AbstractPayment systems need to address a number of security issues in order to be an effective and secure means of transferring payments across the Internet. To be accessible to a wider audience, they also need to be easy to use for their end-users (customers and merchants). Trying to address these issues, we created the Internet Payment System (IPS). IPS tries to combine the advantages of several existing payment systems. While strong emphasis is made on the mobility and ease of use for its customers, IPS still retains strong security properties. It achieves privacy, integrity, authentication and non-repudiation by using different cryptographic algorithms and techniques. To demonstrate that the protocol satisfies the desired security properties, we use a recently proposed tool for formal verification, called AVISPA.