Abstract: Zhou-Gollmann protocol is a fair non-repudiation protocol which is widely discussed in recent years. Kim found a flaw of this protocol on time-limited fairness. Rimming at this f1aw,Kim gave an improved protocol,but the improved schema highly depended on synchronization of network clock. We analyzed Kim's protocol in detail and show that, without clock synchronization, Kim's improved protocol is also unfair. Then we proposed a new schema to improve it further. Without reducing the efficiency of the original protocol, our new method avoids depending on synchronization of the clock, and keeps fairness and non-repudiation of the protocol.

Key words: Fairness, Non-repudiation, Protocol, Clock synchronization