Design and Evaluation of a Blockchain-Integrated Secure Cloud Storage System

Cloud storage is a critical part of modern computer systems that has a centralized structure, which opens sensitive data to security risks, including unauthorized access, insider threats, and data tampering. In order to mitigate these issues, this paper proposed a novel cloud storage system with built-in blockchain, AES-256 encryption, access control using smart contracts, and a dual-hash integrity check system. The encrypted data files are then outsourced to the cloud, and cryptographic hash, ownership, and access control policies are stored on a blockchain ledger. As AES encryption uses randomized initialization vectors, encryption of the same file with subsequent encryption yields a different ciphertext. To prevent falsely identifying integrity failures, the proposed system calculates and stores the hash of the plaintext file on the blockchain prior to the encryption of the file, and ensures that the integrity can be reliably and consistently checked. The system is entirely executed in Python as a hash-linked blockchain with a JSON-based persistence and is tested by the experiments using Cloud Sim-style metrics. It analyses performance metrics such as latency, throughput, AES overhead, and blockchain overhead in three scenarios: cloud-only, cloud with AES encryption, and cloud with AES encryption and blockchain. The experimental result proves that the proposed system substantially improved data confidentiality, integrity, access control, and auditability at the expense of moderate and tolerable performance overhead.