Electronic signatures replace handwritten signatures in electronic processes. In this context, non-repudiation is one of the most desired properties yet in practice it cannot be provided by the signature schemes themselves. Therefore, additional mechanisms in the underlying public key infrastructure are required. In this work, we present a formal treatment of that issue. We extend the formal model for public key infrastructures by Maurer introducing transitions to make it dynamic. We use the extended model to evaluate the relationship between non-repudiation and revocation and prove that backdated revocation always destroys the non-repudiation property. We prove that forward secure signatures can be used to maintain non-repudiation, rendering the costly use of timestamping – as required by all existing solutions – superfluous. We also show how to realize this in practice, introducing a new index reporting protocol. Moreover, we show how this protocol can be used to support detection of malicious key usage, thereby improving the overall security of electronic signing. Besides, the index reporting protocol allows for a convenient realization of pay per use models for certificate pricing.