The requirements for an authentication infrastructure for electronic commerce are explained by identifying the partners involved in e-commerce transactions and the trust relationships required. Related security requirements are also explained, such as authentication, access rights, payment credentials, anonymity (in certain cases), and privacy and integrity of message exchanges. Then several general authentication schemes and specific protocols are reviewed and their suitability for mobile users is discussed. Finally, an improved authentication protocol is presented which can provide trust relationships for mobile e-commerce users. Its analysis and comparison with other proposed authentication protocols indicate that it is a good candidate for use in the context of mobile e-commerce.