The ultimate physical limits of privacy

Abstract

Among those who make a living from the science of secrecy, worry and paranoia are just signs of professionalism. Can we protect our secrets against those who wield superior technological powers? Can we trust those who provide us with tools for protection? Can we even trust ourselves, our own freedom of choice? Recent developments in quantum cryptography show that some of these questions can be addressed and discussed in precise and operational terms, suggesting that privacy is indeed possible under surprisingly weak assumptions.Edgar Allan Poe, an American writer and an amateur cryptographer, once wrote "… it may be roundly asserted that human ingenuity cannot concoct a cipher which human ingenuity cannot resolve …" 1 . Is it true? Are we doomed to be deprived of our privacy, no matter how hard we try to retain it? If the history of secret communication is of any guidance here, the answer is a resounding 'yes'. There is hardly a shortage of examples illustrating how the most brilliant efforts of code-makers were matched by the ingenuity of code-breakers 2 . Even today, the best that modern cryptography can offer are security reductions, telling us, for example, that breaking RSA, one of the most widely used public key cryptographic systems, is at least as hard as factoring large integers 3 . But is factoring really hard? Not with quantum technology. Indeed, RSA, and many other public key cryptosystems, will become insecure once a quantum computer is built 4 . Admittedly, that day is probably decades away, but can anyone prove, or give any reliable assurance, that it is? Confidence in the slowness of technological progress is all that the security of our best ciphers now rests on.This said, the requirements for perfectly secure communication are well understood. When technical buzzwords are stripped away, all we need to construct a perfect cipher is shared private randomness, more precisely, a sequence of random bits known as a 'cryptographic key'. Any two parties who share the key, we call them Alice and Bob (not their real names, of course), can then use it to communicate secretly, using a simple encryption method known as the one-time pad 5 . The key is turned into a meaningful message by one party telling the other, in public, which bits of the key should be flipped. An eavesdropper, Eve, who has monitored the public communication and knows the general method of encryption but not the key will not be able to infer anything useful about the message. It is vital though that the key bits be truly random, never reused, and securely delivered to Alice and Bob, who may be miles apart. This is not easy, but it can be done, and one can only be amazed how well quantum physics lends itself to the task of key distribution.Quantum key distribution, proposed independently by Bennett and Brassard 6 and by Ekert 7 , derives its security either from the Heisenberg uncertainty principle (certain pairs of physical properties are complementary in the sense that knowing one property necessarily precludes knowledge about the othe...

Classical noise, quantum noise and secure communication

“…Quantum Mechanics provides several important ingredients to information communication not present in its classical counterpart 28 . Firstly the information itself sent across a communication channel can be either classical or quantum.…”

Hiding a Quantum Cache in Diamonds

“…Linking multiqubit nodes into a large-scale quantum network [1][2][3][4] will open up exciting opportunities ranging from fundamental tests [5] and enhanced timekeeping [6] to applications in quantum computing and cryptography [1,[7][8][9]. Pioneering experiments with atomic ensembles [3], single atoms trapped in vacuum [2,4,10,11], and spins in solids [12][13][14] have demonstrated entanglement between two optically connected nodes.…”

Random Sampling of Quantum States: a Survey of Methods

“…In the last few years we have been witnessing astonishing theoretical and experimental developments in quantum computing and quantum simulation [5,6,7], and also in others sub-areas of quantum information science [8,9,10,11], with experimental implementations already going beyond the best present classical capabilities [12]. These are the first sights of what will turn out to be a revolution in our science and technology [13,14].…”