Abstract
In the current mobile communication networks, there is no undeniable evidence for calling information in the billing systems. Chen, Jan and Chen proposed a billing system by introducing a trusted third party-Observer and exploiting a hash chain mechanism. In this paper, we first identify some vulnerabilities in their mobile billing system. Then, we propose a fair and secure billing system based on a proper combination of digital signature and hash chain mechanism. The proposed system can achieve authentication, non-repudiation, and fairness, which are desirable security requirements for an undeniable mobile billing system. It is very efficient and applicable not only to the current GSM, but also to the 3GSM.