The omni-presence of the Internet in society today determined an explosive growth for online transactions. However, due to lack of security services such as data secrecy, authentication and integrity, Internet does not provide all the guarantees that a business communication tool should. In this context, protocols to facilitate secure and fair electronic exchange and certified electronic delive...

A computation scheme among n parties is fair if no party obtains the computation result unless all other n− 1 parties obtain the same result. A fair computation scheme is optimistic if n honest parties can obtain the computation result without resorting to a trusted third party. We prove, for the first time, a tight lower bound on the message complexity of optimistic fair computation for n part...

Contract signing is a fundamental service in doing business. The Internet has facilitated the electronic commerce, and it is necessary to find appropriate mechanisms for contract signing in the digital world. From a designing point of view, digital contract signing is a particular form of electronic fair exchange. Protocols for generic exchange of digital signatures exist. There are also specif...

In this paper we consider the optimistic approach of the non-repudiation protocols. We study a non-repudiation protocol with off-line trusted third party and we keep on with the definition of the multi-party non-repudiation, compare it to multi-party fair exchange and show some fundamental differences between these two problems. Finally, we generalize our protocol and propose a multi-party non-...

We describe a novel certified e-mail protocol that is particularly suitable for mobile environments. Our protocol uses an off-line trusted third party (TTP). Protocols with an off-line TTP—also known as optimistic protocols—have numerous practical advantages over protocols with an on-line TTP. Nonetheless, many protocols adopt an on-line TTP primarily because optimistic protocols often entail i...

Priced oblivious transfer (POT) is a two-party protocol between a vendor and a buyer in which the buyer purchases digital goods without the vendor learning what is bought. Although privacy properties are guaranteed, current schemes do not offer fair exchange. A malicious vendor can, e.g., prevent the buyer from retrieving the goods after receiving the payment, and a malicious buyer can also acc...

At the heart of many fair exchange problems is verifiable escrow: a sender encrypts some value using the public key of a trusted party (called the recovery agent), and then must convince the receiver of the ciphertext that the corresponding plaintext satisfies some property (e.g., it contains the sender’s signature on a contract). Previous solutions to this problem are interactive, and often re...

A fair contract-signing protocol allows two potentially mistrusted parities to exchange their commitments (i.e., digital signatures) to an agreed contract over the Internet in a fair way, so that either each of them obtains the other’s signature, or neither party does. Based on the RSA signature scheme, a new digital contract-signing protocol is proposed in this paper. Like the existing RSA-bas...