## 4 SSHies: An Era of Research
### Blockchain and Smart Contracts-Based Techniques in HIE Systems: A Background
The EMRI data set, consisting of 5327 patient records with 29 features, is used to test the DPEM. DPEM combines access control mechanisms and privacy preservation in data sharing, using smart contracts for presetting access permissions. Patients have complete control over their EMRs, and stakeholders can use data expediently without sacrificing patients' privacy. The authors of [92] propose an Ethereum blockchain-based smart contract system to provide patients with decentralized, immutable, transparent, traceable, trustful, and secure control over their medical records. The system consists of Ethereum, proxy re-encryption, trusted oracles, and reputation systems, with system entities including regulatory agencies, hospitals, patients, doctors, and insurance companies. The authors developed smart contracts and proposed algorithms to implement functions, modifiers, and trigger events. They integrated a blockchain-based system with IPFS and trusted reputation-based oracles to securely fetch, store, and retrieve patient health records. A proxy conduct re-encryption scheme was included to preserve medical records' privacy and ensure they can only be shared with intended doctors. The authors of [55] developed a cross-domain image sharing framework that involves two blockchains: one for on-chain transactions, containing a list of imaging studies and their patient, and another for communication. The framework entities include imaging centers, patients, healthcare providers, PHR vendors, and cloud services. The blockchain has two parts: a leader establishing the blockchain's sequencing and integrity and transactions defining the source and study. The authors have produced a distributed database of access permissions, eliminating third-party access to protected health information. The authors conclude that blockchain technology enables consensus without relying on a central authority. The authors of [93] proposed a secure user-centric approach for accessing and controlling access to sensitive medical data using a blockchain-based data sharing framework. The framework uses asymmetric cryptography to encrypt medical data with the recipient's public key, and access is achieved through a smart contract (SC) that maintains pointers to IPFS content identifiers. The SC safeguards permissions to access information by implementing logic for access control. The system handles identity information for each participant using public-key cryptography, and pseudonymous identities are linked with users' social security numbers. The flow of information is orchestrated by a smart contract deployed on a blockchain-based protocol to ensure traceability and data integrity. MedRec, presented in [94], is a decentralized record management system that uses blockchain technology to handle EMRs and data sharing. The system consists of three smart contracts: registrar contract (RC), patient-provider relationship contract (PPR), and summary contract (SC). It has three patient nodes (EMR Manager, database, Backend Lib, DBGateKeeper, Ethereum Client, MedRec Service), and three provider nodes (same components as patients). The system functions by sending requests to the EMR Manager in the provider node, resolving addresses and PPR, linking the SC to PPR, and sending the result to the blockchain. MedRec provides auditability, interoperability, and accessibility via a comprehensive log, demonstrating how principles of decentralization can be applied to large-scale data management in EMR systems. In [95], the authors developed a system for secure, trustable management, sharing, and aggregation of HER data across multiple hospitals. The system uses a permissioned blockchain-based system, with each hospital providing a blockchain node integrated with its own EHR system. A web-based interface is used for patients and doctors to initiate EHR sharing transactions. The system uses a hybrid data management approach, with only management metadata stored on the chain.