Introduction

During the last decades there has been an enormous growth of computing, communication, and storage capacities [HL11]. The reasons for this growth are manifold. Commerce uses the internet as a basis for sales and customer relationship management, the industry computerises their processes, and communication and discussion in the society is changed by social websites, permanent accessibility via mobile devices and omnipresent internet access. In this environment users face the problem that they lose track of what is going on in their devices and which parties they can trust. Given this fact, we need to clarify what we mean by the term

insecure

. While there are lots of scenarios imaginable regarding an insecure environment, we focus on two cases in this work. In the first scenario, we assume the user’s device is not trustworthy, but he nevertheless wants to be able to use it for secure authentications. In the second scenario, a user has a trustworthy device, but does not want to be tracked by the party verifying the authentication. Additionally, the user should not be able to lend his credentials to someone else. In the next paragraphs we discuss both scenarios in more detail.