Fair exchange solutions

Fair Exchange of values is a problem that aims to solve a scenario where N parties want to exchange some item, but none wants to give his item to the others unless he has an assurance that the others will give him the item he is waiting for. It has many applications, like digital signature of contracts, where the items to be exchanged are signatures on contracts, certified delivery of messages, where we exchange a message for evidence of receipt, or a payment process, where we exchange a payment (e-cash, visa, etc.) for digital goods or a receipt.