The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography is a work of nonfiction by British physicist Simon Singh, covering the science and study of codes, ciphers, and cryptology from ancient times to the modern era and beyond. A trained academic who has worked closely with the BBC on a number of informational and documentary programs, Singh presents the subject and his expertise is an accessible way for the general reader. Originally published in 1999, a revised edition was published in 2000.

This study guide uses the revised edition of 2000 published by Anchor Books, a division of Random House publishers.

Plot Summary

The book sets out to achieve two goals: to outline the history and evolution of codes and ciphers and to demonstrate how cryptology is more relevant than it has ever been in the past. The first chapter tells the story of Queen Mary’s treason trial after she was accused of plotting an assassination of Queen Elizabeth. Crucial to the testimony is the evidence that may or may not reveal her participation, and which is only contained in a number of messages written in secret code. The story of Elizabeth and Mary, and the deciphering of the secret messages that eventually led to Mary’s execution, introduces the world of codes, ciphers, and encryption.

Typically, codes and ciphers refer to different ways that messages are hidden in plain sight, using differently encoded alphabets and images that can only be understood by the sender and recipient. Eventually, as Chapter 2 describes, substitution ciphers dominated the landscape. Technology and encryption methods began to evolve in the 18th century with the invention of the telegraph, which made apparent the great need for privacy in communication: When one needs to hand a message to a third party (i.e., the telegraph operator) in order to send it, one sees immediately why privacy is important.

The third chapter gives an overview of the mechanization of the cryptological process. Not only was the world coming into the age of mechanics and industry at the time of the 19th century, but encryption technology was as well. The invention of the radio pushed communication technology to new heights. The radio also raised issues around espionage, as the dissemination of information was now easier than ever, but so was the ability of people to intercept these same messages. In response, mechanical encryption machines were invented, the most famous being the German Enigma, which saw great application in the armed forces and became a major tactical obstacle in the Second World War.

Chapter 4 outlines the process that led to cracking the mystery of the Enigma machine. The Allied forces, led by the team put together by the Polish secret service, was able to crack the code of Enigma by switching to teams led by mathematicians, rather than by linguists—a tactical shift that was quickly adopted by most government agencies globally. Thanks to the work of Alan Turing and a host of colleagues and associates, Enigma was solved, and the work of the Allied cryptologists proved decisive in ending the war much sooner than otherwise would have been possible.

Chapter 5 continues the theme of wartime exploits by narrating the story of the Navajo code talkers. Native speakers of the Navajo language were employed in active combat in order to use their native (and relatively unknown) language as a real-time method of encryption in the battlefield. As the example of the Navajo speakers makes clear, it is not just specifically created codes and ciphers that serve to perplex modern inquirers, but ancient and minority languages as well. Some of the most interesting advances and discoveries in cryptology have come as a result of attempting to decipher forgotten languages of the past.

The sixth chapter introduces the figures of Alice, Eve, and Bob, characters used by cryptologists and analysts to create fictional scenarios by which cryptanalysis can advance. The most significant problem that modern cryptologists have attempted to solve is the problem of using a cipher key to communicate, but which itself needs to be communicated secretly as well. Thus, the problem is how to pass secret messages between parties when the algorithm itself needs to be kept secret. Thanks to this new problem ushered in by the digital mode of communication, the theoretical possibility of asymmetrical cipher keys was discovered, where messages can be encrypted with public information but can only be deciphered by privately held keys.

The development of such systems was, as Chapter 7 reveals, simply a matter of time. The Pretty Good Privacy software program was developed in order to solve the issue of private individuals and businesses—in contrast to military and government applications—needing to encrypt their private information and communication. Released for free to the public online, the Pretty Good Privacy system was widely adopted and led to an increased number of encryption technologies in the years and decades to come.

In Chapter 8, the author introduces quantum technology that is only just beginning to be brought into the encryption process. The future of technology and encryption is an unknown area of progress. However, with the implementation of high-level physics, the future of encryption will likely belong to the physicists, just as the 20th century belonged to mathematicians.