Cryptographic credential infrastructures, such as Public key infrastructure (PKI), allow the building of trust relationships in electronic society and electronic commerce. At the center of credential infrastructures is the methodology of digital signatures. However, methods that assure that credentials and signed messages possess trustworthiness and longevity are not well understood, nor are they adequately addressed in both literature and practice. We believe that, as a basic engineering principle, these properties have to be built into the credential infrastructure rather than be treated as an after-thought since they are crucial to the long term success of this notion. In this paper we present a step in the direction of dealing with these issues. Specifically, we present the basic engineering reasoning as well as a model that helps understand (somewhat formally) the trustworthiness and longevity of digital signatures, and then we give basic mechanisms that help improve these notion...