Privacy-preserving cloud data sharing for healthcare systems with hybrid blockchain

Privacy-preserving cloud data sharing requires user authentication, access control, and privacy provisioning mechanisms. In this paper, we propose two schemes, keyword-based re-encryption (KRE) scheme and oblivious transfer-based re-encryption (OTRE) scheme with hybrid blockchain secure health data sharing. KRE preserves data privacy of patient and provides keyword-based access with reduced key management overhead. OTRE is a superset of KRE that employs OT to preserve privacy of users' data and multiple data sharing. The schemes are collusion-resistant against an honest but curious cloud and malicious users. Hybrid blockchain obviates the need for a single trusted authority and allows large-scale secure data sharing. Blockchain also enables end-user authentication and verifiability of data on the cloud through smart contracts. The privacy provisioning step was responsible for the increased time in the OTRE scheme. We present a concrete construction for KRE and OTRE that satisfies indistinguishability under chosen-plaintext attacks with a random oracle model and formally proves its security. We implemented the schemes using the SageMath tool and hybrid blockchain using Go-Ethereum with Remix. The computational cost, consensus time, latency and throughput in KRE and OTRE schemes demonstrated their efficiency and suitability for healthcare data-sharing applications.