The notion of identity based cryptosystem was introduced by Shamir in 1984, and has attracted much interest since it eliminates the need of certificates and simplify the key management. In this paper, we propose an optimistic fair exchange protocol for identity-based signatures. A semi-trust third party (
) is still involved in our protocol to ensure fairness. However, there is no need for registrations between users and
, and no zero-knowledge proof is needed to provide verifiability. The proposed optimistic fair exchange protocol is much concise and efficient, and can be shown to be secure in the random model with a tight security reduction.