New Imperfect Random Source with Applications to Coin-Flipping

Ari Juels, Markus Jakobsson, Elizabeth Shriver +1 more

We present a new technique for simulating fair coin flips using a biased, stationary source of randomness. Sequences of random numbers are of pervasive importance in cryptography and vital to many other computing applications. Many sources of randomness, such as radioactive or quantum-mechanical sources, possess the property of stationarity. In other words, they produce independent outputs over fixed probability distributions. The output of such sources may be viewed as the result of rolling a biased or loaded die. While a biased die may be a good source of entropy, many applications require input in the form of unbiased bits, rather than biased ones. For this reason, von Neumann (1951) presented a now well-known and extensively investigated technique for using a biased coin to simulate a fair coin. We describe a new generalization of von Neumann's algorithm distinguished by its high level of practicality and amenability to analysis. In contrast to previous efforts, we are able to prove our algorithm optimally efficient, in the sense that it simulates the maximum possible number of fair coin flips for a given number of die rolls. In fact, we are able to prove that in an asymptotic sense our algorithm extracts the full entropy of its input. Moreover, we demonstrate experimentally that our algorithm achieves a high level of computational and output efficiency in a practical setting.

Gene S. Kopp, John D. Wiltshire-Gordon

Gene S. Kopp, John D. Wiltshire-Gordon

Ou Ruan, Zhou Jing, Guohua Cui +1 more

Coin-flipping protocols allow mutually distrustful parties to generate a random bit with uniform distribution. Minimizing bias and keeping fairness are two key requirements in designing such protocols. However, the best previously known fair coin-flipping protocol only guarantees with bias O (1/ r ). In the paper, we introduce a new approach for fair coin-flipping protocol construction by using Garay et al ’s “resource fairness” technique and propose a resource-fair two-party coin-flipping protocol according to the standard real/ideal simulation paradigm. Compared with the other coin-flipping protocols, our protocol enjoys an important advantage that its bias is 0.

Carolin Lunemann, Jesper Buus Nielsen

We propose a coin-flip protocol which yields a string of strong, random coins and is fully simulatable against poly-sized quantum adversaries on both sides. It can be implemented with quantum-computational security without any set-up assumptions, since our construction only assumes mixed commitment schemes which we show how to construct in the given setting. We then show that the interactive generation of random coins at the beginning or during outer protocols allows for quantum-secure realizations of classical schemes, again without any set-up assumptions. As example applications we discuss quantum zero-knowledge proofs of knowledge and quantum-secure two-party function evaluation. Both applications assume only fully simulatable coin-flipping and mixed commitments. Since our framework allows to construct fully simulatable coin-flipping from mixed commitments, this in particular shows that mixed commitments are complete for quantum-secure two-party function evaluation. This seems to be the first completeness result for quantum-secure two-party function evaluation from a generic assumption.

V. I. SERDOBOLSKII