Cryptanalysis is the study of analyzing information systems in order to study the hidden aspects of the systems. Cryptanalysis is used to breach cryptographic security systems and gain access to the contents of encrypted messages if the cryptographic key is unknown. In addition to mathematical analysis of cryptographic algorithms, cryptanalysis includes the study of side-channel attacks that do not target weaknesses in the cryptographic algorithms themselves, but instead exploit weaknesses in their implementation. Though the goal has been the same, the methods and techniques of cryptanalysis have changed drastically through the history of cryptography, adapting to increasing cryptographic complexity, ranging from the pen-and-paper methods of the past, through machines like the British Bombes and Colossus computers at Bletchley Park in World War II, to the mathematically advanced computerized schemes of the present. Methods for breaking modern cryptosystems involve solving constructed problems in pure mathematics, the best-known being integer factorization.
Given some encrypted data, the goal of the cryptanalyst is to gain as much information as possible about the original, unencrypted data. It is useful to consider two aspects of achieving this; the first is breaking the system —, discovering how the encipherment process works. The second is solving the key, unique for a particular encrypted message or group of messages. Attacks can be classified based on; as a basic starting point it is assumed that, for the purposes of analysis, the general algorithm is known. This is a reasonable assumption in practice — throughout history, there are countless examples of secret algorithms falling into wider knowledge, variously through espionage and reverse engineering.: Ciphertext-only: the cryptanalyst has access only to a collection of ciphertexts or codetexts. Known-plaintext: the attacker has a set of ciphertexts to which he knows the corresponding plaintext. Chosen-plaintext: the attacker can obtain the ciphertexts corresponding to an arbitrary set of plaintexts of his own choosing.
Adaptive chosen-plaintext: like a chosen-plaintext attack, except the attacker can choose subsequent plaintexts based on information learned from previous encryptions. Adaptive chosen ciphertext attack. Related-key attack: Like a chosen-plaintext attack, except the attacker can obtain ciphertexts encrypted under two different keys; the keys are unknown. Attacks can be characterised by the resources they require; those resources include: Time -- the number of computation steps. Memory — the amount of storage required to perform the attack. Data — the quantity and type of plaintexts and ciphertexts required for a particular approach. It's sometimes difficult to predict these quantities especially when the attack isn't practical to implement for testing, but academic cryptanalysts tend to provide at least the estimated order of magnitude of their attacks' difficulty, for example, "SHA-1 collisions now 252."Bruce Schneier notes that computationally impractical attacks can be considered breaks: "Breaking a cipher means finding a weakness in the cipher that can be exploited with a complexity less than brute force.
Never mind that brute-force might require 2128 encryptions. The results of cryptanalysis can vary in usefulness. For example, cryptographer Lars Knudsen classified various types of attack on block ciphers according to the amount and quality of secret information, discovered: Total break — the attacker deduces the secret key. Global deduction — the attacker discovers a functionally equivalent algorithm for encryption and decryption, but without learning the key. Instance deduction — the attacker discovers additional plaintexts not known. Information deduction — the attacker gains some Shannon information about plaintexts not known. Distinguishing algorithm — the attacker can distinguish the cipher from a random permutation. Academic attacks are against weakened versions of a cryptosystem, such as a block cipher or hash function with some rounds removed. Many, but not all, attacks become exponentially more difficult to execute as rounds are added to a cryptosystem, so it's possible for the full cryptosystem to be strong though reduced-round variants are weak.
Nonetheless, partial breaks that come close to breaking the original cryptosystem may mean that a full break will follow. In academic cryptography, a weakness or a break in a scheme is defined quite conservatively: it might require impractical amounts of time, memory, or known plaintexts, it might require the attacker be able to do things many real-world attackers can't: for example, the attacker may need to choose particular plaintexts to be encrypted or to ask for plaintexts to be encrypted using several keys related to the secret key. Furthermore
Leopold Samuel "Leo" Marks, MBE was an English writer and cryptographer. During the Second World War he headed the codes office supporting resistance agents in occupied Europe for the secret Special Operations Executive organisation. After the war, Marks became a playwright and screenwriter, writing scripts that utilised his war-time cryptographic experiences, he wrote the script for Peeping Tom, the controversial film directed by Michael Powell which had a disastrous effect on Powell's career, but was described by Martin Scorsese as a masterpiece. In 1998, towards the end of his life, Marks published a personal history of his experiences during the war, Between Silk and Cyanide, critical of the leadership of SOE. Marks was the son of Benjamin Marks, the joint owner of Marks & Co, an antiquarian bookseller in Charing Cross Road, London, he was introduced at an early age to cryptography when his father showed him Edgar Allan Poe's story, The Gold-Bug. From this early interest, he demonstrated his skill at codebreaking by deciphering the secret price codes which his father wrote inside the covers of books.
The bookshop subsequently became famous as a result of the book 84, Charing Cross Road, based on correspondence between American writer Helene Hanff and the shop's chief buyer, Frank Doel. Marks was conscripted in January 1942, trained as a cryptographer. Unlike the rest of his intake, who were sent to the main British codebreaking centre at Bletchley Park, Marks was regarded as a misfit and he was assigned the newly formed Special Operations Executive in Baker Street, set up to train agents to operate behind enemy lines in occupied Europe and to assist local resistance groups. SOE has been described as "a mixture of brilliant brains and bungling amateurs". Marks wrote that he had an inauspicious arrival at SOE when it took him all day to decipher a code he had been expected to finish in 20 minutes, not atypically, SOE had forgotten to supply the cipher key. Marks briefed many Allied agents sent into occupied Europe, including Noor Inayat Khan, the Grouse/Swallow team of four Norwegian Telemark saboteurs and his own close friend'Tommy' Yeo-Thomas, nicknamed "the White Rabbit".
In an interview which accompanied the DVD of the film Peeping Tom, Marks quoted General Eisenhower as saying that his group's work shortened the war by three months, saving countless lives. Marks was portrayed by Anton Lesser in David Morley's BBC Radio drama A Cold Supper Behind Harrods; the fiction play was inspired by conversations between Marks and David Morley and real events in SOE. It featured David Jason, Stephanie Cole as Vera Atkins. One of Marks's first challenges was to phase out double transposition ciphers using keys based on preselected poems; these poem ciphers had the limited advantage of being easy to memorise, but significant disadvantages, including limited cryptographic security, substantial minimum message sizes, the fact that the method's complexity caused encoding errors. Cryptographic security was enhanced by Marks's innovations "worked-out keys", he was credited with inventing the letter one-time pad, but while he did independently discover the method, he found it in use at Bletchley.
While attempting to relegate poem codes to emergency use, he enhanced their security by promoting the use of original poems in preference to known ones, forcing a cryptanalyst to work it out the hard way for each message instead of guessing an agent's entire set of keys after breaking the key to a single message Marks wrote many poems used by agents, the most famous being one he gave to the agent Violette Szabo, The Life That I Have, which gained popularity when it was used in the 1958 film about her, Carve Her Name With Pride. According to his book, Marks wrote the poem in Christmas 1943 about a girlfriend, who had died in an air crash in Canada; the life that I have Is all that I have And the life that I have Is yours. The love that I have Of the life that I have Is yours and yours. A sleep I shall have A rest I shall have Yet death will be but a pause. For the peace of my years In the long green grass Will be yours and yours. Gestapo signal tracers endangered clandestine radio operators, their life expectancy averaged about six weeks.
Therefore and less frequent transmissions from the codemaster were of value. The pressure could cause agents to make mistakes encoding messages, the practice was for the home station to tell them to recode it and retransmit it. In response to this problem, Marks established and trained a group based at Grendon Underwood, Buckinghamshire to cryptanalyse garbled messages so they could be dealt with in England without forcing the agent to risk retransmitting from the field. Other innovations of his simplified encoding in the field, which reduced errors and made shorter messages possible, both of which reduced transmission time; the Germans did not execute captured radio operators out of hand. The goal was to extract enough information to imitate them. For the safety of entire underground "circuits", it was important to determine if an operator was genuine and still free, but means of independently checking were primitive. Marks claims that he became convinced that their agents in the Netherlands had been compromised by the German count
In cryptography, a Caesar cipher known as Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, so on; the method is named after Julius Caesar. The encryption step performed by a Caesar cipher is incorporated as part of more complex schemes, such as the Vigenère cipher, still has modern application in the ROT13 system; as with all single-alphabet substitution ciphers, the Caesar cipher is broken and in modern practice offers no communications security. The transformation can be represented by aligning two alphabets. For instance, here is a Caesar cipher using a left rotation of three places, equivalent to a right shift of 23: Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ Cipher: XYZABCDEFGHIJKLMNOPQRSTUVW When encrypting, a person looks up each letter of the message in the "plain" line and writes down the corresponding letter in the "cipher" line.
Plaintext: THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG Ciphertext: QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD Deciphering is done in reverse, with a right shift of 3. The encryption can be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme, A → 0, B → 1... Z → 25. Encryption of a letter x by a shift n can be described mathematically as, E n = mod 26. Decryption is performed D n = mod 26; the replacement remains the same throughout the message, so the cipher is classed as a type of monoalphabetic substitution, as opposed to polyalphabetic substitution. The Caesar cipher is named after Julius Caesar, according to Suetonius, used it with a shift of three to protect messages of military significance. While Caesar's was the first recorded use of this scheme, other substitution ciphers are known to have been used earlier. "If he had anything confidential to say, he wrote it in cipher, that is, by so changing the order of the letters of the alphabet, that not a word could be made out.
If anyone wishes to decipher these, get at their meaning, he must substitute the fourth letter of the alphabet, namely D, for A, so with the others." His nephew, Augustus used the cipher, but with a right shift of one, it did not wrap around to the beginning of the alphabet: "Whenever he wrote in cipher, he wrote B for A, C for B, the rest of the letters on the same principle, using AA for Z." Evidence exists that Julius Caesar used more complicated systems, one writer, Aulus Gellius, refers to a treatise on his ciphers: "There is a rather ingeniously written treatise by the grammarian Probus concerning the secret meaning of letters in the composition of Caesar's epistles." It is unknown how effective the Caesar cipher was at the time, but it is to have been reasonably secure, not least because most of Caesar's enemies would have been illiterate and others would have assumed that the messages were written in an unknown foreign language. There is no record at that time of any techniques for the solution of simple substitution ciphers.
The earliest surviving records date to the 9th century works of Al-Kindi in the Arab world with the discovery of frequency analysis. A Caesar cipher with a shift of one is used on the back of the mezuzah to encrypt the names of God; this may be a holdover from an earlier time. The letters of the cryptogram themselves comprise a religiously significant "divine name" which Orthodox belief holds keeps the forces of evil in check. In the 19th century, the personal advertisements section in newspapers would sometimes be used to exchange messages encrypted using simple cipher schemes. Kahn describes instances of lovers engaging in secret communications enciphered using the Caesar cipher in The Times; as late as 1915, the Caesar cipher was in use: the Russian army employed it as a replacement for more complicated ciphers which had proved to be too difficult for their troops to master. Caesar ciphers can be found today in children's toys such as secret decoder rings. A Caesar shift of thirteen is performed in the ROT13 algorithm, a simple method of obfuscating text found on Usenet and used to obscure text, but not used as a method of encryption.
The Vigenère cipher uses a Caesar cipher with a different shift at each position in the text. If the keyword is as long as the message, chosen random, never becomes known to anyone else, is never reused, this is the one-time pad cipher, proven unbreakable; the conditions are so difficult. Keywords shorter than the message, introduce a cy
The Playfair cipher or Playfair square or Wheatstone-Playfair cipher is a manual symmetric encryption technique and was the first literal digram substitution cipher. The scheme was invented in 1854 by Charles Wheatstone, but bears the name of Lord Playfair for promoting its use; the technique encrypts pairs of letters, instead of single letters as in the simple substitution cipher and rather more complex Vigenère cipher systems in use. The Playfair is thus harder to break since the frequency analysis used for simple substitution ciphers does not work with it; the frequency analysis of bigrams is possible, but more difficult. With 600 possible bigrams rather than the 26 possible monograms, a larger cipher text is required in order to be useful; the Playfair cipher was the first cipher to encrypt pairs of letters in cryptologic history. Wheatstone invented the cipher for secrecy in telegraphy, but it carries the name of his friend Lord Playfair, first Baron Playfair of St. Andrews, who promoted its use.
The first recorded description of the Playfair cipher was in a document signed by Wheatstone on 26 March 1854. It was rejected by the British Foreign Office when it was developed because of its perceived complexity. Wheatstone offered to demonstrate that three out of four boys in a nearby school could learn to use it in 15 minutes, but the Under Secretary of the Foreign Office responded, "That is possible, but you could never teach it to attachés."It was however used for tactical purposes by British forces in the Second Boer War and in World War I and for the same purpose by the British and Australians during World War II. This was because Playfair is reasonably fast to use and requires no special equipment - just a pencil and some paper. A typical scenario for Playfair use was to protect important but non-critical secrets during actual combat e.g. the fact that an artillery barrage of smoke shells would commence within 30 minutes to cover soldiers' advance towards the next objective. By the time enemy cryptanalysts could decode such messages hours such information would be useless to them because it was no longer relevant.
During World War II, the Government of New Zealand used it for communication among New Zealand, the Chatham Islands, the coastwatchers in the Pacific Islands. Playfair is no longer used by military forces because of the advent of digital encryption devices; this cipher is now regarded as insecure for any purpose, because modern computers could break it within seconds. The first published solution of the Playfair cipher was described in a 19-page pamphlet by Lieutenant Joseph O. Mauborgne, published in 1914; the Playfair cipher uses a 5 by 5 table containing a key phrase. Memorization of the keyword and 4 simple rules was all, required to create the 5 by 5 table and use the cipher. To generate the key table, one would first fill in the spaces in the table with the letters of the keyword fill the remaining spaces with the rest of the letters of the alphabet in order; the key can be written in the top rows of the table, from left to right, or in some other pattern, such as a spiral beginning in the upper-left-hand corner and ending in the center.
The keyword together with the conventions for filling in the 5 by 5 table constitute the cipher key. To encrypt a message, one would break the message into digrams such that, for example, "HelloWorld" becomes "HE LL OW OR LD"; these digrams will be substituted using the key table. Since encryption requires pairs of letters, messages with an odd number of characters append an uncommon letter, such as "X", to complete the final digram; the two letters of the digram are considered opposite corners of a rectangle in the key table. To perform the substitution, apply the following 4 rules, in order, to each pair of letters in the plaintext: If both letters are the same, add an "X" after the first letter. Encrypt the new pair and continue; some variants of Playfair use "Q" instead of "X", but any letter, itself uncommon as a repeated pair, will do. If the letters appear on the same row of your table, replace them with the letters to their immediate right respectively. If the letters appear on the same column of your table, replace them with the letters below respectively.
If the letters are not on the same row or column, replace them with the letters on the same row but at the other pair of corners of the rectangle defined by the original pair. The order is important – the first letter of the encrypted pair is the one that lies on the same row as the first letter of the plaintext pair. To decrypt, use the inverse of the last 3 rules, the first as-is. There are several minor variations of the original Playfair cipher. Using "playfair example" as the key, the table becomes: P L A Y F I R E X M B C D G H K N O Q S T U V W Z Encrypting the message "Hide the gold in the tree stump": HI DE TH EG OL DI NT HE TR EX ES TU MP ^ BM OD ZB XD NA BE KU DM UI XM MO UV IF Thus the message "Hide the gold in the tree stump" becomes "BMODZ BXDNA BEKUD MUIXM MOUVI F". (Break
Jean-Baptiste Poquelin, known by his stage name Molière, was a French playwright and poet regarded as one of the greatest writers in the French language and universal literature. His extant works include comedies, tragicomedies, comédie-ballets and more, his plays have been translated into every major living language and are performed at the Comédie-Française more than those of any other playwright today. His influence is such that the French language itself is referred to as the "language of Molière". Born into a prosperous family and having studied at the Collège de Clermont, Molière was well suited to begin a life in the theatre. Thirteen years as an itinerant actor helped him polish his comic abilities while he began writing, combining Commedia dell'arte elements with the more refined French comedy. Through the patronage of aristocrats including Philippe I, Duke of Orléans—the brother of Louis XIV—Molière procured a command performance before the King at the Louvre. Performing a classic play by Pierre Corneille and a farce of his own, The Doctor in Love, Molière was granted the use of salle du Petit-Bourbon near the Louvre, a spacious room appointed for theatrical performances.
He was granted the use of the theatre in the Palais-Royal. In both locations Molière found success among Parisians with plays such as The Affected Ladies, The School for Husbands and The School for Wives; this royal favour brought a royal pension to the title Troupe du Roi. Molière continued as the official author of court entertainments. Despite the adulation of the court and Parisians, Molière's satires attracted criticism from churchmen. For Tartuffe's impiety, the Catholic Church denounced this study of religious hypocrisy followed by the Parliament's ban, while Don Juan was withdrawn and never restaged by Molière, his hard work in so many theatrical capacities took its toll on his health and, by 1667, he was forced to take a break from the stage. In 1673, during a production of his final play, The Imaginary Invalid, Molière, who suffered from pulmonary tuberculosis, was seized by a coughing fit and a haemorrhage while playing the hypochondriac Argan, he finished the performance but died a few hours later.
Molière was born in Paris, the son of Jean Poquelin and Marie Cressé, the daughter of a prosperous bourgeois family. Upon seeing him for the first time, a maid exclaimed, "Le nez!", a reference to the infant's large nose. Molière was called "Le Nez" by his family from that time, he lost his mother when he was ten and he does not seem to have been close to his father. After his mother's death, he lived with his father above the Pavillon des Singes on the rue Saint-Honoré, an affluent area of Paris, it is that his education commenced with studies at a Parisian elementary school. In 1631, Jean Poquelin purchased from the court of Louis XIII the posts of "valet de chambre ordinaire et tapissier du Roi", his son assumed the same posts in 1641. The title required an initial cost of 1,200 livres. Molière studied as a provincial lawyer some time around 1642 in Orléans, but it is not documented that he qualified. So far he had followed his father's plans. In June 1643, when Molière was 21, he decided to abandon his social class and pursue a career on the stage.
Taking leave of his father, he joined the actress Madeleine Béjart, with whom he had crossed paths before, founded the Illustre Théâtre with 630 livres. They were joined by Madeleine's brother and sister; the new theatre troupe went bankrupt in 1645. Molière had become head of the troupe, due in part to his acting prowess and his legal training. However, the troupe had acquired large debts for the rent of the theatre, for which they owed 2000 livres. Historians differ as to whether the lover of a member of his troupe paid his debts, it was at this time that he began to use the pseudonym Molière inspired by a small village of the same name in the Midi near Le Vigan. It was likely that he changed his name to spare his father the shame of having an actor in the family. After his imprisonment, he and Madeleine began a theatrical circuit of the provinces with a new theatre troupe. Few plays survive from this period; the most noteworthy are Le Docteur Amoureux. In the course of his travels he met Armand, Prince of Conti, the governor of Languedoc, who became his patron, named his company after him.
This friendship ended when Armand, having contracted syphilis from a courtesan, turned towards religion and joined Moliè
World War II
World War II known as the Second World War, was a global war that lasted from 1939 to 1945. The vast majority of the world's countries—including all the great powers—eventually formed two opposing military alliances: the Allies and the Axis. A state of total war emerged, directly involving more than 100 million people from over 30 countries; the major participants threw their entire economic and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 50 to 85 million fatalities, most of whom were civilians in the Soviet Union and China, it included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, the only use of nuclear weapons in war. Japan, which aimed to dominate Asia and the Pacific, was at war with China by 1937, though neither side had declared war on the other. World War II is said to have begun on 1 September 1939, with the invasion of Poland by Germany and subsequent declarations of war on Germany by France and the United Kingdom.
From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, formed the Axis alliance with Italy and Japan. Under the Molotov–Ribbentrop Pact of August 1939, Germany and the Soviet Union partitioned and annexed territories of their European neighbours, Finland and the Baltic states. Following the onset of campaigns in North Africa and East Africa, the fall of France in mid 1940, the war continued between the European Axis powers and the British Empire. War in the Balkans, the aerial Battle of Britain, the Blitz, the long Battle of the Atlantic followed. On 22 June 1941, the European Axis powers launched an invasion of the Soviet Union, opening the largest land theatre of war in history; this Eastern Front trapped most crucially the German Wehrmacht, into a war of attrition. In December 1941, Japan launched a surprise attack on the United States as well as European colonies in the Pacific. Following an immediate U. S. declaration of war against Japan, supported by one from Great Britain, the European Axis powers declared war on the U.
S. in solidarity with their Japanese ally. Rapid Japanese conquests over much of the Western Pacific ensued, perceived by many in Asia as liberation from Western dominance and resulting in the support of several armies from defeated territories; the Axis advance in the Pacific halted in 1942. Key setbacks in 1943, which included a series of German defeats on the Eastern Front, the Allied invasions of Sicily and Italy, Allied victories in the Pacific, cost the Axis its initiative and forced it into strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained its territorial losses and turned toward Germany and its allies. During 1944 and 1945 the Japanese suffered major reverses in mainland Asia in Central China, South China and Burma, while the Allies crippled the Japanese Navy and captured key Western Pacific islands; the war in Europe concluded with an invasion of Germany by the Western Allies and the Soviet Union, culminating in the capture of Berlin by Soviet troops, the suicide of Adolf Hitler and the German unconditional surrender on 8 May 1945.
Following the Potsdam Declaration by the Allies on 26 July 1945 and the refusal of Japan to surrender under its terms, the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki on 6 and 9 August respectively. With an invasion of the Japanese archipelago imminent, the possibility of additional atomic bombings, the Soviet entry into the war against Japan and its invasion of Manchuria, Japan announced its intention to surrender on 15 August 1945, cementing total victory in Asia for the Allies. Tribunals were set up by fiat by the Allies and war crimes trials were conducted in the wake of the war both against the Germans and the Japanese. World War II changed the political social structure of the globe; the United Nations was established to foster international co-operation and prevent future conflicts. The Soviet Union and United States emerged as rival superpowers, setting the stage for the nearly half-century long Cold War. In the wake of European devastation, the influence of its great powers waned, triggering the decolonisation of Africa and Asia.
Most countries whose industries had been damaged moved towards economic expansion. Political integration in Europe, emerged as an effort to end pre-war enmities and create a common identity; the start of the war in Europe is held to be 1 September 1939, beginning with the German invasion of Poland. The dates for the beginning of war in the Pacific include the start of the Second Sino-Japanese War on 7 July 1937, or the Japanese invasion of Manchuria on 19 September 1931. Others follow the British historian A. J. P. Taylor, who held that the Sino-Japanese War and war in Europe and its colonies occurred and the two wars merged in 1941; this article uses the conventional dating. Other starting dates sometimes used for World War II include the Italian invasion of Abyssinia on 3 October 1935; the British historian Antony Beevor views the beginning of World War II as the Battles of Khalkhin Gol fought between Japan and the fo
A book cipher is a cipher in which the key is some aspect of a book or other piece of text. Books, being common and available in modern times, are more convenient for this use than objects made for cryptographic purposes, it is essential that both correspondents not only have the same book, but the same edition. Traditionally book ciphers work by replacing words in the plaintext of a message with the location of words from the book being used. In this mode, book ciphers are more properly called codes; this can have problems. An alternative approach which gets around this problem is to replace individual letters rather than words. One such method, used in the second Beale cipher, substitutes the first letter of a word in the book with that word's position. In this case, the book cipher is properly a cipher — a homophonic substitution cipher. However, if used this technique has the side effect of creating a larger ciphertext and increases the time and effort required to decode the message; the main strength of a book cipher is the key.
The sender and receiver of encoded messages can agree to use any book or other publication available to both of them as the key to their cipher. Someone intercepting the message and attempting to decode it, unless they are a skilled cryptographer, must somehow identify the key from a huge number of possibilities available. In the context of espionage, a book cipher has a considerable advantage for a spy in enemy territory. A conventional codebook, if discovered by the local authorities incriminates the holder as a spy and gives the authorities the chance of deciphering the code and sending false messages impersonating the agent. On the other hand, a book, if chosen to fit with the spy's cover story, would seem innocuous; the drawback to a book cipher is. The book must not be of the sort that would look out of place in the possession of those using it and it must be of a type to contain any words required. Thus, for example, a spy wishing to send information about troop movements and numbers of armaments would be unlikely to find a cookbook or romance novel useful keys.
Another approach is to use a dictionary as the codebook. This guarantees that nearly all words will be found, makes it much easier to find a word when encoding; this approach was used by George Scovell for the Duke of Wellington's army in some campaigns of the Peninsular War. In Scovell's method, a codeword would consist of a number, a letter, a number indicating which entry of the column was meant. However, this approach has a disadvantage: because entries are arranged in alphabetical order, so are the code numbers; this can give strong hints to the cryptanalyst. The wide distribution and availability of dictionaries present a problem; the Bible is a available book, always printed with chapter and verse markings making it easy to find a specific string of text within it, making it useful for this purpose. The code version of a "book cipher" is just like any other code, but one in which the trouble of preparing and distributing the codebook has been eliminated by using an existing text; however this means, as well as being attacked by all the usual means employed against other codes or ciphers, partial solutions may help the cryptanalyst to guess other codewords, or to break the code by identifying the key text.
This is, not the only way a book cipher may be broken. It is still susceptible to other methods of cryptanalysis, as such is quite broken without sophisticated means, without the cryptanalyst having any idea what book the cipher is keyed to. If used the cipher version is much stronger, because it acts as a homophonic cipher with an large number of equivalents. However, this is at the cost of a large ciphertext expansion. A famous use of a book cipher is in the Beale ciphers, of which document no. 2 uses the United States Declaration of Independence as the key text. In the American Revolution, Benedict Arnold used a book cipher, sometimes known as the Arnold Cipher, which used Sir William Blackstone's Commentaries on the Laws of England as a key text. Book ciphers have been used throughout the Cicada 3301 mystery. In Colony, a television series, the resistance movement uses a book cipher to communicate between members of the cells. In the novel Monsieur Lecoq, published in 1868, Monsieur Lecoq finds that a prisoner is communicating with his confederate using a double book cipher.
Since the prisoner has only one book, The songs of Béranger, the solution is discovered. In The Valley of Fear, Sherlock Holmes decrypts a message enciphered with a book cipher by deducing which book had been used as a key text; the name of Ken Follett's World War II thriller The Key to Rebecca refers to a German spy in Cairo using Daphne du Maurier's novel Rebecca as the basis of a code. In A Presumption of Death, Lord Peter Wimsey, on assignment for British Intelligence in World War II Nazi-occupied Europe, uses a code based on the works of John Donne; the Germans, suspecting that an intelligence service in which Oxonians have a major role would choose a classical work o