine or mine?

Thou shalt not be saved by works: Thou hast been a sinner too: Ruined trunks on withered forks, Empty scarecrows, I and you!

Fill the cup, and fill the can: Have a rouse before the morn: Every moment dies a man, Every moment one is born.

These are verses from a poem by Alfred Tennyson ent.i.tled "The Vision of Sin." The keyword happens to be the first name of Tennyson's wife, Emily Sellwood. I chose to use a section from this particular poem as an example for crypta.n.a.lysis because it inspired some curious correspondence between Babbage and the great poet. Being a keen statistician and compiler of mortality tables, Babbage was irritated by the lines "Every moment dies a man, Every moment one is born," which are the last lines of the plaintext above. Consequently, he offered a correction to Tennyson's "otherwise beautiful" poem: example for crypta.n.a.lysis because it inspired some curious correspondence between Babbage and the great poet. Being a keen statistician and compiler of mortality tables, Babbage was irritated by the lines "Every moment dies a man, Every moment one is born," which are the last lines of the plaintext above. Consequently, he offered a correction to Tennyson's "otherwise beautiful" poem: It must be manifest that if this were true, the population of the world would be at a standstill...I would suggest that in the next edition of your poem you have it read-"Every moment dies a man, Every moment 1[image] is born."...The actual figure is so long I cannot get it onto a line, but I believe the figure 1 is born."...The actual figure is so long I cannot get it onto a line, but I believe the figure 1[image] will be sufficiently accurate for poetry. will be sufficiently accurate for poetry.

I am, Sir, yours, etc., Charles Babbage.

Babbage's successful crypta.n.a.lysis of the Vigenere cipher was probably achieved in 1854, soon after his spat with Thwaites, but his discovery went completely unrecognized because he never published it. The discovery came to light only in the twentieth century, when scholars examined Babbage's extensive notes. In the meantime, his technique was independently discovered by Friedrich Wilhelm Kasiski, a retired officer in the Prussian army. Ever since 1863, when he published his crypta.n.a.lytic breakthrough in Die Geheimschriften und die Dechiffrir-kunst Die Geheimschriften und die Dechiffrir-kunst ("Secret Writing and the Art of Deciphering"), the technique has been known as the Kasiski Test, and Babbage's contribution has been largely ignored. ("Secret Writing and the Art of Deciphering"), the technique has been known as the Kasiski Test, and Babbage's contribution has been largely ignored.

And why did Babbage fail to publicize his cracking of such a vital cipher? He certainly had a habit of not finis.h.i.+ng projects and not publis.h.i.+ng his discoveries, which might suggest that this is just one more example of his lackadaisical att.i.tude. However, there is an alternative explanation. His discovery occurred soon after the outbreak of the Crimean War, and one theory is that it gave the British a clear advantage over their Russian enemy. It is quite possible that British Intelligence demanded that Babbage keep his work secret, thus providing them with a nine-year head start over the rest of the world. If this was the case, then it would fit in with the long-standing tradition of hus.h.i.+ng up codebreaking achievements in the interests of national security, a practice that has continued into the twentieth century.

From Agony Columns to Buried Treasure Thanks to the breakthroughs by Charles Babbage and Friedrich Kasiski, the Vigenere cipher was no longer secure. Cryptographers could no longer guarantee secrecy, now that crypta.n.a.lysts had fought back to regain control in the communications war. Although cryptographers attempted to design new ciphers, nothing of great significance emerged during the latter half of the nineteenth century, and professional cryptography was in disarray. However, this same period witnessed an enormous growth of interest in ciphers among the general public.

The development of the telegraph, which had driven a commercial interest in cryptography, was also responsible for generating public interest in cryptography. The public became aware of the need to protect personal messages of a highly sensitive nature, and if necessary they would use encryption, even though this took more time to send, thus adding to the cost of the telegram. Morse operators could send plain English at speeds of up to 35 words per minute because they could memorize entire phrases and transmit them in a single burst, whereas the jumble of letters that make up a ciphertext was considerably slower to transmit, because the operator had to continually refer back to the sender's written message to check the sequence of letters. The ciphers used by the general public would not have withstood attack by a professional crypta.n.a.lyst, but they were sufficient to guard against the casual snooper.

As people became comfortable with encipherment, they began to express their cryptographic skills in a variety of ways. For example, young lovers in Victorian England were often forbidden from publicly expressing their affection, and could not even communicate by letter in case their parents intercepted and read the contents. This resulted in lovers sending encrypted messages to each other via the personal columns of newspapers. These "agony columns," as they became known, provoked the curiosity of crypta.n.a.lysts, who would scan the notes and try to decipher their t.i.tillating contents. Charles Babbage is known to have indulged in this activity, along with his friends Sir Charles Wheatstone and Baron Lyon Playfair, who together were responsible for developing the deft Playfair cipher Playfair cipher (described in (described in Appendix E Appendix E). On one occasion, Wheatstone deciphered a note in Wheatstone deciphered a note in The Times The Times from an Oxford student, suggesting to his true love that they elope. A few days later, Wheatstone inserted his own message, encrypted in the same cipher, advising the couple against this rebellious and rash action. Shortly afterward there appeared a third message, this time unencrypted and from the lady in question: "Dear Charlie, Write no more. Our cipher is discovered." from an Oxford student, suggesting to his true love that they elope. A few days later, Wheatstone inserted his own message, encrypted in the same cipher, advising the couple against this rebellious and rash action. Shortly afterward there appeared a third message, this time unencrypted and from the lady in question: "Dear Charlie, Write no more. Our cipher is discovered."

In due course a wider variety of encrypted notes appeared in the newspapers. Cryptographers began to insert blocks of ciphertext merely to challenge their colleagues. On other occasions, encrypted notes were used to criticize public figures or organizations. The Times The Times once unwittingly carried the following encrypted notice: once unwittingly carried the following encrypted notice: "The Times "The Times is the Jeffreys of the press." The newspaper was being likened to the notorious seventeenth-century Judge Jeffreys, implying that it was a ruthless, bullying publication which acted as a mouthpiece for the government. is the Jeffreys of the press." The newspaper was being likened to the notorious seventeenth-century Judge Jeffreys, implying that it was a ruthless, bullying publication which acted as a mouthpiece for the government.

Another example of the public's familiarity with cryptography was the widespread use of pinp.r.i.c.k encryption. The ancient Greek historian Aeneas the Tactician suggested conveying a secret message by p.r.i.c.king tiny holes under particular letters in an apparently innocuous page of text, just as there are dots under some letters in this paragraph. Those letters would spell out a secret message, easily read by the intended receiver. However, if an intermediary stared at the page, they would probably be oblivious to the barely perceptible pinp.r.i.c.ks, and would probably be unaware of the secret message. Two thousand years later, British letter writers used exactly the same method, not to achieve secrecy but to avoid paying excessive postage costs. Before the overhaul of the postage system in the mid-1800s, sending a letter cost about a s.h.i.+lling for every hundred miles, beyond the means of most people. However, newspapers could be posted free of charge, and this provided a loophole for thrifty Victorians. Instead of writing and sending letters, people began to use pinp.r.i.c.ks to spell out a message on the front page of a newspaper. They could then send the newspaper through the post without having to pay a penny.

The public's growing fascination with cryptographic techniques meant that codes and ciphers soon found their way into nineteenth-century literature. In Jules Verne's Journey to the Center of the Earth Journey to the Center of the Earth, the decipherment of a parchment filled with runic characters prompts the first step on the epic journey. The characters are part of a subst.i.tution cipher which generates a Latin script, which in turn makes sense only when the letters are reversed: "Descend the crater of the volcano of Sneffels when the shadow of Scartaris comes to caress it before the calends of July, audacious voyager, and you will reach the center of the Earth." In 1885, Verne also used a cipher as a pivotal element in his novel a Latin script, which in turn makes sense only when the letters are reversed: "Descend the crater of the volcano of Sneffels when the shadow of Scartaris comes to caress it before the calends of July, audacious voyager, and you will reach the center of the Earth." In 1885, Verne also used a cipher as a pivotal element in his novel Mathias Sandorff Mathias Sandorff. In Britain, one of the finest writers of cryptographic fiction was Sir Arthur Conan Doyle. Not surprisingly, Sherlock Holmes was an expert in cryptography and, as he explained to Dr. Watson, was "the author of a trifling monograph upon the subject in which I a.n.a.lyze one hundred and sixty separate ciphers." The most famous of Holmes's decipherments is told in The Adventure of the Dancing Men The Adventure of the Dancing Men, which involves a cipher consisting of stick-men, each pose representing a distinct letter.

On the other side of the Atlantic, Edgar Allan Poe was also developing an interest in crypta.n.a.lysis. Writing for Philadelphia's Alexander Weekly Messenger Alexander Weekly Messenger, he issued a challenge to readers, claiming that he could decipher any monoalphabetic subst.i.tution cipher. Hundreds of readers sent in their ciphertexts, and he successfully deciphered them all. Although this required nothing more than frequency a.n.a.lysis, Poe's readers were astonished by his achievements. One adoring fan proclaimed him "the most profound and skillful cryptographer who ever lived."

In 1843, keen to exploit the interest he had generated, Poe wrote a short story about ciphers, which is widely acknowledged by professional cryptographers to be the finest piece of fictional literature on the subject. "The Gold Bug" tells the story of William Legrand, who discovers an unusual beetle, the gold bug, and collects it using a sc.r.a.p of paper lying nearby. That evening he sketches the gold bug upon the same piece of paper, and then holds his drawing up to the light of the fire to check its accuracy. However, his sketch is obliterated by an invisible ink, which has been developed by the heat of the flames. Legrand examines the characters that have emerged and becomes convinced that he has in his hands the encrypted directions for finding Captain Kidd's treasure. The remainder of the story is a cla.s.sic demonstration of frequency a.n.a.lysis, resulting in the decipherment of Captain Kidd's clues and the discovery of his buried treasure. his sketch is obliterated by an invisible ink, which has been developed by the heat of the flames. Legrand examines the characters that have emerged and becomes convinced that he has in his hands the encrypted directions for finding Captain Kidd's treasure. The remainder of the story is a cla.s.sic demonstration of frequency a.n.a.lysis, resulting in the decipherment of Captain Kidd's clues and the discovery of his buried treasure.

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Figure 19 A section of the ciphertext from A section of the ciphertext from The Adventure of the Dancing Men The Adventure of the Dancing Men, a Sherlock Holmes adventure by Sir Arthur Conan Doyle.

Although "The Gold Bug" is pure fiction, there is a true nineteenth-century story containing many of the same elements. The case of the Beale ciphers involves Wild West escapades, a cowboy who ama.s.sed a vast fortune, a buried treasure worth $20 million and a mysterious set of encrypted papers describing its whereabouts. Much of what we know about this story, including the encrypted papers, is contained in a pamphlet published in 1885. Although only 23 pages long, the pamphlet has baffled generations of crypta.n.a.lysts and captivated hundreds of treasure hunters.