Efficient electronic cash with restricted privacy

Abstract

In this paper we propose a coin-based electronic payment system suitable for small payments. It is derived from Brands’ scheme presented at Crypto’93, in the sense that the coins are built using the representation problem. The main contribution of our solution consists of the speedup of the withdrawal protocol. The gain of efficiency is achieved preserving the same level of integrity for user, shop and bank. A coin remains untraceable with respect to the user. This feature is fulfilled even if one assumes that the bank has unlimited computing power and colludes with shops in order to trace a coin to a specific user. However, a set of coins axe linkable to a pseudonym of the user, restricting in this way his privacy. This drawback can be limited by “rotating” coins derived from different pseudonyms in a set of consecutive payment transactions.