Cryptography Summary

  • Last updated on November 11, 2022

Cryptography encompasses use of letters, numbers, symbols, and words to form coded messages.


Cryptography encompasses use of letters, numbers, symbols, and words to form coded messages. Military personnel utilize cryptography to transmit orders to officers and troops on land, sea, or in air as well as to mislead enemies who intercept messages. Historians have analyzed the role of cryptography in warfare, often soon after major conflicts occurred, with scholars revising interpretations as information regarding secret code-breaking work became declassified and participants divulged their contributions. World War II and espionage were the focus of much historical scholarship examining cryptography in the late twentieth century. Early twenty-first century histories discussed digital aspects of encrypting military information and assessed cyber vulnerabilities affecting military forces.CryptographyCoded messagesCiphersEncryptionCryptographyCoded messagesCiphersEncryptionIntelligence gathering;cryptography


Since ancient times, military forces have benefited from various forms of cryptography, which allows sensitive information to be transmitted without informing the enemy and which can also deliberately misinform the enemy, in the effort to win battles and wars. Codes disguising military information have enabled victories over enemies who were unaware when and where troops would attack, their strength, and other crucial facts. Moreover, the ability to intercept and decipher enemies’ encrypted messages has alerted commanders to invasions so they can plan defenses and revise strategies. Military cryptanalysts have deciphered enemy messages regarding destruction of supply lines necessary for transportation of both military and civilian resources so officers could order strikes to stop enemies before they could act. Military leaders unaware of their opponents’ plans have often experienced defeat.

History of Cryptography

A four-rotor Enigma machine.

(Greg Goebel)

Warfare has been influenced by cryptography for centuries. Although applications have varied, military forces in different eras have appropriated universal aspects of cryptography to transmit secret information. Basic ciphers often involved substitution of letters in a word or the rearrangement of their order. The frequency of specific letters and patterns has alerted cryptanalysts to the enemy’s encoding key, so they could convert the remaining letters. Some cryptographers assigned words unique codes, which they recorded in code books accessible to people composing messages and translating them; code books were vulnerable to being misplaced or theft by enemies. Knowledge of keys became essential for effective cryptography.

Ciphers and techniques associated with cryptography advanced as people recognized more complex ways to conceal messages with elaborate combinations of codes and sophisticated technology, such as machines and computers, devised to generate or decipher coded messages. Military cryptographers have constantly sought more secure encryption methods to outwit code breakers. CryptanalystsCryptanalysts honed their skills to comprehend meanings in otherwise nonsensical text. Military code specialists developed strategies to prevent enemy cryptographers from realizing their codes had been broken unless such awareness could be manipulated to confuse enemy officers. Codes associated with warfare throughout history have rarely proved impossible for enemies to decipher.

Ancient World

Humans in ancient civilizations first utilized cryptography to protect secrets in communications from economic and political rivals, particularly during combat. Early methods often relied on people’s insights regarding how to confuse enemies. Caesar, JuliusCaesar, JuliusGallic Wars, The (Caesar) Julius Caesar (100-44 b.c.e.) explained in Comentarii debello Gallico (52-51 b.c.e. ; The Gallic Wars, in his Commentaries, 1609), that he had disguised a communication to his Roman military officers fighting in Europe to prevent enemies from comprehending the message if they secured access to it. Aware that Cicero, overwhelmed by opposing forces, was thinking of surrendering, Caesar prepared a message to reassure his officer that he was sending reinforcements. Concerned about the enemy learning that more Romans were en route, Caesar wrote his message with Latin vocabulary formed with Greek letters. Cicero, fluent in both languages, announced Caesar’s news to his soldiers, who rebounded to resist enemy attacks. Caesar also used substitution ciphers, in which pairs of letters corresponding with each other could be used to encode words. The "Caesar shift"[Caesar shift] “Caesar shift” that ancient historian Suetonius describes involved correlating letters with those three positions away, such as writing the cipher letter D for the text letter A.

Polybius PolybiusPolybius (Greek historian)created a grid with the alphabet placed in five columns and numbers from one through five written along the top and also descending on the left side to designate the rows and columns in which letters were located. The two numbers associated with each letter formed the cipher. Polybius stated that signalers could consecutively hold specific amounts of torches representing letters to send messages coded with his system to troops on the battlefield.

Other Romans used transposition ciphers, which rearranged letters to create nonsensical words or entire sentences that confused enemy readers. Most ancient cryptographic systems were vulnerable to being unraveled by the enemy, who occasionally would decode messages when recognizing the correct order of letters in a jumbled word or analyzing messages for patterns of the most common vowels and consonants, which could help determine the cipher technique that had been applied to a message.

Ancient historians such as Plutarch and Herodotus recorded incidents involving secret messages and cryptographic devices associated with warfare. For example, the Spartans;coded messagesSpartans in the fifth century b.c.e. provided military leaders with a wooden device called the Scytale (coding device)scytale, which they wrapped with a parchment or leather strip circling it along the length of the scytale. A message was then written on the parchment or leather, with letters spanning the different wrapped strips. The strip was removed from the scytale and delivered by a courier to the military official for whom it was intended, who would then wrap the strip around his own scytale, which had to be of the exact same diameter. Without a corresponding scytale, the writing on the strip was indecipherable. Military victories attributed to scytale communications included that of Spartan military general Pausanias over Persian forces after he received troops requested through this form of encryption.

Demaratus, DemaratusDemaratusthe ruler of Sparta exiled in Persia, alerted Greeks that the Persian ruler Xerxes’ forces were planning an invasion. Demaratus etched his message on pieces of wood, which were coated in wax to hide his words. Persian guards did not suspect anything strange about those boards en route to the recipients. Demaratus’s clever approach succeeded in preparing Greeks to repel Persian efforts to conquer their territory.

In Aeneas on Siegecraft Aineiou poliorketika (after 357 b.c.e. ; Aeneas on Siegecraft, 1927), Aeneas the Tactician described placing holes in disks in patterns to conceal messages that could be deciphered by threading a cord in the holes.

Medieval World

During the Middle Ages, mathematicians and scientists created methods of encryption that were more complex than their ancient predecessors. Many of these encoded messages were used in military communications to outwit increasingly adept code breakers. By the late fourteenth century, governments were using ciphers for diplomatic correspondence in an effort to thwart spies.

In Italy, architect and Engineers;Italianengineer Alberti, Leon BattistaAlberti, Leon BattistaLeon Battista Alberti (1404-1472) devised a disk consisting of two rings with the alphabet printed on both. A person encoding a message set the rings and coded a few words with the corresponding letters, then moved the rings to code more text. Recipients deciphered messages by using a similar cipher disk and awareness of how they needed to adjust their device as they translated.Treatise on Ciphers, A (Alberti) Alberti innovated polyalphabetic cipher methods and discussed cryptography in his text De componendis cifris (c. 1466; A Treatise on Ciphers, 1997).

The Italian city-states sought cipher experts to create keys for codes and read rivals’ messages, appointing people to positions of cipher secretary and cryptanalyst. In VeniceVenice, the Council of Ten (Venice)Council of Ten and its secret police force maintained power and selected cryptanalyst Soro, GiovanniSoro, GiovanniGiovanni Soro (died 1544) in 1506 as Venice’s cipher secretary. He skillfully cracked codes, including one used in a request that Holy Roman Empire army commander Colonna, Mark AnthonyColonna, Mark AnthonyMark Anthony Colonna had sent to Emperor Maximilian IMaximilian I (Holy Roman Emperor)[Maximilian 01]Maximilian I, telling him he needed more funds, thus revealing that force’s weakness.

In Polygraphia (Trithemius) Polygraphia (1518), Trithemius, JohannesTrithemius, Johannes Johannes Trithemius (1462-1516) described a method of altering cipher keys as each letter was enciphered to produce more secure messages. Vigenère, Blaise deVigenère, Blaise de Blaise de Vigenère (1523-1596), in Traicté des chiffres (tract on ciphers), examined contemporary cryptography and described coding messages with his tableau technique, which used twenty-six rows and columns in which letters shifted to the next position in each succeeding column and row. About 1550, Italian mathematician Cardano, GerolamoCardano, Gerolamo Gerolamo Cardano (1501-1575) publicized a concept in which the key for enciphering words in a message changed for every word following the first. Also trained as a physician, Cardano created masks with slots for writing portions of a secret communication on paper. The message, concealed when other text was written around it, was revealed only if a mask with the proper slots was available to the recipient. The Knights Templar Knights Templar used ciphers to write letters representing credit because they did not carry currency when they traveled on military crusades to the Holy Land.

Modern World

In the seventeenth century, French cryptologistRossignol, AntoineRossignol, AntoineAntoine Rossignol (1600-1682) contributed his skills to create and crack codes for KingLouis XIIILouis XIII (king of France)[Louis 13]Louis XIII. In 1626, Rossignol examined an intercepted encoded letter that Huguenot leaders in Réalmont had written during their siege of that city. Rossignol decoded the letter, which revealed that the Huguenots were considering surrendering. Rossignol gave French representatives the deciphered message to show the Huguenots their dire situation was known, thus securing Réalmont for the French army. Rossignol continued his cryptographic services for the king and military. His son, Rossignol, BonaventureRossignol, BonaventureBonaventure Rossignol, also pursued cryptography. The pair devised a cipher using syllables instead of letters to encode royal messages. They emphasized capturing enemies’ coded messages for military purposes, resulting in the creation of the Cabinet NoirCabinet Noir, a group of cryptanalysts devoted to decoding intercepted diplomatic communications. Other European nations established similar cryptography services, which provided useful military intelligence during warfare.

A letter of recommendation for a Navajo enlistee emphasizes his ability to speak the Navajo dialect, which is “completely unintelligible to all other tribes and all other people.”


By the nineteenth century, technological advances were having a great impact on military cryptography. The Telegraphtelegraph resulted in officers ordering cryptographers to encrypt messages prior to their subsequent transcription into Morse codeMorse code. Kerckhoffs, AugusteKerckhoffs, AugusteCryptographie militaire, La (Kerckhoffs) Auguste Kerckhoffs (1835-1903) contributed articles about cryptography to the Journal of Military Science, which were compiled into the text La Cryptographie militaire (1883; military cryptography). Kerckhoffs sought more secure ways to telegraph messages during wars, emphasizing that military ciphers should use keys that could be easily memorized, could be adapted for changing situations, and could remain secret.

Comanche code talkers for the Fourth Signal Company, U.S. Army Signal Center, Ft. Gordon.

(U.S. Army)

Modern warfare involved numerous cryptography experts and events. During World War I (1914-1918)[World War 01];cryptographyWorld War I, French code breaker Painvin, Georges-JeanPainvin, Georges-JeanGeorges-Jean Painvin (1886-1980) worked in the Bureau du Chiffre (cipher bureau) to decipher German codes during crucial military operations in spring, 1918. Painvin evaluated German messages transmitted during combat in northern France and detected patterns of letters and digits that helped him discover the cipher used. Yardley, Herbert O.Yardley, Herbert O.Herbert O. Yardley (1889-1958) developed the World War I Cipher Bureau to support the U.S. military. He interacted with European cryptanalysts, including Painvin, to enhance American cryptography methods. American Black Chamber, The (Yardley) Yardley wrote The American Black Chamber (1931), which revealed how code breaking enhanced U.S. military intelligence during warfare.

The British Government Code and Cypher School (Great Britain)Government Code and Cypher School (GCCS) established its headquarters at Bletchley ParkBletchley Park. Most Axis countries (Germany, Italy, and Japan) encoded their military communications with the so-called Enigma machineEnigma machine, which could created millions of ciphers. In the late 1930’s, Polish mathematician Rejewski, MarianRejewski, MarianMarian Rejewski (1905-1980) and associates told British and French officials how their technology helped decipher Enigma messages during the interwar period. World War II Bletchley Park cryptanalysts, mostly linguists and mathematicians such asTuring, AlanTuring, AlanAlan Turing, focused on the more complex Enigma ciphers German military branches used for orders, particularly those directing U-boat missions, which were disrupting North Atlantic Allied shipping. Engineers;BritishEngineer Flowers, Thomas H.Flowers, Thomas H.Thomas H. Flowers (1905-1998) built a digital computer, Colossus, to process encrypted German messages. UsingColossus computersColossus computers, Bletchley Park cryptanalysts decoded more than 2.5 million communications during the war, which helped the Allied military prepare maneuvers in Europe, including the June, 1944, Normandy invasion.

In the Pacific, Rosen, LeoRosen, LeoLeo Rosen created a facsimile of Japan’s cipher machine. Friedman, William F.Friedman, William F.William F. Friedman (1891-1969), the U.S. Army Signals Intelligence ServiceArmy Signals Intelligence Service chief, and Rowlett, FrankRowlett, FrankFrank Rowlett (1908-1998) cracked Purple (Japanese cipher)Purple, the Japanese cipher used for diplomatic communications. Access to decoded Japanese military orders enabled U.S. naval pilots to hit the plane transporting Admiral Isoroku, YamamotoIsoroku, YamamotoYamamoto Isoroku, the Imperial Japanese Navy’s Combined Fleet commander. Cryptanalysts’ work contributed to the American victory in the Midway, Battle of (1942)Battle of Midway in 1942. Approximately 420 Code talkersNavajo code talkersNavajos served as code talkers, using their complex language to encipher communications in battles the U.S. Marines fought on Pacific islands. Officers credited the Navajos for American troops successfully securing Iwo Jima, among other strategic victories, which helped the Allies defeat Japan. The Japanese were unable to break the Navajo code.

Bletchley Park, north of London, where the Enigma codes were cracked.

Military cryptography embraced emerging technological advances, such as those of the digital revolution. Code experts applied mathematical functions, such as algorithms, to encode and decipher information digitally. The U.S. Military Academy’s mathematical science department began publishing the journal Cryptologia in 1977. Codes were used to protect nuclear materials, electronic data associated with military procedures and records, and the Milstar satellites deployed for military communications.

Cryptography was utilized in the Vietnam War (1961-1975);cryptographyVietnam War (1961-1975) and played an important role in the 1964 Gulf of Tonkin incident (1964)Tonkin Gulf incident (1964)Gulf of Tonkin incident, where it was used to obtain congressional approval for nearly unlimited U.S. action in Vietnam. A part of the verification process that supported the idea that North Vietnamese torpedo boats attacked two American destroyers was the use of deciphered North Vietnamese communication. A National Security Agency report, declassified in 2006, revealed that it was likely that the communications were incorrectly deciphered.

Modern communication monitoring really hit its stride with the Persian Gulf War (1990-1991);cryptogrphyGulf War (1990-1991), as “traffic analysis” allowed Americans listening to massive amounts of communication to decipher Iraqi war plans. However, American cryptographic experts may have eventually become victims of their own success, as nations wishing to avoid American eavesdropping operations have returned to lower-tech ways of personally delivering messages.CryptographyCoded messagesCiphersEncryptionIntelligence gathering;cryptography

Books and Articles
  • Churchhouse, Robert F. Codes and Ciphers: Julius Caesar, the Enigma, and the Internet. New York: Cambridge University Press, 2002. A chronological discussion of cryptography from its ancient origins through the early twenty-first century, noting military and espionage applications.
  • Copeland, B. Jack, ed. Colossus: The Secrets of Bletchley Park’s Codebreaking Computers. New York: Oxford University Press, 2006. Articles written by cryptography experts include perspectives from such prominent figures as Thomas H. Flowers, describing technological developments to decipher Enigma messages.
  • Kahn, David. The Reader of Gentlemen’s Mail: Herbert O. Yardley and the Birth of American Codebreaking. New Haven, Conn.: Yale University Press, 2004. In this pioneering military cryptanalyst’s biography, a renowned cryptography historian offers insights and corrects errors in the cryptography literature that are often reiterated.
  • Kozaczuk, Władysław, and Jerzy Straszak. Enigma: How the Poles Broke the Nazi Code. New York: Hippocrene Books, 2004. Examines Polish mathematicians’ cryptography training and accomplishments, the Polish Cipher Bureau, and their impact on British cryptanalysts.
  • Meadows, William C. The Comanche Code Talkers of World War II. Austin: University of Texas Press, 2002. Comprehensive study of Native Americans who served Allied military forces by using their languages to encipher and translate messages.
  • Showell, Jak P. Mallmann. German Naval Code Breakers. Annapolis, Md.: Naval Institute Press, 2003. This illustrated history presents details unavailable in most secondary sources regarding the German Naval Radio Monitoring Service intercepting Allied communications in warfare.


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