Abstract—The proposed system introduces a blockchain-federated security system and a zero-knowledge adaptive trust system that would allow secure and auditable access control on multi-cloud platforms. There is a need to overcome the issues of heterogeneity of infrastructure, dynamic threats, and uninteroperable and privacy-sensitive trust management in securing access in a multi-cloud environment. The suggested model incorporates a federated identity management system, which combines biometric authentication and cryptographic identity that runs on blockchain to provide non-repudiable identity management. The system involves a blockchain trust authority that handles cryptographic keys and a dynamic on-chain trust score, which facilitates privacy-preserving authentication employing zero-knowledge proofs. Hybrid cryptography provides secure data access and sharing by combining fine-grained access control with homomorphic encryption, based on attribute-based encryption and homomorphic encryption, respectively. The combination of a blockchain and differential privacy allows executing queries in an auditable and privacy-guaranteed manner with clear data owner policies. To achieve cross-cloud interoperability, a trust-aware blockchain-aided proxy re-encryption scheme permits protected information sharing with adjustable trust levels, and trust re-assessment and blockchain-aided key refreshing schemes are resistant to dynamic threats. Experiments conducted on a federated multi-cloud testbed have shown that the proposed framework obtains the following security guarantee and efficiency, as compared to existing schemes authentication delay of 180 ms, a transaction throughput of 220 tps, and an accuracy of trust evaluation of 94%. Results of the experiment indicate that the proposed framework is scalable, privacy-protecting, and reliable in terms of access control to federated multi-cloud systems.
 
Keywords—multi-cloud environments, blockchain trust authority, adaptive trust scoring and attribute-based encryption


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