# A Review of Blockchain-Based Secure Sharing of Healthcare Data
## 3 Blockchain in Healthcare Data
### Blockchain-Based Data Security Storage and Access  
The emergence of EMR has brought convenience as well as privacy issues. Subject to security issues, medical data cannot be shared freely. Some blockchain-based models have been proposed [22]. A blockchain-based'medichain' model was proposed by [PERSON] et al. [23]. This model uses the blockchain as a database to store the complete case information of the patient in the block. The transaction records are hashed to store the obtained hash values in the Merkle tree to ensure the security of the data and prevent tampering, thus reducing errors in clinical decision-making. To address the problem of a wide range of sources and diverse structures of medical data, the data of all fields are combined into a single hyperfield stored in the proposed framework. The method uses on-chain storage. However, the blockchain is less scalable. On-chain storage is also expensive. [PERSON] introduces a patient-oriented privacy-preserving access control model into the process of access control of private information in healthcare systems [24]. Then, blockchain technology is used to build a private information storage platform, and standard cryptographic algorithms are used to realize information transmission. In this process, the privacy information is also secured by a file authorization contract to further prevent the theft of medical privacy information. The model proposes a fine-grained privacy-preserving access control method that grants different privileges to users by judging their types. EMR information is stored in the cloud database and hosted by a third-party cloud service organization. When data are stored on the cloud, a hash of that data is generated. Then the hash is stored on the blockchain. When the data in the cloud is tampered with, it can be compared by the hash value on the chain. In this model, the consensus algorithm is POW, which requires a lot of invalid computations by the nodes. [PERSON] et al. propose a lightweight blockchain-based model for sharing and protecting medical data [25]. The model uses proxy re-encryption technology to enable data sharing among physicians in different hospitals. The hash function used is hard to collide. Therefore, stored medical information is almost impossible to be tampered with. The traditional delegated proof of interest is improved to obtain a new consensus algorithm that is more secure and reliable. A disease-matching mechanism is designed where patients suffering from the same disease can communicate with each other. After mutual authentication, session keys can be set between patients. This mechanism can help patients to exchange disease information. The private chain is fast in transactions but less decentralized. It is more suitable for applications within companies or institutions. It is not applicable when there are many patients and hospitals. A hybrid chain-based EHR sharing scheme  
Figure 2: Smart contracts. is proposed by [PERSON] et al. to store the private part of the electronic case in the federated chain and the non-private part in the public chain [26]. Only licensed users can access the private part, and the non-private part can be shared with scientific institutions for medical development. The model also uses off-chain storage, and only data hashes are stored on the chain to prevent data tampering, and smart contracts can automatically manage the EMR request, approval and usage process. The hybrid chain approach applied by the model is very novel. However, no attributes are granted to the nodes, and coarse-grained access control is used. [PERSON] et al. have designed a new chain structure to avoid the forking problem and proposed a trust-based consensus mechanism to resist Byzantine attacks [27]. Medical institutions can accumulate trust points through continuous mining in exchange for EMR. The proposed reputation system needs to accumulate reputation scores through a large number of invalid calculations. A large amount of energy is consumed in order to obtain voting rights. [PERSON] et al.