Book cipher
Encyclopedia
A book cipher is a cipher
Cipher
In cryptography, a cipher is an algorithm for performing encryption or decryption — a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. In non-technical usage, a “cipher” is the same thing as a “code”; however, the concepts...

 in which the key is some aspect of a book or other piece of text; books being common and widely available in modern times, users of book ciphers take the position that the details of the key is sufficiently well hidden from attackers in practice. This is in some ways an example of security by obscurity. It is typically essential that both correspondents not only have the same book, but the same edition
Edition
In printmaking, an edition is a number of prints struck from one plate, usually at the same time. This is the meaning covered by this article...

.

Traditionally book ciphers work by replacing words in the plaintext
Plaintext
In cryptography, plaintext is information a sender wishes to transmit to a receiver. Cleartext is often used as a synonym. Before the computer era, plaintext most commonly meant message text in the language of the communicating parties....

 of a message with the location of words from the book being used. In this mode, book ciphers are more properly called code
Code (cryptography)
In cryptography, a code is a method used to transform a message into an obscured form, preventing those who do not possess special information, or key, required to apply the transform from understanding what is actually transmitted. The usual method is to use a codebook with a list of common...

s.

This can have problems; if a word appears in the plaintext but not in the book, it cannot be encoded. 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
Beale ciphers
The Beale ciphers are a set of three ciphertexts, one of which allegedly states the location of a buried treasure of gold, silver and jewels estimated to be worth over USD$63 million as of September, 2011. The other two ciphertexts allegedly describe the content of the treasure, and list the names...

, 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 — specifically, a homophonic substitution cipher. However, if used often, this technique has the side effect of creating a larger ciphertext (typically 4 to 6 digits being required to encipher each letter or syllable).

Dictionary

Another approach is to use a dictionary as the codebook. This guarantees that nearly all words will be found, and also makes it much easier to find a word when encoding. This approach was used by George Scovell
George Scovell
General Sir George Scovell was a member of the quartermaster's staff of the British Army in Iberia during the Peninsular War.-Military career:Scovell was commissioned as cornet and adjutant in the 4th Queen's Own Dragoons in 1798....

 for the Duke of Wellington
Arthur Wellesley, 1st Duke of Wellington
Field Marshal Arthur Wellesley, 1st Duke of Wellington, KG, GCB, GCH, PC, FRS , was an Irish-born British soldier and statesman, and one of the leading military and political figures of the 19th century...

's army in some campaigns of the Peninsular War
Peninsular War
The Peninsular War was a war between France and the allied powers of Spain, the United Kingdom, and Portugal for control of the Iberian Peninsula during the Napoleonic Wars. The war began when French and Spanish armies crossed Spain and invaded Portugal in 1807. Then, in 1808, France turned on its...

. In Scovell's method, a codeword would consist of a number (indicating the page of the dictionary), a letter (indicating the column on the page), and finally a number indicating which entry of the column was meant. However, this approach also has a disadvantage: because entries are arranged in alphabetical order, so are the code numbers. This can give strong hints to the cryptanalyst
Cryptanalysis
Cryptanalysis is the study of methods for obtaining the meaning of encrypted information, without access to the secret information that is normally required to do so. Typically, this involves knowing how the system works and finding a secret key...

 unless the message is superenciphered. The wide distribution and availability of dictionaries also present a problem; it is likely that anyone trying to break such a code is also in possession of the dictionary which can be used to read the message.

Bible Cipher

The Bible
Bible
The Bible refers to any one of the collections of the primary religious texts of Judaism and Christianity. There is no common version of the Bible, as the individual books , their contents and their order vary among denominations...

 is a widely available book that is almost always printed with chapter and verse markings making it easy to find a specific string of text within it, making it particularly useful for this purpose.

Security

Essentially, 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
Codebook
A codebook is a type of document used for gathering and storing codes. Originally codebooks were often literally books, but today codebook is a byword for the complete record of a series of codes, regardless of physical format.-Cryptography:...

 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
Cryptanalysis
Cryptanalysis is the study of methods for obtaining the meaning of encrypted information, without access to the secret information that is normally required to do so. Typically, this involves knowing how the system works and finding a secret key...

 to guess other codewords, or even to completely break the code by identifying the key text. This is, however, not the only way a book cipher may be broken. It is still susceptible to other methods of cryptanalysis, and as such is quite easily broken, even without sophsitcated means, without the cryptanalist having any idea what book the cipher is keyed to.

If used carefully, the cipher version is probably much stronger, because it acts as a homophonic cipher with an extremely large number of equivalents. However, this is at the cost of a very large ciphertext expansion.

In the context of espionage
Espionage
Espionage or spying involves an individual obtaining information that is considered secret or confidential without the permission of the holder of the information. Espionage is inherently clandestine, lest the legitimate holder of the information change plans or take other countermeasures once it...

, a book cipher has a considerable advantage for an agent in enemy territory. A conventional codebook, if discovered by the local authorities, instantly 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 carefully to fit with the spy's cover story, would seem entirely innocuous.

Examples

  • A famous use of a book cipher is in the Beale ciphers
    Beale ciphers
    The Beale ciphers are a set of three ciphertexts, one of which allegedly states the location of a buried treasure of gold, silver and jewels estimated to be worth over USD$63 million as of September, 2011. The other two ciphertexts allegedly describe the content of the treasure, and list the names...

    , of which document no. 2 uses a (variant printing of) the United States Declaration of Independence
    United States Declaration of Independence
    The Declaration of Independence was a statement adopted by the Continental Congress on July 4, 1776, which announced that the thirteen American colonies then at war with Great Britain regarded themselves as independent states, and no longer a part of the British Empire. John Adams put forth a...

     as the key text.
  • Richard Sorge's
    Richard Sorge
    Richard Sorge was a German communist and spy who worked for the Soviet Union. He has gained great fame among espionage enthusiasts for his intelligence gathering during World War II. He worked as a journalist in both Germany and Japan, where he was imprisoned for spying and eventually hanged....

     spy ring in Japan used a book cipher which the Japanese were unable to cryptanalyze even after capturing both Sorge and his radio operator / code clerk. It used an edition of a statistical handbook of Germany as the key text.
  • In the American Revolution
    American Revolution
    The American Revolution was the political upheaval during the last half of the 18th century in which thirteen colonies in North America joined together to break free from the British Empire, combining to become the United States of America...

    , Benedict Arnold
    Benedict Arnold
    Benedict Arnold V was a general during the American Revolutionary War. He began the war in the Continental Army but later defected to the British Army. While a general on the American side, he obtained command of the fort at West Point, New York, and plotted to surrender it to the British forces...

     used a book cipher, sometimes known as the Arnold Cipher
    Arnold Cipher
    The Arnold Cipher was a book cipher used by John André and Benedict Arnold during the negotiations that led to Arnold's failed attempt to surrender West Point to the British in 1780.-Background:...

    , which used Sir William Blackstone
    William Blackstone
    Sir William Blackstone KC SL was an English jurist, judge and Tory politician of the eighteenth century. He is most noted for writing the Commentaries on the Laws of England. Born into a middle class family in London, Blackstone was educated at Charterhouse School before matriculating at Pembroke...

    's Commentaries on the Laws of England
    Commentaries on the Laws of England
    The Commentaries on the Laws of England are an influential 18th-century treatise on the common law of England by Sir William Blackstone, originally published by the Clarendon Press at Oxford, 1765–1769...

    as a key text.

In fiction

  • In "The Valley of Fear
    The Valley of Fear
    The Valley of Fear is the fourth and final Sherlock Holmes novel by Sir Arthur Conan Doyle. The story was first published in the Strand Magazine between September 1914 and May 1915, and the first book edition was published in New York on 27 February 1915.- Part I: The Tragedy of Birlstone...

    ", Sherlock Holmes
    Sherlock Holmes
    Sherlock Holmes is a fictional detective created by Scottish author and physician Sir Arthur Conan Doyle. The fantastic London-based "consulting detective", Holmes is famous for his astute logical reasoning, his ability to take almost any disguise, and his use of forensic science skills to solve...

     decrypts a message enciphered with a book cipher by deducing which book had been used as a key text.
  • The name of Ken Follet's World War II
    World War II
    World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

     thriller The Key to Rebecca
    The Key to Rebecca
    The Key to Rebecca is a novel by British author Ken Follett. Published in 1980 by Pan Books , it was a noted bestseller that achieved popularity both in the United Kingdom and worldwide...

    refers to a German spy
    Abwehr
    The Abwehr was a German military intelligence organisation from 1921 to 1944. The term Abwehr was used as a concession to Allied demands that Germany's post-World War I intelligence activities be for "defensive" purposes only...

     in Cairo
    Cairo
    Cairo , is the capital of Egypt and the largest city in the Arab world and Africa, and the 16th largest metropolitan area in the world. Nicknamed "The City of a Thousand Minarets" for its preponderance of Islamic architecture, Cairo has long been a centre of the region's political and cultural life...

     using Daphne du Maurier
    Daphne du Maurier
    Dame Daphne du Maurier, Lady Browning DBE was a British author and playwright.Many of her works have been adapted into films, including the novels Rebecca and Jamaica Inn and the short stories "The Birds" and "Don't Look Now". The first three were directed by Alfred Hitchcock.Her elder sister was...

    's novel Rebecca
    Rebecca (novel)
    Rebecca is a novel by Daphne du Maurier. When Rebecca was published in 1938, du Maurier became – to her great surprise – one of the most popular authors of the day. Rebecca is considered to be one of her best works...

    as the basis of a code.
  • In A Presumption of Death
    A Presumption of Death
    A Presumption of Death is a mystery novel by Jill Paton Walsh, based loosely on The Wimsey Papers by Dorothy L. Sayers. The Wimsey Papers were a series of articles published by Sayers during World War II, purporting to be letters written between the various Wimseys during the war A Presumption of...

    , Lord Peter Wimsey
    Lord Peter Wimsey
    Lord Peter Death Bredon Wimsey is a bon vivant amateur sleuth in a series of detective novels and short stories by Dorothy L. Sayers, in which he solves mysteries; usually, but not always, murders...

    , on assignment for British Intelligence in WWII Nazi-occupied Europe, uses a code based on the works of John Donne
    John Donne
    John Donne 31 March 1631), English poet, satirist, lawyer, and priest, is now considered the preeminent representative of the metaphysical poets. His works are notable for their strong and sensual style and include sonnets, love poetry, religious poems, Latin translations, epigrams, elegies, songs,...

    . The Germans, suspecting that an intelligence service in which Oxonian
    Oxonian
    An Oxonian is a member of the University of Oxford, England. The term is derived from Oxonia, the Latin form of Oxenford or Oxford. The term can also refer to an inhabitant of the city of Oxford, but is less used in this context.The matching word for Cambridge and the University of Cambridge is...

    s have a major role would choose a classical work of English literature, systematically try such works until hitting the right one and breaking the code, coming near to catching the spy. Wimsey then improvises a new code, based on an unpublished text known only to himself and his wife.
  • Graham Greene
    Graham Greene
    Henry Graham Greene, OM, CH was an English author, playwright and literary critic. His works explore the ambivalent moral and political issues of the modern world...

    's protagonists often use book codes. In The Human Factor
    The Human Factor
    The Human Factor is an espionage novel by Graham Greene, first published in 1978 and adapted into a 1979 film, directed by Otto Preminger using a screenplay by Tom Stoppard.-Plot summary:...

    , several books are used, and an edition of Charles Lamb's Tales from Shakespeare
    Tales from Shakespeare
    Tales from Shakespeare was an English children's book written by Charles Lamb with his sister Mary Lamb in 1807. It was illustrated by Arthur Rackham in 1899 and 1909....

    is used in Our Man in Havana
    Our Man in Havana
    Our Man In Havana is a novel by British author Graham Greene, where he makes fun of intelligence services, especially the British MI6, and their willingness to believe reports from their local informants....

    .
  • A book cipher plays an important role in the TV version of Sharpe's Sword
    Sharpe's Sword (TV programme)
    Sharpe's Sword is a 1995 British television drama, part of a series screened on the ITV network that follows the career of Richard Sharpe, a fictional British soldier during the Napoleonic Wars...

    . The key text is Voltaire's Candide
    Candide
    Candide, ou l'Optimisme is a French satire first published in 1759 by Voltaire, a philosopher of the Age of Enlightenment. The novella has been widely translated, with English versions titled Candide: or, All for the Best ; Candide: or, The Optimist ; and Candide: or, Optimism...

    .
  • In the 2004 film National Treasure
    National Treasure (film)
    National Treasure is a 2004 mystery adventure heist film from the Walt Disney Studios under Walt Disney Pictures. It was written by Jim Kouf, Ted Elliott, Terry Rossio, Cormac Wibberley, and Marianne Wibberley, produced by Jerry Bruckheimer, and directed by Jon Turteltaub...

    , an Ottendorf cipher is discovered on the back of the U. S. Declaration of Independence
    United States Declaration of Independence
    The Declaration of Independence was a statement adopted by the Continental Congress on July 4, 1776, which announced that the thirteen American colonies then at war with Great Britain regarded themselves as independent states, and no longer a part of the British Empire. John Adams put forth a...

    , using the "Silence Dogood
    Silence Dogood
    Silence Dogood was a false persona used by Benjamin Franklin to get his work published.-History:As a teenager, Franklin worked as an apprentice in his older brother James' printing shop in Boston, where The New-England Courant was printed....

    " letters as the key text.
  • The protagonists of the Matthew Reilly
    Matthew Reilly
    Matthew John Reilly is an Australian action thriller writer. His novels are noted for their fast pace, twisting plots and intense action.- Biography :...

     novel The Six Sacred Stones
    The Six Sacred Stones
    The Six Sacred Stones is a novel by Australian thriller author Matthew Reilly. It is a sequel to Seven Ancient Wonders and The Five Greatest Warriors is its sequel...

    used a book cipher to send confidential messages to each other. The key text was the Harry Potter
    Harry Potter
    Harry Potter is a series of seven fantasy novels written by the British author J. K. Rowling. The books chronicle the adventures of the adolescent wizard Harry Potter and his best friends Ron Weasley and Hermione Granger, all of whom are students at Hogwarts School of Witchcraft and Wizardry...

     books, but the messages were sent via a The Lord of the Rings
    The Lord of the Rings
    The Lord of the Rings is a high fantasy epic written by English philologist and University of Oxford professor J. R. R. Tolkien. The story began as a sequel to Tolkien's earlier, less complex children's fantasy novel The Hobbit , but eventually developed into a much larger work. It was written in...

    forum to make the key text harder to identify.
  • In Lost
    Lost (TV series)
    Lost is an American television series that originally aired on ABC from September 22, 2004 to May 23, 2010, consisting of six seasons. Lost is a drama series that follows the survivors of the crash of a commercial passenger jet flying between Sydney and Los Angeles, on a mysterious tropical island...

    : Mystery of the Island
    , a series of four jigsaw puzzle
    Jigsaw puzzle
    A jigsaw puzzle is a tiling puzzle that requires the assembly of numerous small, often oddly shaped, interlocking and tessellating pieces.Each piece usually has a small part of a picture on it; when complete, a jigsaw puzzle produces a complete picture...

    s released in 2007, Ottendorf cipher was used on each puzzle's box to hide spoilers and reveal information about the show to the fans.
  • "The Fisher King", a two-part episode of Criminal Minds
    Criminal Minds
    Criminal Minds is an American police procedural drama that premiered September 22, 2005, on CBS. The series follows a team of profilers from the FBI's Behavioral Analysis Unit based in Quantico, Virginia. The BAU is part of the FBI National Center for the Analysis of Violent Crime...

    , features an Ottendorf cipher brought to the Behavioral Analysis Unit
    Behavioral Analysis Unit
    The Behavioral Analysis Unit is a component of the United States Federal Bureau of Investigation's National Center for the Analysis of Violent Crime that uses behavioral sciences to assist in criminal investigations...

     by the UNSUB via Agent Hotchner's wife. The cypher was part of a larger puzzle to find a girl who had been missing for two years. The key text was The Collector
    The Collector
    The Collector is the title of a 1963 novel by John Fowles. It was made into a movie in 1965.- Plot summary :The novel is about a lonely young man, Frederick Clegg, who works as a clerk in a city hall, and collects butterflies in his spare time...

    by John Fowles
    John Fowles
    John Robert Fowles was an English novelist and essayist. In 2008, The Times newspaper named Fowles among their list of "The 50 greatest British writers since 1945".-Birth and family:...

    .
  • Burn Notice
    Burn notice
    A burn notice is an official statement issued by one intelligence agency to other agencies. It states that an individual or a group is unreliable for one or more reasons...

    (episodes "Where There's Smoke" and "Center of the Storm", 2010): Michael Westen steals a Bible from a safe deposit box that is the code book of Simon. This becomes part of the season plot to track an organization starting wars for profit as Michael tries to arrange an interview with Simon.
  • In the film Unknown (2011), Prof. Bressler's passwords are obscured by an Ottendorf cipher.
  • In The Unit
    The Unit
    The Unit is an American action-drama television series that focuses on a top-secret military unit modeled after the real-life U.S. Army special operations unit commonly known as Delta Force...

    episode "Paradise Lost", Jonas Blane
    Jonas Blane
    Sergeant Major Jonas Blane is a fictional character on the CBS television series The Unit. He is portrayed by Dennis Haysbert...

     (aka Snake Doctor) uses a book code from the poem Paradise Lost
    Paradise Lost
    Paradise Lost is an epic poem in blank verse by the 17th-century English poet John Milton. It was originally published in 1667 in ten books, with a total of over ten thousand individual lines of verse...

    to communicate to his wife, Molly, that he has arrived safely in Panama.
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