Fair Loss-Tolerant Quantum Coin Flipping

TL;DR

This paper introduces a new quantum coin flipping protocol that is completely resistant to losses, ensuring fairness and optimal cheating strategies for both players, addressing practical implementation issues in quantum cryptography.

Contribution

The paper presents a loss-tolerant quantum coin flipping protocol that guarantees fairness and provides explicit optimal cheating strategies for both players.

Findings

Protocol is impervious to quantum channel losses

Ensures fairness with equal cheating success probabilities

Provides explicit optimal cheating strategies

Abstract

Coin flipping is a cryptographic primitive in which two spatially separated players, who in principle do not trust each other, wish to establish a common random bit. If we limit ourselves to classical communication, this task requires either assumptions on the computational power of the players or it requires them to send messages to each other with sufficient simultaneity to force their complete independence. Without such assumptions, all classical protocols are so that one dishonest player has complete control over the outcome. If we use quantum communication, on the other hand, protocols have been introduced that limit the maximal bias that dishonest players can produce. However, those protocols would be very difficult to implement in practice because they are susceptible to realistic losses on the quantum channel between the players or in their quantum memory and measurement apparatus. In this paper, we introduce a novel quantum protocol and we prove that it is completely impervious to loss. The protocol is fair in the sense that either player has the same probability of success in cheating attempts at biasing the outcome of the coin flip. We also give explicit and optimal cheating strategies for both players.