"The Code Book" by Simon Singh

Toward the end of August, an international team of computer scientists succeeded in breaking a 155-digit number into its two prime factors. The feat was significant because public-key cryptography -- the system of encryption that ostensibly guards the privacy of most Internet traffic today -- depends on the difficulty of factoring a very large number. True, it took 292 computers seven months to factor that number. But the mere fact that the problem was solvable in a finite period -- as opposed to, say, 43 times the age of the universe -- was enough to show that the current level of security can, and ultimately will, be penetrated.

This latest blow in the struggle between code makers and code breakers comes as a timely epilogue to "The Code Book," Simon Singh's fascinating and remarkably accessible history of cryptography. Tracing the development of secret writing from the fifth century B.C. to the present and beyond, Singh recounts a series of theoretical and practical breakthroughs, each followed by a matching counterstroke in an ongoing intellectual tennis match. The "evolution" of his subtitle, in fact, is analogous to the biological arms race that develops over time between predators and their prey, or pathogens and their hosts. Each time a technological advance makes a new level of secrecy in encrypted writing possible, the code makers have the upper hand; but eventually the code breakers locate a flaw in the new system, making secure communications an iffy proposition again.

Singh begins his story with the simple alphabetic cipher, a letter-by-letter encoding that lives on today in the humble cryptogram. (It's amazing to consider that what is now an amusement in the daily newspaper was used as recently as the Renaissance to conceal top-level diplomatic correspondence.) Cryptograms, of course, can be easily broken by "frequency analysis" -- assuming, for instance, that the most common letter in a long English message stands for E. Singh goes on to the Vigenhre cipher, hailed in the 19th century as "the indecipherable cipher," which improved on that system by using several different cipher alphabets in rotation, but it, too, ultimately proved fallible. He then proceeds to describe the penetration of the Germans' Enigma code during World War II and, ultimately, with the rise of the computer and electronic communications, the 1970s development of public-key cryptography. He also includes a compelling detour into the great achievements in historical code breaking: the decipherings of the Rosetta Stone and the Linear B inscriptions of the Minoan civilization.

Singh, the British science journalist who wrote the popular "Fermat's Enigma," has a gift for explaining the nuts and bolts of even the most intricate cryptographic systems; it is nothing short of astonishing to watch the Enigma cipher crumble before your eyes through a series of logical deductions that a lay reader can easily follow. At the end of the book, in a sort of promotional-gimmick-cum-final-exam, Singh offers a $15,000 prize to the first reader who can crack a series of coded messages -- one in each of the increasingly sophisticated encryption schemes he details in the book. (It's a measure of just how clear his explanations are that the challenge seems well within the realm of possibility.)

Singh goes beyond the technical details of cryptography to profile the people behind the codes; his book is full of enlivening biographical details and deft portraits of some of the quirkier figures in the history of mathematics, computer science, archaeology and diplomacy. He is scrupulous, too, about giving credit not just to the marquee names but to the lesser-known figures in the story of cryptography as well: His account of the Enigma decoding, for instance, recognizes the work of the British intelligence service and the brilliant mathematician Alan Turing, but also explicates the essential foundations laid by an obscure Polish statistician named Marian Rejewski.

The book's lone false note comes at the end, when Singh contemplates the cryptographic advances based on quantum physics that may appear in the 21st century. He seems to take seriously the likelihood that a truly unbreakable cipher is on the horizon -- this after 300 pages detailing the history of just such a fantasy. It's possible, of course, that complete privacy may yet be achievable. But nothing in "The Code Book" supports the idea.