Fair exchange turns out to be an increasingly important topic due to the rapid growth of electronic commerce An exchange is deemed to be fair if at the end of exchange either each party receives the expected item or neither party receives any useful information about the other s item Several protocols for fair exchange have been proposed in recent years In this paper we rst examine a newly publ...

Verifiably encrypted signatures are used when Alice wants to sign a message for Bob but does not want Bob to possess her signature on the message until a later date. Such signatures are used in optimistic contact signing to provide fair exchange. Partially blind signature schemes are an extension of blind signature schemes that allows a signer to sign a partially blinded message that include pr...

We investigate how the behaviors of malicious trusted parties affect participants of optimistic non-repudiation protocols. We introduce a notion of risk balance for exchange protocols. Intuitively, risk balance refers to fairness in the amount of protection a protocol offers to the participants against malicious trustees. We explore how risk balance relates to the notions of accountable trustee...

Mobile agents have been advocated to support electronic commerce over the Internet. While being a promising paradigm, many intricate problems need to be solved to make this vision reality. The problem of fair exchange between two agents is one such fundamental problem. Informally speaking, this means to exchange two electronic items in such a way that neither agent suffers a disadvantage. We st...

A verifiably encrypted signature can convince the verifier that a given cipher-text is the encryption of a signature on a given message. It is often used as a building block to construct optimistic fair exchange. Recently, Gu et.al gave an ID-based verifiably encrypted signature scheme and claimed that their scheme was secure in random oracle model. Unfortunately, in this works, we show that th...

As a value-added service for standard email systems, a certified email scheme allows a sender to deliver a message to a receiver in a fair way in the sense that either the sender obtains a receipt from the receiver and the receiver accesses the content of the email simultaneously, or neither party gets the expected item. In this paper, we first point out some weaknesses in several existing sche...

In asynchronous systems where processes are prone to crash failures, we show that fair exchange is incomparable to distributed consensus. By incomparability we mean there exist failure detector classes that solve fair exchange and not distributed consensus, and vice versa. Remarkably, this is in contrast to the folklore belief that solving fair exchange is generally harder than solving distribu...

Fair exchange constitutes a fundamental mechanism for electronic commerce. Fair exchange assumes that it is possible to verify the correctness of the digital items to be exchanged. However, when an item is indescribable, i.e., it cannot be formally specified, its verification becomes difficult, if not impossible. This hinders a practical deployment of fair exchange. We propose a methodology for...

Although fair exchange protocols are being widely implemented, there are few formal methods able to verify them. This work introduces the strand spaces method for verifying certified mail delivery protocols, a subclass of fair exchange protocols. Three fair exchange properties are verified: effectiveness, verifiability of TTP and timeliness. For effectiveness and verifiability we used the FPH p...