Book summary “The codebreakers”: summary

Introduction

You often look at history through the lens of generals, treaties, and battles, missing the silent war of wits that actually decides outcomes. Secret intelligence and the methodical interception of messages wield immense influence over nations, yet the mechanisms behind this hidden realm remain largely unknown to you. You might assume that espionage and spying drive the fate of empires, but the true arbiter of history is cryptology, the eternal duel between the creation of secrets and their ruthless destruction.

David Kahn demonstrates in The Codebreakers that this invisible struggle spans thousands of years, shaping diplomacy, warfare, and modern technology. You will discover how ancient writing systems evolved into complex mechanical ciphers and how brilliant minds eventually pierced every conceivable shield. The book traces this expansive arc from early Middle Eastern scribes to massive twentieth-century intelligence agencies.

• How did ancient civilizations attempt to hide their most sensitive messages?
• What breakthrough allowed scholars to read texts encrypted in unknown languages?
• How did the industrialization of codebreaking alter the course of world wars?
• Why do modern digital systems rely on mathematics to secure global communications?

1. The First Three Thousand Years

The history of cryptology begins in ancient Egypt, driven initially by a desire to impress readers and add solemnity rather than to achieve military secrecy. Around 1900 B.C., a master scribe carved unusual hieroglyphic symbols into the tomb of Khnumhotep II. As the civilization developed, scribes used acrophony, rebuses, and variant forms of letters not to hide text from enemies, but to revive flagging reader interest in funerary epitaphs or to show off their mastery. The tomb of Khnumhotep II at Menet Khufu features the oldest known deliberate transformation of writing in its final columns across 222 total columns.

Cryptology originated in antiquity for aesthetic pride rather than military secrecy, evolving through Hebrew protocryptography, Spartan devices, and the brilliant Arab breakthrough of frequency analysis.

Ancient Hebrew texts utilized early letter substitution systems like atbash and albam, which served as textual transformations without a strict secrecy motive. Scribes and biblical authors applied systematic letter exchanges, such as atbash where the alphabet's last letter replaces the first, resulting in substitutes like Sheshach for Babylon and Leb Kamai for Chaldeans. These transformations arose largely from scribal playfulness, pride, and a wish for immortality through textual alterations. In Jeremiah 25:26, the name Sheshach appears as a substitute for Babel through the application of atbash.

The Spartans developed the skytale as the earliest mechanical device for transposition ciphers in military communications. The skytale consisted of a wooden staff around which a strip of papyrus, leather, or parchment was tightly wrapped. The message was written down the length of the staff, and the disconnected letters only made sense when unwound and wrapped around a baton of identical thickness. Thucydides and Plutarch recorded the use of the skytale by Spartan rulers to recall ambitious commanders like Pausanias and Lysander around 475 B.C.

Polybius devised a square grid arrangement that converts letters into coordinate numbers, creating the foundational checkerboard widely used in later systems. By arranging the alphabet in a square and numbering the rows and columns, each letter can be represented by a pair of numbers designating its position. Polybius originally suggested transmitting these numbers via torches over long distances. Polybius suggested using one torch in the right hand and five in the left to signal letter coordinates, such as 15 for the letter e and 51 for the letter v on a 5 x 5 square.

Julius Caesar established a historic cipher standard by substituting plaintext letters with those three places further down the alphabet. Suetonius noted that Caesar routinely used a cipher where D replaced a, E replaced b, and so on. Any standard sequential cipher alphabet of this type is named a Caesar alphabet even if it begins on a different letter. Julius Caesar sent a message written in Greek characters attached to a hurled spear to alert Cicero of his advancing legions.

Cryptology as a complete science was born among the Arabs, who were the first to discover and systematically record the methods of cryptanalysis. Building on intensive grammatical, lexicographical, and phonetic studies of the Koran, Arab scholars like al-Khalil and later Ibn ad-Duraihim developed the science of solving ciphers. Al-Khalil solved a Greek cryptogram sent by the Byzantine emperor by assuming the message began with 'In the name of God'. You can follow the traditional Arab methodology for solving ciphers by applying these practical steps:

• Begin by counting the letters and how many times each symbol is repeated, setting down totals individually.
• If word-divisions are concealed, work out the symbol dividing words by testing a letter as a divider against possible letter combinations.
• Look for the most frequent letters in the message and compare them with the standard language frequency pattern, assuming the highest is alif and the next is lam.
• Try to work out two-lettered words first by estimating feasible letter combinations, then write down their equivalents.
• Apply the same principle successively to three-, four-, and five-lettered words.
• Posit two or three conjectures whenever there is doubt and write each down until confirmed by another word.

Qalqashandi completed his 14-volume encyclopedia Subh al-a 'sha in the 1400s, while an Algerian individual spent 15 years to decode a 1600 monalphabetic dispatch. Cryptanalysis of monalphabetic substitution relies on the distinct statistical personalities, frequencies, and contact preferences of language letters. Letters maintain predictable frequency tiers, with high-frequency vowels and common consonants like e, t, a, o, n, i, r, s, and h appearing distinctively. Furthermore, contact charts tracking which letters precede and follow a subject letter reveal clear relational structures, such as vowels avoiding each other or specific consonants appearing near vowels. Parker Hitt's Manual for the Solution of Military Ciphers uses a 280-letter sample cryptogram starting with 'Success in dealing with unknown ciphers...' compared against a 200-letter English frequency sample to demonstrate manual cryptanalysis.

2. The Rise of the West and Black Chambers

Renaissance states institutionalized cryptography through nomenclators and advanced secret bureaus, while European black chambers systematically intercepted and read foreign diplomatic mail.

The nomenclator dominated European and American security for 450 years. Gabrieli di Lavinde compiled the oldest extant Western key in 1379 for the antipope Clement VII, creating individual keys for 24 correspondents. These systems combined letter substitution with small code vocabularies. Russian nomenclators in the 1700s expanded to 2,000 or 3,000 syllables and words.

The Duchy of Mantua used the earliest known Western homophonic substitution cipher in 1401 to give vowels several possible equivalents. Cicco Simonetta set down thirteen rules for solving monalphabetic substitution ciphers in Pavia on July 4, 1474. Matteo Argenti compiled a 135-page manual detailing papal cryptographic security methods:

• Write out the keyword, omitting any repeated letters, and follow it with the remaining letters of the alphabet.
• Merge predictable letter sequences like u after q into a single unit.
• Delete the second letter of frequent doubled consonants within words.
• Strew nulls throughout the cryptogram at a rate of no less than three to eight per line.
• Mix single digits with pairs while assigning single digits to high-frequency plaintext equivalents.

Venice established an elaborate cryptologic organization under the Council of Ten. Giovanni Soro served as Venice's cipher secretary after his appointment in 1506. Marco Rafael received a reward of 100 ducats in 1525 for a new method of invisible writing. Venice paid cipher secretaries 10 to 12 ducats a month.

Spain deployed nomenclators through the Despacho Universal. Hernán Cortés used a small nomenclator in a letter from Mexico on June 25, 1532. Philip II wrote to his uncle introducing a new general cipher on May 24, 1556. François Viète solved a letter from Commander Juan de Moreo dated October 28, 1589, and delivered the solution to Henry IV on March 15. Philip's nomenclator given to Moreo contained 413 terms. Philip van Marnix broke Don Juan of Austria's Spanish nomenclator in July 1577. Thomas Phelippes intercepted Mary, Queen of Scots, on July 17, 1586, and she was executed on February 8, 1587.

Antoine Rossignol solved an enciphered message at the siege of Réalmont in 1628, earning immense wealth from the French crown. The Austrian Geheime Kabinets-Kanzlei handled 80 to 100 letters a day on a strict schedule of 7 a.m., 9:30 a.m., 10 a.m., 2 p.m., 11 a.m., 4 p.m., and 6:30 p.m. An Austrian cabinet polyglot received a bonus of 500 florins for learning Armenian in a few months. Abbot Georgel paid 1,000 ducats to obtain copies from a masked man in Vienna.

John Wallis broke a 700-number cipher for Parliament in 1643 and received a joint annual grant of £100 a year in 1701. Edward Willes became Canon of Westminster with a salary of £500, later sharing a salary of £1,000 a year with his son Edward, Jr. Benjamin Tallmadge compiled a one-part nomenclator of 800 elements for the Culper ring, while Robert A. Livingston printed 1,700 forms for colonial nomenclators. James Lovell solved British dispatches that helped secure victory for 16,000 American men and 24 French ships of the line at Yorktown in September 1781. Public outrage finally ended these practices when Britain discontinued interceptions and dissolved its Decyphering Branch in 1844, followed by the closure of the Austrian Geheime Kabinets-Kanzlei and the French Cabinet Noir in 1848.

3. Polyalphabetic Ciphers and Mechanical Devices

Polyalphabetic substitution broke the limits of classical monalphabetic systems. Leon Battista Alberti founded Western polyalphabetic substitution and cryptanalysis using a two-plate copper cipher disk. You can build this system by following precise mechanical steps:

• Make two circles out of copper plates, the stationary outer circle having a diameter one-ninth greater than the movable inner circle.
• Divide the circumference of each circle into 24 equal parts called cells.
• Inscribe capital letters in red in the outer circle in usual order omitting H, K, and Y, and place numbers 1 to 4 in the remaining four spaces.
• Inscribe small letters in black at random in the 24 cells of the inner circle.
• Place the smaller circle upon the larger with a needle through their centers as an axis.
• Agree upon an index letter in the movable disk and place it against a letter of the outer disk to start encipherment.

Alberti demonstrated his system to Leonardo Dato in the Vatican gardens and wrote the West's oldest extant text on cryptanalysis in 1466 or 1467.

The invention of polyalphabetic substitution and sliding alphabets revolutionized cryptology, eventually leading to mechanical cipher devices like the wheel cypher and the Enigma.

Johannes Trithemius introduced letter-by-letter encipherment and the square table or tableau to polyalphabetic cryptography. He published the first printed book on cryptology, titled Polygraphiae libri sex, in 1518.

Giovan Batista Belaso introduced the use of a literal, easily changed key or countersign to control polyalphabetic substitution alphabets. Giovanni Battista Porta unified previous cryptologic concepts, introduced the probable word method, and designed early cryptanalytic attacks. Girolamo Cardano invented the first autokey system using plaintext as its own key, as well as the physical steganographic cipher grille. Blaise de Vigenère perfected the autokey system using a priming key and continuous running text, though history misnamed a degenerate standard-alphabet keyword cipher after him.

Thomas Jefferson invented the wheel cypher, an exceptionally advanced mechanical encryption device that remained secure for over a century. You can construct this historical machine using specific materials:

• Turn a cylinder of white wood of about 2 inches diameter and 6 or 8 inches long.
• Bore through its center a hole sufficient to receive an iron spindle.
• Divide the periphery into 26 equal parts and draw parallel lines.
• Cut the cylinder crosswise into pieces about 1/6 of an inch thick resembling backgammon men.
• Put all the letters of the alphabet in jumbled order on the periphery of each wheel.
• String them in numerical order on an iron axis with a nut and screw to hold them firm.

Jefferson described his wheel cypher in detail and calculated that a six-inch cylinder with thirty-six wheels could produce 371,993,326,789,901,217,467,999,448,150,835,200,000,000 combinations.

Friedrich Kasiski solved the 300-year-old problem of polyalphabetic ciphers by exploiting text repetitions to determine keyword length. August Kercokhoffs wrote La Cryptographie militaire in 1883 and formulated the fundamental principle that security must reside entirely in keys. Gilbert S. Vernam invented on-line encipherment in 1917, while Major Joseph O. Mauborgne combined random key generation with nonrepetition to create the unbreakable one-time system. Boris Hagelin created the M-209 converter, and Polish mathematicians broke the Enigma before World War II.

4. World War I and the Intercept Revolution

The advent of radio transformed cryptology from an internal, academic exercise into an external weapon of war by providing a constant, voluminous stream of enemy intercepts.

On the first day of World War I, Director of Naval Intelligence Rear Admiral Henry F. Oliver mentioned accumulating intercepted code messages to Sir Alfred Ewing. Ewing recruited four teachers from naval colleges who lacked prior cryptanalytic experience, establishing a bureau in the Admiralty that eventually moved to Room 40. Ewing called in A. G. Denniston, W. H. Anstie, E. J. C. Green, and G. L. N. Hope to inspect incomprehensible lines of letters around a table in his office.

Cryptanalytic progress accelerated dramatically when British allies and daring operations recovered physical enemy codebooks from wrecks and sunken vessels. Shipwright E. C. Miller repeatedly dove into sunken U-boats, navigating past corpses and conger eels, to retrieve iron boxes containing naval codes. Captain William Reginald Hall took over direction of naval intelligence in late 1914. Nicknamed Blinker due to a nervous tic, Hall possessed exceptional intuitive genius for espionage.

Room 40 cryptanalysts successfully broke German diplomatic codes, such as Code 0075 and Code 13040, by analyzing recurring structures, stop groups, and stereotyped phrasing. Montgomery and de Grey solved fragments of Code 0075 over a six-month period, eventually exposing secret diplomatic cables routed through neutral Sweden and the United States. You can crack similar diplomatic traffic using specific analytical rules:

• Identify the groups meaning stop by looking for candidates that recur near the end of telegrams.
• Use stereotyped diplomatic expressions such as I have the honor to report to Your Excellency as initial clues.
• Enter tentative identifications in pencil as pencil groups for easy erasing until confirmed as ink groups.

Hall engineered a sophisticated deception to expose the intercepted Zimmermann telegram to American authorities without revealing British cryptanalytic capabilities or compromising intelligence sources. Agent T secured a copy of the message from the Mexico City telegraph office sent via Western Union, proving it had been re-encoded in Code 13040 because the local minister lacked Code 0075. You can execute a similar intelligence maneuver using standard operational steps:

• Obtain a copy of the target message from a remote telegraph office where it was re-encoded in an older, more vulnerable code.
• Verify that serial numbers, dates, and preambles differ to establish a plausible narrative of local theft or interception.
• Present the decoded text to allied diplomatic liaisons while withholding the underlying method of codebreaking.

The public exposure of the Zimmermann telegram galvanized American public opinion and directly propelled the United States into World War I. Rollin Kirby published a political cartoon titled Exploding in his Hands in The New York World immediately after the telegram was made public.

5. World War II Intelligence and Modern Operations

Global conflict drove massive organizational expansion in codebreaking and traffic analysis, directly altering military outcomes at Midway, in the Atlantic, and across the Pacific theater.

Led by William F. Friedman, the Army's Signal Intelligence Service laboriously deduced the mechanism of the Japanese 97-shiki O-bun In-ji-ki machine through statistical methods, isomorphism, and team analysis, eventually building a duplicate mechanical apparatus that cracked dispatches within hours. After 18 to 20 months of intensive analysis, S.I.S. handed in the first complete PURPLE solution in August 1940, while British and German cryptanalytic services remained completely baffled.

The distribution of decrypted intelligence, designated as MAGIC, required an extreme balancing act between operational utility and rigorous security measures. You must protect sources while sharing actionable data with top commanders.

• Lock intelligence in a zippered pouch with a padlock.
• Deliver via pouch directly to the recipient.
• Have the recipient read it and put it back in the pouch.
• Return all copies for fiery immolation except for strictly filed archival copies.

On November 7, 1941, President Roosevelt demanded to see the original intercept documents rather than summaries, prompting a new system where the Navy furnished MAGIC to the White House and the Army to the State Department. Ten top-ranking officials were named in the original January 23, 1941, distribution list, and 14 copies of intercepts were distributed by December 1941.

Radio intelligence combined direction-finding and traffic analysis to maintain continuous, long-range surveillance of Japanese fleet movements. You can pinpoint transmitters by following standard procedures:

• Take bearings on a signal with multiple direction-finders from different outposts.
• Draw lines of direction on a map to find the crossing point marking the transmitter position.
• Deduce force organization by ascertaining which radios communicate with each other.

The Mid-Pacific Strategic Direction-Finder Net utilized stations from Cavite to Dutch Harbor to pinpoint locations, such as fixing ship KUNA 1 at 10.7 degrees north latitude, 166.7 degrees east longitude on October 16. Sixty to 70 officers and men staffed these outposts in 1941 to monitor 200 radio circuits operated within the Japanese Navy.

American naval cryptanalysts at Pearl Harbor unlocked Japanese naval codes and provided crucial tactical intelligence that enabled the U.S. fleet to ambush and defeat the Japanese at Midway. Lieutenant Commander Joseph Rochefort led the Combat Intelligence Unit in the windowless basement of Pearl Harbor.

Cryptanalysis enabled the Allies to intercept Admiral Isoroku Yamamoto's exact inspection itinerary, leading to a planned aerial ambush that successfully shot down his aircraft and devastated Japanese morale. Major John W. Mitchell and Captain Thomas G. Lanphier, Jr. led P-38 Lightnings from Henderson Field on Bougainville, intercepting Yamamoto's Betty bombers right on schedule and shooting them down into the jungle on April 18, 1943.

6. The Anatomy and Future of Cryptology

Claude Shannon's information theory anchored cryptology in mathematics, while modern computing, public-key cryptography, and the National Security Agency ushered in an era of unbreakable algorithms.

Cryptography and cryptanalysis are reciprocal sciences that differ fundamentally in their methods and logical nature. Cryptography is theoretical, abstract, and mathematical, operating on analytic statements that are universally true. Cryptanalysis is empirical, concrete, and relies on the physical sciences through observation, experiment, and synthetic statements.

Dr. A. Adrian Albert stated that abstract cryptography is identical with abstract mathematics, while Maurits de Vries described secrecy transformations using linear algebraic transformations like y = ax + b.

Claude Shannon's information theory explains cryptanalysis and language structure through the concept of redundancy. Redundancy means transmitting more symbols than strictly necessary because languages are burdened by grammatical, phonetic, and etymological rules. This built-in predictability is what enables cryptanalysis and error correction.

Claude Shannon built a maze-solving mouse and a chess-playing machine at Bell Telephone Laboratories while concurrently developing information theory and the communication theory of secrecy systems.

Modern computational complexity and mathematically unbreakable ciphers mean that mass codebreaking is largely becoming a thing of the past.

Because properly implemented modern cryptosystems create computational infeasibility where doubling combinations forces an exponential square increase in trial effort, codebreakers can no longer systematically solve robust algorithms, leaving theft and betrayal as the primary remaining avenues.

Herbert O. Yardley predicted in 1931 that the widespread adoption of unbreakable systems would cause professional cryptography as a codebreaking profession to die.

10 Key Ideas

1. Cryptology originated in antiquity for aesthetic pride rather than military secrecy, evolving through Hebrew protocryptography, Spartan devices, and the brilliant Arab breakthrough of frequency analysis.
2. Renaissance states institutionalized cryptography through nomenclators and advanced secret bureaus, while European black chambers systematically intercepted and read foreign diplomatic mail.
3. The invention of polyalphabetic substitution and sliding alphabets revolutionized cryptology, eventually leading to mechanical cipher devices like the wheel cypher and the Enigma.
4. The advent of radio transformed cryptology from an internal, academic exercise into an external weapon of war by providing a constant, voluminous stream of enemy intercepts.
5. Global conflict drove massive organizational expansion in codebreaking and traffic analysis, directly altering military outcomes at Midway, in the Atlantic, and across the Pacific theater.
6. Ancient Hebrew texts utilized early letter substitution systems like atbash and albam, which served as textual transformations without a strict secrecy motive.
7. Thomas Jefferson invented the wheel cypher, an exceptionally advanced mechanical encryption device that remained secure for over a century.
8. Room 40 cryptanalysts successfully broke German diplomatic codes, such as Code 0075 and Code 13040, by analyzing recurring structures, stop groups, and stereotyped phrasing.
9. Claude Shannon's information theory explains cryptanalysis and language structure through the concept of redundancy.
10. Modern computational complexity and mathematically unbreakable ciphers mean that mass codebreaking is largely becoming a thing of the past.