## 2 Literature Review
### Literature Review on the Methods  
To achive security using blockchain, PHR and EMR sharing has been widely applied within a decade [31]. In order to access the PHR, URLs of the PHRs are stored on the blockchain using HE techniques. The encrypted data is stored in the hospitals and healthcare institutions database which is more secure due to the HE encryption. HE technique is considered as light weight encryption techniques and hence its communication and computational cost is low as compared to ring and group signature [32]. Each digital hospital and clinic keeps PHR that creates a fractional or complete patient history to be investigated by clinicians. Digital healthcare frequently pacts with critical healthcare data about patient medical diagnosis which are more vulnerable to security breaches if would kept on centralized system [33]. There exist many common access control models based on objects, identity, role, attributes, time, domain, and trust [34]. Various cryptographic techniques have been designed and applied for access control of encrypted data. In the traditional symmetric key model, encryption is carried out using symmetric keys. The data owner divides the data into some groups and then encrypts these groups using the symmetric key [35]. Users who have the private key can decode the encrypted data. In this scheme, authorized users are listed in the ACL. The major drawback of this scheme is that the number of keys grows linearly as the number of data groups increases. In addition, if any change occurs in the user and data owner relationship, then it will affect other users in the ACL [36]. So, in summary, this scheme is not in practical use in different scenarios. Object-based models are called as DAC and MAC models. In the DAC model, an object is directly connected to the subject using the relationship between these two entities. The MAC model is the improved form of the DAC in a way that it uses the security attributes of the subject and objects to grant access. MAC (Mandatory Access Control) is considered as a standard and well established approach in cryptography. It was first designed for military purposed for controlling information. MAC indeed based on a lattice-based information flow model. MAC has two further versions, such as Bell-LaPdula and Biba models. The Bell-LaPadula model provides information flow and confidentiality, whereas the Biba model is concerned with maintaining the integrity of the data. In MAC, there is no concept of ownership. In other words, it can be described that in MAC, user rights and privileges are not resource-centric [37]. In order to explore this model more, the workings of the MAC model, partial orders, and lattices must be understood. Partial orders use mathematical sets and set properties. Partial order modeling in MAC is used to match and order resources \begin{table}
\begin{tabular}{l l} \hline \hline
**List of Acronyms** & **Explanation** \\ \hline DAC & Discretionary Access Control \\ MAC & Mandatory Access Control \\ ABE & Attribute Based Encryption \\ ABAC & Attribute Based Access Control \\ N & Number of Transactions \\ Dn & Dead Nodes \\ DL & Deep Learning \\ HE & Homomorphic Encryption \\ BC & Blockchain \\ PHR & Personal Health Records \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of Proposed Acronym used. Figure 1: Classification of Homomorphic Encryption. and users' attributes properly. Lattices are used in MAC when the information flow is almost critical. That is the main reason it was purposely designed for the defense sector. MAC uses lattices to follow the information flow policy [38]. Information flow policy deals with the flow of information from one security layer to another security layer. The flow of information can be monitored and maintained by assigning each object a security level or class. Several methods, such as machine learning and data mining techniques, rule-based models, statistical approaches, and deep-learning (DL) methods, are used to design effective security solutions [39].