Paper 2017/389

Decentralized Blacklistable Anonymous Credentials with Reputation

Rupeng Yang, Man Ho Au, Qiuliang Xu, and Zuoxia Yu

Abstract

Blacklistable anonymous credential systems provide service providers with a way to authenticate users according to their historical behaviors, while guaranteeing that all users can access services in an anonymous and unlinkable manner, thus are potentially useful in practice. Traditionally, to protect services from illegal access, the credential issuer, which completes the registration with users, must be trusted by the service provider. However, in practice, this trust assumption is usually unsatisfied. Besides, to better evaluate users, it is desired to use blacklists, which record historical behaviors of users, of other service providers, but currently, this will threaten the security unless a strong trust assumption is made. Another potential security issue in current blacklistable anonymous credential systems is the blacklist gaming attack, where the service provider attempt to compromise the privacy of users via generating blacklist maliciously. In this paper, we solve these problems and present the decentralized blacklistable anonymous credential system with reputation, which inherits nearly all features of the BLACR system presented in Au et.al. (NDSS'12). However, in our new system, no trusted party is needed to register users. Moreover, blacklists from other service providers can be used safely in the new system assuming a minimal trust assumption holds. Besides, the new system is also partially resilient to the blacklist gaming attack. Technically, the main approach to solving these problems is a novel use of the blockchain technique, which serve as a public append-only ledger and are used to store credentials and blacklists. To simplify the construction, we also present a generic framework for constructing our new system. The general framework can be instantiated from three different types of cryptographic systems, including the RSA system, the classical DL system, and the pairing based system, and all these three types of instantiations can be supported simultaneously in the framework. To demonstrate the practicability of our system, we also give a proof of concept implementation for the instantiation under the RSA system. The experiment results indicate that when authenticating with blacklists of reasonable size, our implementation can fulfill practical efficiency demands, and when authenticating with empty blacklists, it is more efficient than that of Garman et al. (NDSS'14), which presents a decentralized anonymous credential system without considering revocation.