Opaak

Abstract

Trust and anonymity are both desirable properties on the Internet. However, online services and users often have to make the trade off between trust and anonymity due to the lack of usable frameworks for achieving them both. We propose Opaak, a practical anonymous authentication framework. Opaak enables its users to establish identities with different online services while ensuring that these identities cannot be linked with each other or their real identity. In addition, Opaak allows online service providers to control the rate at which users utilize their services while preserving their anonymity. Hence, allowing the service providers to prevent abuse in the form of spam or Sybil attacks, which are prevalent in such online services that offer anonymity. Opaak leverages the mobile phone as a scarce resource combined with anonymous credentials in order to provide these features. We target two kinds of applications for Opaak and identify their requirements in order to achieve both trust and anonymity. We develop efficient protocols for these applications based on anonymous credentials. In addition, we design an architecture that facilitates integration with existing mobile and web applications and allows application developers to transparently utilize our protocols. We implement a prototype on Android and evaluate its performance to demonstrate the practicality of our approach.