Quantum coin hedging, and a counter measure
This addresses a security issue in quantum cryptography for protocol designers, but the findings are incremental as they extend prior work on quantum hedging.
The paper tackled the problem of quantum hedging in cryptographic protocols, specifically showing that perfect quantum hedging occurs in quantum coin flipping, which can introduce serious challenges when protocols are composed. It also proved that hedging cannot occur in sequential two-outcome board games by deriving a formula for their value based on a single game's optimal value.
A quantum board game is a multi-round protocol between a single quantum player against the quantum board. Molina and Watrous discovered quantum hedging. They gave an example for perfect quantum hedging: a board game with winning probability < 1, such that the player can win with certainty at least 1-out-of-2 quantum board games played in parallel. Here we show that perfect quantum hedging occurs in a cryptographic protocol - quantum coin flipping. For this reason, when cryptographic protocols are composed, hedging may introduce serious challenges into their analysis. We also show that hedging cannot occur when playing two-outcome board games in sequence. This is done by showing a formula for the value of sequential two-outcome board games, which depends only on the optimal value of a single board game; this formula applies in a more general setting, in which hedging is only a special case.