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Science

Simple Noise May Stymie Spies Without Quantum Weirdness

Adrian Y. Cho

Published Sep 29, 2005• 61 citations
Publisher

Abstract

With the grand ambition of sending unbreakable coded messages, some physicists are using exotic tools—streams of individual photons and quantum mechanics—to shut out prying eyes. But a wire and a few resistors may convey a message as securely, says a physicist who has devised a simple and—he claims—uncrackable scheme. The idea shows that "classical" methods might compete with budding "quantum cryptography," others say. "I believe in beautiful and simple ideas, and this is one of them," says János Bergou, a theorist at Hunter College of the City University of New York.

DOI
10.1126/science.309.5744.2148b

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With the grand ambition of sending unbreakable coded messages, some physicists are using exotic tools—streams of individual photons and quantum mechanics—to shut out prying eyes. But a wire and a few resistors may convey a message as securely, says a physicist who has devised a simple and—he claims—uncrackable scheme. The idea shows that "classical" methods might compete with budding "quantum cryptography," others say. "I believe in beautiful and simple ideas, and this is one of them," says János Bergou, a theorist at Hunter College of the City University of New York.
A product is on the way that relies on the quantum properties of photons to keep out eavesdroppers. All of them. The battle between codemakers and codebreakers is centuries old, but at the start of the 21st century, could it finally be drawing to a close? Physicists are putting the finishing touches to a method of encrypting messages that is more secure than anything that has gone before. Unlike the ciphers of the past, this new method has the potential to be absolutely unbreakable, not just practically unbreakable, but theoretically unbreakable. Mathematicians believe they can prove it. Central to the technique are the strange laws of quantum mechanics. Already experimental messages encrypted using quantum mechanics are being sent over tens of kilometers of optical fibers and received securely. Researchers are confident that it will not be long before ultra-secret messages are routinely transmitted in this way. But while quantum cryptography may be perfect in theory, practical considerations introduce security loopholes that an eavesdropper can exploit. The seriousness of these is still unclear, and physicists believe they can plug most of the holes with more efficient equipment. Still, enough cracks remain to maintain a healthy interest among cryptographers.
For thousands of years, code-makers and code-breakers have been competing for supremacy. Their arsenals may soon include a powerful new weapon: quantum mechanics. Cryptography — the art of code-making — has a long history of military and diplomatic applications, dating back to the Babylonians. In World World Two, the Allies ’ feat of breaking the legendary German code, Enigma, contributed greatly to the final Allied victory. Nowadays, cryptography is becoming increasingly important in commercial applications for electronic business and electronic commerce. Sensitive data such as credit card numbers and personal identification numbers (PINs) are routinely transmitted in encrypted form. Quantum mechanics is a new tool for both code-breakers and code-makers in their eternal arms race. It has the potential to revolutionize cryptography both by creating perfectly secure codes and by breaking standard encryption schemes. The most well-known application of cryptography is secure communication [1]. Suppose Alice would like to send a message to Bob, but there is an eavesdropper, Eve, who is wiretapping the channel. To prevent Eve from knowing the message, Alice and Bob may perform encryption, i.e., transform the message to something that is not intelligible to Eve during the communication. On receiving the message, Bob inverts the transformation and recovers the message (see figure 1). Bob’s advantage over Eve lies in his knowledge of a secret, commonly called the key, that he shares with Alice. The key tells him how to decode the message. For example: The rumble of Soviet tanks shook the Prague hotel room (number 117) as secret agent John Blond finished decoding his orders from his superior N. He tore the used page from the codebook and immediately burned it with his lighter. Blond is using a perfectly unbreakable cipher, a “one-time pad. ” The secret codebook allows N and Blond to share a long secret binary string — the key —
As we have said, quantum mechanics has become the standard framework for most of what is done in physics and indeed has played this role for three-quarters of a century. For just as long, physicists and philosophers have, as we have already suggested, raised and discussed questions about the interpretation of quantum mechanics: Why do we single out measurement as a special kind of interaction that evokes a probabilistic, irreversible response from nature, when other kinds of interaction cause deterministic, reversible changes? How should we talk about quantum superpositions? What does entanglement tell us about the nature of reality? These are interesting questions and researchers still argue about the answers. However, in the last couple of decades researchers have also been thinking along the following line: Let us accept that quantum objects act in weird ways, and see if we can use this weirdness technologically. The two best examples of potential uses of quantum weirdness are quantum computation and quantum cryptography. One of the first quantum cryptographic schemes to appear in the literature, and the only one we consider in detail in this book, was introduced by Charles Bennett and Gilles Brassard in 1984. Their idea is not to use quantum signals directly to convey secret information. Rather, they suggest using quantum signals to generate a secret cryptographic key shared between two parties. Thus the Bennett–Brassard scheme is an example of “quantum key distribution.”
Quantum computing, quantum encryption and quantum teleportation: today's physics journals sound morelike Star Trek than ever. Based on the weird and counterintuitive features of quantum mechanics, these concepts have been brought to fruition in recent years by theorists and experimentalists alike. Indeed, these ideas are now leaving physics laboratories and being used by industry to make real products. Who knows what revolutions await in computation and communications from these budding breakthroughs?