Improving fairness and privacy of Zhou-Gollmann´s fair non-repudiation protocol

We deal with two claws of Zhou-Gollmann´s fair non-repudiation protocol. Firstly, their protocol divides a message into 2 parts, i.e., a key K and a ciphertext C. Then, C is delivered to the recipient, while K is submitted to TTP (Trusted Third Party). If the originator doesn´t submit K to TTP, then the protocol appears to have no dispute between the originator and the recipient. However the protocol depends on his action on whether the originator really submits K to TTP or not. We show that the originator can make the protocol unfair by using his advantageous position, and present how to improve the fairness of the protocol. Secondly, the protocol doesn´t provide the message privacy. This means that additional protocols are required to transfer an important message in private. We propose an improved version of the protocol to guarantee the message privacy