Optimistic Gradual Fair Exchange by Using Threshold Cryptography

An optimistic fair exchange protocol in distributed settings without a single trusted third party is proposed. In this protocol, the exchange process consists of secret share ciphertext exchange phase and key gradual exchange phase. Each party is able to stop releasing the rest secret shares in case of cheat, which can be detected with high probability during the process. A threshold decryption group is involved only when unfair behavior occurs in the last exchange round. The proposed protocol does not rely on equal computing power assumption or a trusted third party to guarantee fairness. It also has equivalent computation complexity and smaller communication complexity compared with previous gradual release schemes.