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Quantum Coin Hedging, and a Counter Measure Ganz, Maor Sattath, Or quantum coin hedging quantum board games 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 in parallel, 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 of possible target functions, in which hedging is only a special case. Schloss Dagstuhl – Leibniz-Zentrum für Informatik Maor Ganz and Or Sattath 2018 Is Part Of LIPIcs, Volume 73, 12th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2017) InProceedings Text doc-type:ResearchArticle publishedVersion application/pdf doi:10.4230/LIPIcs.TQC.2017.4 urn:nbn:de:0030-drops-85767 https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.TQC.2017.4 eng https://creativecommons.org/licenses/by/3.0/legalcode