Novel Design of Fair Exchange Protocol for Semi-trusted Server and Its Application in Cloud Environment

The messages can be exchanged in a fair manner if after the protocol, both exchanging parties can simultaneously obtain their desired messages or none of them can get useful information. Since the network communication is time consuming, the major approach for this issue is to develop a off-line trusted third party (off-line TTP) which gets involved into the exchange procedure only if at least one party is dishonest for delivering her/his messages. This paper focuses on a new design for the semi-trusted server that can be easily implemented and eliminates the assumption of private channel between the two parties compared with Franklin and Reiter's approach. Furthermore, this kind of design can also be used for the encrypted message fair exchange in cloud computing environment; that is, if two parties want to exchange the messages stored in cloud with encrypted forms, the TTP then acts as a proxy server to help them by fair re-encryptions of these two ciphertexts in an efficient way.