Scored Anonymous Credentials

Abstract

<p> Securely maintaining “credits” of users judging their behavior in past authenticated sessions is vital to encourage user participation, but doing it over anonymous credentials is non-trivial, especially when users would avoid claiming negative credit and escape from blocklisting. Prevalent designs impose an authentication cost linear in the blocklist size or a stringent requirement of <em>sequential and timely</em> judgment of each session without retrospective adjustment, as a single unjudged session curbs the authentication of <em>all</em> users. We propose scored anonymous credentials, a new design storing a number of <em>active</em> sessions with volatile scores downgradable before finalized. Sessions can be judged in any order and at varying times without affecting all users. Any backlog of unjudged sessions only affects the users behind them. We achieve efficiency and flexibility using verifiable shuffle, which is hardly used in existing anonymous blocklisting/reputation enforcement systems. <br></p>