A sound logic for analysing electronic commerce protocols

We present an authentication logic which is capable to analyse one of the most important features of electronic commerce protocols, i.e., the accountability of transactions. By referring to a formal semantics we prove that the calculus is correct. We apply our logic to the payment systems SET and Payword. The formal analysis explains why micropayment systems based on chains of hashvalues do not lead to the same level of assurance as usual payment systems.