CPG: closed pseudonymous groups

This paper presents the design and implementation of Closed Pseudonymous Groups (CPG), a pseudonymous communication system for a closed user community (e.g., a class of students, team of employees, residents of a neighborhood). In CPG, each legitimate user is known by a pseudonym that, while unlinkable to a true identity, enables service providers to link users' behavior and blacklist any abuser of the system. This system is useful for providing honest feedback without fear of reprisals (e.g., instructor/course ratings, employee comments, community feedback for local politics). CPG is designed to be easy to understand, to implement (using existing techniques), and to use. This paper also presents the results of an initial user study that resulted in an important design change.