Quantum state targeting

We introduce a new primitive for quantum communication that we term "state targeting" wherein the goal is to pass a test for a target state even though the system upon which the test is performed is submitted prior to learning the target state's identity. Success in state targeting can be described as having some control over the outcome of the test. We show that increasing one's control above a minimum amount implies an unavoidable increase in the probability of failing the test. This is analogous to the unavoidable disturbance to a quantum state that results from gaining information about its identity, and can be shown to be a purely quantum effect. We provide some applications of the results to the security analysis of cryptographic tasks implemented between remote antagonistic parties. Although we focus on weak coin flipping, the results are significant for other two-party protocols, such as strong coin flipping, partially binding and concealing bit commitment, and bit escrow. Furthermore, the results have significance not only for the traditional notion of security in cryptography, that of restricting a cheater's ability to bias the outcome of the protocol, but also on a novel notion of security that arises only in the quantum context, that of cheat-sensitivity. Finally, our analysis of state targeting leads to some interesting secondary results, for instance, a generalization of Uhlmann's theorem and an operational interpretation of the fidelity between two mixed states

Dilemma that cannot be resolved by biased quantum coin flipping

We show that a biased quantum coin flip (QCF) cannot provide the performance of a black-boxed biased coin flip, if it satisfies some fidelity conditions. Although such a QCF satisfies the security conditions of a biased coin flip, it does not realize the ideal functionality, and therefore, does not fulfill the demands for universally composable security. Moreover, through a comparison within a small restricted bias range, we show that an arbitrary QCF is distinguishable from a black-boxed coin flip unless it is unbiased on both sides of parties against insensitive cheating. We also point out the difficulty in developing cheat-sensitive quantum bit commitment in terms of the uncomposability of a QCF.Comment: 5 pages and 1 figure. Accepted versio

Strong Cryptography: The Global Tide of Change

Encryption technology allows people using electronic networks to ensure that the messages they send remain private--secure from hackers, industrial espionage, government wiretap abuses, and spies. Encryption technology will prove vital to the future of electronic commerce. For example, thefts of nuclear secrets from U.S. national laboratories would be much less likely if the labs' commercial software had built-in encryption features that could be used to limit unauthorized access--a type of security product discouraged by export controls. For years the U.S. government has struggled unsuccessfully to control the export of encryption technology from this country. Those ineffectual controls do, however, adversely affect the competitive position of the U.S. software industry and national security. Despite the controls, powerful encryption products are increasingly available around the world. Those products include Pretty Good Privacy, which offers 128-bit encryption, and many others. This paper provides a list of Web sites where such products may be found, thus establishing beyond doubt the futility of controls. Although some of the Web sites may from time to time disappear, others will spring up in their place