Coin tossing is strictly weaker than bit commitment

We define cryptographic assumptions applicable to two mistrustful parties who each control two or more separate secure sites between which special relativity ensures a time lapse in communication. We show that, under these assumptions, unconditionally secure coin tossing can be carried out by exchanges of classical information. We then show that, under standard cryptographic assumptions, coin tossing is strictly weaker than bit commitment. That is. no unconditionally secure bit commitment protocol can be built from a finite number of invocations of a secure coin-tossing black box together with finitely many additional classical or quantum information exchanges.