Coin Tossing is Weaker Than Bit Commitment
Adrian Kent · arXiv (Cornell University) · 1998
We define cryptographic assumptions applicable to two mistrustful parties who each control two or more separate secure sites. We show that, under these assumptions, unconditionally secure coin tossing can be carried out by exchanges of classical information. We show also, following Mayers, Lo and Chau, that unconditionally permanently secure bit commitment cannot be carried out by finitely many exchanges of classical or quantum information. Finally we show that, under standard cryptographic assumptions, coin tossing is strictly weaker than bit commitment. That is, no permanently secure classical or quantum bit commitment protocol can be built from a finite number of invocations of a secure coin tossing black box together with finitely many additional information exchanges.