Provable Versioned Inter-Cloud Auditing via Block-Level Redactable Blockchain

As a core infrastructure for data governance, trustworthy cloud storage demands secure and efficient auditing mechanisms, which are essential for unlocking data value.However, existing public auditing schemes suffer from critical limitations, including collusion risks introduced by third-party auditors, storage bloat due to the immutability of blockchain, and inefficiencies in version traceability. In this paper, we propose PVICA, a provable versioned inter-cloud auditing scheme to address these challenges. PVICA adopts a decentralized mutual auditing architecture across multiple clouds based on verifiable delay functions, which inherently prevent precomputation attacks and eliminate potential collusion between clouds or between a cloud and its auditor. To mitigate blockchain-induced storage overhead while ensuring controlled redactability, we design a block-level redactable blockchain with double-trapdoor keys control. By separating ephemeral and long-term trapdoor key authorities, PVICA reduces redundant outsourced logs and enables efficient audit updates. Moreover, we integrate RSA accumulators to achieve constant-time cross-version verification, thereby reconciling the conflict between audit log privacy and regulatory compliance. Formal security analysis demonstrates that PVICA achieves key guarantees such as storage correctness and tamper detection. Experimental results show that its block-level redacting mechanism significantly improves audit throughput over traditional approaches, highlighting its practicality for real-world deployment.