Abstract — This paper discusses the design, implementation and deployment of a secure and practical payment system for electronic commerce on the Internet. The system is basedontheiKP family of protocols – i =1, 2, 3 – developed at IBM Research. The protocols implement credit cardbased transactions between buyers and merchants while the existing financial network is used for payment clearing and authorization. The protocols are extensible and can be readily applied to other account-based payment models, such as debit cards. They are based on careful and minimal use of public-key cryptography and can be implemented in either software or hardware. Individual protocols differ in both complexity and degree of security. In addition to being both a pre-cursor and a direct ancestor of the well-known SET standard, iKP-based payment systems have been in continuous operation on the Internet since mid-1996. This longevity – as well as the security and relative simplicity of the underlying mechanisms – make the iKP experience unique. For this reason, this paper also reports on, and addresses, a number of practical issues arising in the course of implementation and real-world deployment of a secure payment system.