A privacy-preserving cloud storage framework with hybrid encryption, homomorphic keyword search, and blockchain-based integrity verification

Cloud storage outsourcing introduces critical privacy risks as Cloud Service Providers (CSPs) can access unencrypted user data, while encryption eliminates essential functionality such as searchability and remote integrity verification. Existing solutions face a trilemma: fully homomorphic encryption (FHE) remains impractical with seconds-per-operation overhead, searchable encryption leaks access patterns exploitable through inference attacks, and blockchain-based auditing schemes using public chains suffer from low throughput (7–15 TPS), high costs ($1–50 per transaction), and pseudonymous-but-traceable privacy. We present a privacy-preserving cloud storage framework that synergistically combines hybrid AES-ECC encryption for data confidentiality with HMAC-based authentication, Paillier homomorphic encryption (PHE) for probabilistic keyword encryption, and Ethereum-based smart contracts for decentralized integrity verification. Our hybrid protocol achieves IND-CPA security for bulk encryption with authenticated integrity through HMAC-SHA256, delivering 245.7ms encryption time for 1MB files and 7.3% storage overhead—21.2% faster than comparable systems. The Paillier-based keyword encryption prevents frequency analysis through probabilistic ciphertext generation, with search operations over 100 encrypted files completing in 847ms under the Decisional Composite Residuosity assumption. Leveraging Ethereum smart contracts with dynamic third-party auditor selection, our system achieves 287ms consensus latency on the Ganache test network with empirical collusion resistance measurements ($$R_c = 0.049$$, consistent with hypergeometric model predictions). The distributed architecture demonstrates 85 TPS per group throughput while maintaining O(n) encryption complexity and O(1) blockchain operations validated through empirical log-log regression ($$R^2 > 0.98$$) across file sizes from 1KB to 32KB, establishing feasibility for consortium-based privacy-critical cloud deployments. Production deployment on permissioned blockchain networks such as Hyperledger Fabric would provide additional privacy features including transaction unlinkability through identity management.