The Code Book

SIMON SINGH

The Code Book

Simon Singh received his Ph.D. in physics from Cambridge University. A former BBC producer, he directed an award-winning documentary film on Fermat’s Last Theorem that aired on PBS’s
Nova
series and wrote the bestselling book,
Fermat’s Enigma
. He lives in London, England.

Also by Simon Singh

Fermat’s Enigma

FIRST ANCHOR BOOKS EDITION, SEPTEMBER 2000

Copyright © 1999 by Simon Singh

All rights reserved under International and Pan-American Copyright Conventions. Published in the United States by Anchor Books, a division of Random House, Inc., New York, and simultaneously in Canada by Random House of Canada Limited, Toronto. Originally published in hardcover in the United States by Doubleday, a division of Random House, Inc., New York, and in the United Kingdom by the Fourth Estate, London, in 1999.

Anchor Books and colophon are registered trademarks of Random House, Inc.

The Library of Congress has cataloged the Doubleday edition as follows:
Singh, Simon.
The code book : the evolution of secrecy from Mary Queen of Scots to quantum cryptography / Simon Singh. –1st ed.
p. cm.
1. Cryptography–History.   2. Data encryption (Computer science)–History.   I. Title.
Z103.S56    1999
652′.8′09–dc21          99-35261

eISBN: 978-0-307-78784-2

Author photo © Nigel Spalding

www.anchorbooks.com

v3.1

For my mother and father,

Sawaran Kaur and Mehnga Singh

The urge to discover secrets is deeply ingrained in human nature; even the least curious mind is roused by the promise of sharing knowledge withheld from others. Some are fortunate enough to find a job which consists in the solution of mysteries, but most of us are driven to sublimate this urge by the solving of artificial puzzles devised for our entertainment. Detective stories or crossword puzzles cater for the majority; the solution of secret codes may be the pursuit of a few.

John Chadwick
The Decipherment of Linear B

Contents

Cover

About the Author

Other Books by This Author

Title Page

Copyright

Dedication

Epigraph

Introduction

1 The Cipher of Mary Queen of Scots

2 Le Chiffre Indéchiffrable

3 The Mechanization of Secrecy

4 Cracking the Enigma

5 The Language Barrier

6 Alice and Bob Go Public

7 Pretty Good Privacy

8 A Quantum Leap into the Future

The Cipher Challenge

Appendices

Glossary

Acknowledgments

Further Reading

Picture Credits

Introduction

F
or thousands of years, kings, queens and generals have relied on efficient communication in order to govern their countries and command their armies. At the same time, they have all been aware of the consequences of their messages falling into the wrong hands, revealing precious secrets to rival nations and betraying vital information to opposing forces. It was the threat of enemy interception that motivated the development of codes and ciphers: techniques for disguising a message so that only the intended recipient can read it.

The desire for secrecy has meant that nations have operated codemaking departments, responsible for ensuring the security of communications by inventing and implementing the best possible codes. At the same time, enemy codebreakers have attempted to break these codes, and steal secrets. Codebreakers are linguistic alchemists, a mystical tribe attempting to conjure sensible words out of meaningless symbols. The history of codes and ciphers is the story of the centuries-old battle between codemakers and codebreakers, an intellectual arms race that has had a dramatic impact on the course of history.

In writing
The Code Book
, I have had two main objectives. The first is to chart the evolution of codes. Evolution is a wholly appropriate term, because the development of codes can be viewed as an evolutionary struggle. A code is constantly under attack from codebreakers. When the codebreakers have developed a new weapon that reveals a code’s weakness, then the code is no longer useful. It either becomes extinct or it evolves into a new, stronger code. In turn, this new code thrives only until the codebreakers identify its weakness, and so on. This is analogous to the situation facing, for example, a strain of infectious bacteria. The bacteria live, thrive and survive until doctors discover an antibiotic that exposes a weakness in the bacteria and kills them. The bacteria are forced to evolve and outwit the antibiotic, and, if successful, they will thrive once again and reestablish themselves. The bacteria are continually forced to evolve in order to survive the onslaught of new antibiotics.

The ongoing battle between codemakers and codebreakers has inspired a whole series of remarkable scientific breakthroughs. The codemakers have continually striven to construct ever-stronger codes for defending communications, while codebreakers have continually invented more powerful methods for attacking them. In their efforts to destroy and preserve secrecy, both sides have drawn upon a diverse range of disciplines and technologies, from mathematics to linguistics, from information theory to quantum theory. In return, codemakers and codebreakers have enriched these subjects, and their work has accelerated technological development, most notably in the case of the modern computer.

History is punctuated with codes. They have decided the outcomes of battles and led to the deaths of kings and queens. I have therefore been able to call upon stories of political intrigue and tales of life and death to illustrate the key turning points in the evolutionary development of codes. The history of codes is so inordinately rich that I have been forced to leave out many fascinating stories, which in turn means that my account is not definitive. If you would like to find out more about your favorite tale or your favorite codebreaker then I would refer you to the list of further reading, which should help those readers who would like to study the subject in more detail.

Having discussed the evolution of codes and their impact on history, the book’s second objective is to demonstrate how the subject is more relevant today than ever before. As information becomes an increasingly valuable commodity, and as the communications revolution changes society, so the process of encoding messages, known as encryption, will play an increasing role in everyday life. Nowadays our phone calls bounce off satellites and our e-mails pass through various computers, and both forms of communication can be intercepted with ease, so jeopardizing our privacy. Similarly, as more and more business is conducted over the Internet, safeguards must be put in place to protect companies and their clients. Encryption is the only way to protect our privacy and guarantee the success of the digital marketplace. The art of secret communication, otherwise known as cryptography, will provide the locks and keys of the Information Age.

However, the public’s growing demand for cryptography conflicts with the needs of law enforcement and national security. For decades, the police and the intelligence services have used wire-taps to gather evidence against terrorists and organized crime syndicates, but the recent development of ultra-strong codes threatens to undermine the value of wire-taps. As we enter the twenty-first century, civil libertarians are pressing for the widespread use of cryptography in order to protect the privacy of the individual. Arguing alongside them are businesses, who require strong cryptography in order to guarantee the security of transactions within the fast-growing world of Internet commerce. At the same time, the forces of law and order are lobbying governments to restrict the use of cryptography. The question is, which do we value more—our privacy or an effective police force? Or is there a compromise?