This paper explores user authentication schemes for banking systems implemented over mobile phone networks in the developing world. We analyze an authentication scheme currently deployed by an Indian mobile banking service provider which uses a combination of PINs and printed codebooks for authenticating users. As a first step, we report security weaknesses in that scheme and show that it is susceptible to easy and efficient PIN recovery attacks. We then propose a new scheme which offers better secrecy of PINs, while still maintaining the simplicity and scalability advantages of the original scheme. Finally, we investigate the usability of the two schemes with a sample of 34 current and potential customers of the banking system. Our findings suggest that the new scheme is more efficient, less susceptible to human error and better preferred by the target consumers.

[The scheme proposed in this paper, and its variants, were jointly developed by Microsoft researchers and Eko India Financial Services Ltd.]