## 4 SSHies: An Era of Research
### Blockchain and Smart Contracts-Based Techniques in HIE Systems: A Background
The K-anonymity technique makes re-identification infeasible by hiding K objects in the same group, preprocessing medical data for blurring identity information. A smart contract implements attribute-based access control, guaranteeing data access only by users with proper attributes. The consortium blockchain is leveraged among different medical institutions to provide the trust layer and host the smart contract. The encrypted keywords are stored in remote clouds. The authors of [90] proposed a MeDShare system for medical data sharing among big data custodians in a trust-less environment. The system uses four layers: user, data query, data structuring and provenance, and existing database infrastructure. The threat model identifies data violations without the data owner's knowledge, risking privacy and data value. The latency increases with the number of cloud service providers, but MeDShare uses metrics like blockchain, identity management, decentralized access, centralized access, distant access, tamper-proof data audit, and data access revocation to secure data provenance and auditing. The authors designed a data sharing model using blockchain, smart contracts, and access control mechanisms to trace data behavior and revoke access to violated rules and permissions. The proposed architecture in [20] includes a private Ethereum blockchain and multi SC functions. It consists of two modules: the linkage module, which links EHR databases with the blockchain, and the request module, which allows patients to grant permission to clinicians to access their data through the blockchain. Data exchange involves data encryption and use of the blockchain system to send and retrieve decryption keys. The blockchain adapter retrieves decryption keys and hashes from a smart contract, hashing the data and comparing the hash with the original hash to identify mismatched records. The paper concludes that a blockchain model can protect data security, patients' privacy, and provide patients with full control over their health records. The authors of [91] incorporated proxy re-encryption technology to securely store medical information, safeguarding it against tampering attempts. Furthermore, an enhanced consensus mechanism was introduced, refining the traditional delegated proof of stake. The scheme involves three key participants: a trusted third party (SM), a hospital (HO), and a user (U). The SM is responsible for generating the master key and system parameters, while the HO registers and creates its private and public keys. Upon visiting HO-i, U establishes their private and public keys and doctors can access U's records by applying to the SM. The proposed scheme fulfills security, privacy, computational, and communication requirements, rendering it a promising solution for enhancing the electronic health system of hospitals. The authors of [63] introduced a blockchain-based framework designed to facilitate the secure sharing of healthcare information among healthcare providers in KSA. The framework consists of three stages: a literature review, data collection pertaining to data sharing, and the categorization of factors into specific categories. The Sharing Data between Healthcare Providers Framework (SDHPF) was divided into three main categories: healthcare system factors encompassing aspects such as decentralization, cost, efficiency, culture, risk policy, and ease of use; security factors including data integrity, privacy, confidentiality, transparency, and anonymity; and blockchain factors comprising availability, accuracy, immutability, tamper-proofing, interoperability, and access control. The authors of [65] proposed a blockchain-based decentralized privacy-preserving EMR management (DPEM) framework for patient-centric healthcare. The framework consists of four layers: data preparation, access control and security, data sharing, and data storage. DPEM uses a new elliptic curve-based content extraction signature (EC-CES) to exclude sensitive information from EMRs, using blockchain smart contracts to define access permissions.