Usability Challenges of PKI

Security mechanisms can only be effective if they are usable as well. Above all, this statement applies to systems based on public key infrastructures (PKI) due to their high complexity. In practice PKI has been found to be a source for user errors, which could easily lead to security breaches. On the other hand usability problems hinder the usage and deployment of PKI-enabled applications and services. The dissertation at hand describes challenges specific for PKI technology with regard to usability and presents possible solutions. A general multi-layer model of methods to promote usable security is introduced as well as a generic tool for the analysis of PKI-enabled applications of any kind. The thesis subsequently exemplifies several applications of these concepts: Beside technical protection mechanisms, user sensitization and training are important pillars of IT security since a lot of security problems can be traced back to human error or negligence. We develop a new approach for security awareness campaigns aiming at long-term and sustainable changes in behaviour by means of direct user involvement. Delegating security-critical tasks is one possibility to face usability issues. Here we focus on PKI outsourcing, i.e. the operation of a trustcenter by a service provider. In this setting, a secure certificate enrolment process is of particular importance. We point out a weakness of the standard protocol and show how to repair it with the help of a four-eyes principle realized by distributed digital signatures. Our protocol requires a distributed key generation by two parties. For this purpose we present a new, efficient and provably secure algorithm for RSA. Most email programs already support encryption and digital signatures, but these features are hardly used in practice. An important reason for this is the unfavourable cost-benefit ratio from the user's point of view. We show how to improve this ratio with the help of opportunistic security. This allows using public key cryptography without certificates and the costs associated with them. PKI-based authentication schemes suffer from the restriction that, due to security reasons, cryptographic keys cannot be temporarily delegated to a proxy for instance during holidays. We present a uniform and seamless solution for the World Wide Web that supports all common authentication mechanisms