Document Type

Technical Report

Publication Date


Technical Report Number



Several credential systems have been proposed in which users can authenticate to service providers anonymously. Since anonymity can give users the license to misbehave, some variants allow the selective deanonymization (or linking) of misbehaving users upon a complaint to a trusted third party (TTP). The ability of the TTP to revoke a user's privacy at any time, however, is too strong a punishment for misbehavior. To limit the scope of deanonymization, systems have been proposed in which users are deanonymized if they authenticate ``too many times,'' such as ``double spending'' with electronic cash. While useful in some applications, it is not possible to generalize such techniques to more subjective definitions of misbehavior, e.g., it is not possible to block users who ``deface too many webpages'' on a website. We present BLAC, the first anonymous credential system in which service providers can revoke the credentials of repeatedly misbehaving users without relying on a TTP. Since revoked users remain anonymous, misbehaviors can be judged subjectively without users fearing arbitrary deanonymization by a TTP. Finally, our construction supports a $d$-strikes-out revocation policy, whereby users who have been subjectively judged to have repeatedly misbehaved at least $d$ times are revoked from the system.


The preliminary part of this report is based on our work originally published in the proceedings of the 14th ACM Conference on Computer and Communication Security (CCS '07), October 2007, under the title of "Blacklistable Anonymous Credentials: Blocking Misbehaving Users without TTPs". At the same period of time, we published an extended version of the CCS '07 paper as Dartmouth Computer Science technical report TR2007-601, which contained further details that did not fit into the conference paper. In this report, we make a significant additional contribution by extending our original construction of BLAC to provide more flexible revocation --- SPs can specify a d-strikes-out revocation policy, so that users can authenticate anonymously only if they have not misbehaved d or more times. Such a policy forgives a few (i.e., up to d-1) misbehaviors, but then blocks users who misbehave repeatedly. Following authentication, users remain anonymous, and SPs learn only whether a user has crossed the threshold of d misbehaviors. The original construction of BLAC is a special case with d=1.