Simple and fast optimistic protocols for fair electronic exchange

Assume each of two parties has something the other wants. Then, a fair exchange is an electronic protocol guaranteeing that either both parties get what they want, or none of them does. Protocols relying on traditional trusted parties easily guarantee such exchanges, but are inefficient (because a trusted party must be part of every execution) and expensive (because trusted parties want to be paid for each execution).In this paper we• Quickly review fair exchanges that are optimistic, that is, relying on a trusted party that (1) does not participate at all in an honest execution, and yet (2) guarantees the fairness of all executions; and• Present some older and surprisingly simple optimistic protocols for fair certified e-mail and contract signing that never appeared in the scientific literature.