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With the growing need for privacy and self-sovereign identity, traditional identity management relying on centralized data registries not only represents single points of failure but also lacks transparency and control over users’ identity information. With the built-in tamper-proofness and transparency, blockchain has been widely studied to accommodate the challenges in traditional identity management. Still, it usually comes with privacy concerns due to its public accessibility. Anonymous credentials take advantage of the recent progress in zero-knowledge proof, allowing the unlinkable presentation of only the necessary attributes for a service to guarantee anonymity. However, the existing anonymous credentials require a secondary issuer to verify and manage the anonymized credentials, which compromises the overall transparency and causes indirect management of the user’s identity. In this paper, we propose GrAC, a blockchain-based identity management system based on a novel identity graph, which allows users and identity providers to securely store and manage identity information on the blockchain without intermediate entities. GrAC also includes an anonymous authentication protocol suite based on zero-knowledge proof, allowing users to generate one-time anonymous credentials that selectively reveal minimal information to the service provider for authentication. The analysis and evaluations show that GrAC has a reasonable overhead and provides adequate anonymity protection while removing the need for intermediate issuers. 

The primary strengths of blockchain systems come from strong guarantees of immutability and reliability, and these systems can be extended with programmatic logic through Smart Contracts. Many such programmatic use cases would benefit from the use of external data. However, the method of bringing that data onto the blockchain needs to be trustworthy and secure. Otherwise, the benefits of blockchain, namely distributed trust, no single points of failure, and immutability, would be at risk. Blockchain oracles are proposed as a conceptual solution to this problem. A blockchain oracle acts as a trusted intermediary to external data. In this work we analyze existing proposals for Discernible event oracle mechanisms, which seek to address oracle queries with a broad knowledgeable answering population, to characterize problems related to the incentives imposed by external value that depends on oracle outcomes. In doing so, we focus on continued challenges to voting based oracles, including trust limitations and the need to match questions to knowledgeable answering parties. To address these difficulties, we propose an extension to existing oracle protocols to utilize reputation as a tool to measure value and as a tool to associate questions with context. By providing a method to track contextual knowledge, our proposal allows for context-based query matching and enables higher probability of correctness for a given population size as well as stronger participation incentives.