We study the ultimate physical limits of privacy. We are interested in both fundamental and technological aspects of information security. 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. We study the power of quantum computation and its impact on cryptography and cryptanalysis. Quantum effects which were once thought to be of only philosophical significance have turned out to have applications. Indeed, there are probably very few other places in science where the fundamental and the technological live so closely side by side.