Weighted Threshold Anonymous Credentials with Redactable Fine-Grained Blind Signature for Auditable Lending System in Blockchains

Anonymous credentials are ideal for decentralized systems like blockchains, as they enhance privacy, security, and regulatory compliance while maintaining flexibility and adaptability. These decentralized systems often require features such as weighted threshold issuance and tracing for elasticity of decision-making, fine-grained blindness for user privacy control, and issuer-hiding to reduce potential external threats. However, the latest advancements in anonymous credential schemes cannot meet all of these essential properties for blockchain systems. To address these challenges, we propose weighted threshold anonymous credentials with redactable fine-grained blindsignature (WTAC). First, by leveraging unlinkable redactable signatures (URS) and functional encryption techniques, our redactable fine-grained blind signature supports selective disclosure and permits credential issuers to learn a particular function value related to the attributes during issuance without revealing the actual attribute content. Second, we utilize weighted ramp secret-sharing (WRSS) and randomizable signatures to achieve weighted threshold anonymous credentials, which are issuer-hiding to both users and verifiers. Moreover, we provide a concrete construction instantiated by the Fiat-Shamir paradigm and demonstrate its application, a privacy-preserving auditable lending system in blockchain scenarios, achieving the integration of user privacy and regulatory compliance. Finally, we give a formal security proof of anonymity, fine-grained blindness, traceability, non-frameability, and issuer-hiding. Performance evaluation shows the practicability and efficiency of our proposed WTAC.