Cryptology is the study of both cryptography, the use of messages concealed by codes or ciphers, and cryptanalysis, or the breaking of coded messages. It is nearly as old as civilization itself, although ciphers and codes prior to the late medieval period in western Europe tended to be extremely simple by today’s standards. Advances in mathematics made possible the development of ever more sophisticated systems. Further improvements in cryptology accompanied the creation of modern standing armies and intelligence services during the nineteenth century. Following the world wars and the creation of the computer, cryptology entered a far more advanced stage, resulting in the creation of codes and ciphers so sophisticated that virtually no amount of human genius unaided by computer technology can break them.

Ancient Cryptology

Early examples of cryptology can be found in the work of Mesopotamian, Egyptian, Chinese, and Indian scribes. In those four cradles of civilization, which emerged during the period between 3500 and 2000 B.C., few people could read and write, therefore, written language was a secret code in itself. Further concealment of meaning behind opaque hieroglyphs, cuneiform, or ideograms served to narrow the intended audience even further.

The specialization of writing skills served, in two cases, to prevent the transmission of these skills to later generations. Knowledge of hieroglyphic writing in Egypt died out, and without the discovery and deciphering of the Rosetta Stone in the early nineteenth century, translation of Egyptian texts would probably have not occurred until the computer age—if at all. The fact that the written language of the Indus River valley civilizations in ancient India remains to be translated serves as proof that computers cannot solve all cryptologic questions without a crib or key.

Greece and Rome.

Modern scholars know a great deal more about cryptologic systems in Greece and Rome than in earlier civilizations. The Spartans in about 400 B.C. used a cryptographic system called a scytale, whereby a sheet of papyrus was wrapped around a staff, a message was written down the length of the staff, and then the papyrus was unwrapped. In order to read the message properly, the recipient had to have a staff of exactly the same diameter.

Two centuries later, the Greek historian Polybius introduced what became known as the Polybius square, a 5 x 5 grid that used the 24 letters of the Greek alphabet—a model for the ADFGX cipher used by the Germans in World War I. Julius Caesar in the first century B.C. employed one of the first known ciphers, a system that involved a shift three letters to the right: for example, a plain text Z would become a C, an A a D, and so on.

Medieval Cryptology

Progress in cryptology—as with most other areas of study— came to a virtual standstill between the decline of the Roman Empire in the third century and the rise of Islam in the seventh. Arab scholars pioneered cryptanalysis, the solving of ciphers or codes without the aid of a key, from the eighth century onward. In 1412, al-Kalka-shandi published a treatise in which he introduced the technique, later made famous to popular audiences by Edgar Allan Poe in “The Gold Bug,” of solving a cipher based on the relative frequency of letters in the language.

By that time, cryptology had begun to advance again in Europe, where the Italian city states used secret codes for their diplomatic messages in the fourteenth century. Messages were carried on horseback, and even in peacetime, the roads of Europe were plagued with highway robbers, so secrecy in communication was of the utmost importance.

Progress in mathematical learning from the twelfth century onward aided these advances. In the early thirteenth century, Italian mathematician Leonardo Fibonacci introduced the Fibonacci sequence, wherein each number is the sum of the previous two: 1, 1, 2, 3, 5, 8, and so on. Fibonacci’s sequence would prove highly influential in cryptology: even in the late twentieth century, some cryptologic systems relied on an electronic machine called a Fibonacci generator, which produced numbers in a Fibonacci sequence.

In the late fifteenth century, another influential Italian mathematician, Leon Battista Alberti, published a work in which he introduced the idea of a cipher disk. The latter is a device for encoding and decoding messages by use of concentric wheels imprinted with alphabetic and numeric characters. Even in the late nineteenth century, cryptographers were using cipher disks based on the model pioneered by Alberti.

The Early Modern Era (1500–1900)

Due to its secret nature, cryptography—a word based on Greek roots meaning “secret writing”—had long been associated with the occult, and one occultist who advanced the art was the early sixteenth-century German monk Trithemius. Trithemius developed a table in which each row contained all the letters of the alphabet, but each successive row was shifted over by one letter. The first letter of plain text would be encrypted using the first line, the second letter using the second line, and so on. Late in the 1500s, French cryptographer Blaise de Vigenère adapted the Trithemius table for his own Vigenère table, which in the twentieth century became the basis for the widely used data encryption standard, or DES.

By the eighteenth and early nineteenth centuries, cryptography had become widely used in Europe, where governments employed special offices called “black chambers” to decipher intercepted communications. In America, Thomas Jefferson developed an early cipher wheel, and in the 1840s, Samuel F. B. Morse introduced a machine that would have a vast impact on cryptology: the telegraph. Up to this time, all encoded or enciphered communication had been written and carried by hand, and the telegraph marked the first means of remote transmission. It also employed one of the most famous codes in the world, the Morse code, and helped influence widespread popular interest in cryptography. (It is no accident that Poe’s fictional writing on cryptology coincided with this era.)

In the 1850s, Charles Wheatstone and Lyon Playfair introduced the Playfair system, which used a Polybius square and encrypted letters in pairs. This pairing made deciphering more difficult, since it was less easy to see how frequently certain letters appeared. The Playfair system proved so effective that the Allies used it in limited form against the Japanese during World War II. Despite these advances of the era, cryptography was still far from advanced during the American Civil War. The Confederacy was so disadvantaged in the realm of cryptanalysis that its government sometimes published undeciphered Union messages in newspapers with a request for readers’ help in deciphering them.