# Distributed Authentication and Authorization Models in Cloud Computing Systems: A Literature Review
## 4 Discussion  
This paper's goal was to review the requirements and mechanisms used by researchers designing ACMs for the cloud computing environment. These requirements are considered as evaluation criteria for the assessment of the proposed ACMs and as a mechanism for identifying gaps in their design. Researchers and designers of security solutions for the cloud environment should be mindful of requirements regarding the dynamic and complex nature of cloud computing. Cloud services, for example, can be elastically provisioned and released (in certain circumstances automatically) in response to demands. Provisioning resources appear to be limitless to the user and can be used in any amount at any time and are compatible with the addition of new virtual machines (VMs) with specified computing resources [10]. The ability to quickly assess the security of new VMs and to determine whether newly added VMs are qualified to carry out a particular activity (R9) is a challenge to the design of ACMs [10]. Despite the importance of this requirement, only three manuscripts considered it in the design of ACMs. This review indicated that some articles did not take into account cloud architecture requirements in designing a cloud access control model. For example, the multi-tenancy cloud architecture requirement (R6) is only considered in five articles. The importance of this cloud architecture requirement is represented when several tenants share the same resources. In addition, this requirement is directly related to the privacy and security of the customer's data requirement (R1), which is essential to prevent a tenant from accessing the resources of others in a multi-tenancy cloud environment. Therefore, multiple tenants can share a single application without compromising the security of the others. Another requirement related to the cloud architecture is data sharing and resource isolation (R11). To facilitate data sharing, this review showed that the trust and risk concepts were used. Risk, for example, was used as a mechanism to mitigate uncertainty in cloud access control models generated by a non-dynamic access control system policy, or by a non-flexible access control decision support mechanism. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Requirements**} & \multicolumn{8}{c}{**Cloud Access Control Models**} \\ \cline{2-10}  & **MAC** & **DAC** & **RBAC** & **ABAC** & **ABAC** & **IBEAC** & **TBAC** & **RISKBAC** & **MTAC** \\ \hline R1 & N & Y & Y & Y & Y & Y & Y & Y & Y \\ R2 & N & N & N & Y & Y & N & Y & Y & Y \\ R3 & N & N & N & Y & Y & N & Y & Y & Y \\ R4 & N & N & N & Y & Y & N & Y & Y & Y \\ R5 & N & N & N & N & Y & N & N & N & N \\ R6 & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & Y \\ R7 & N & N & N & N & N & N & N & N & Y \\ R8 & N & Y & N & N & N & N & Y & Y & Y \\ R9 & N/A & N/A & Y & N/A & Y & Y & Y & Y & Y \\ R10 & N/A & N/A & N & N & N/A & Y & Y & N/A & Y \\ R11 & N & N & N & N & Y & Y & Y & Y & Y \\ R12 & N & N & N & Y & Y & N & N & Y & Y \\ R13 & N & N & Y & Y & Y & Y & Y & Y & Y \\ R14 & N & N & Y & Y & Y & Y & Y & Y & Y \\ R15 & N & Y & Y & N & N & N & N & N & N \\ R16 & N & N & N & Y & Y & Y & Y & Y & Y \\ R17 & N & Y & Y & Y & Y & Y & Y & Y & Y \\ R18 & N & N & N & Y & Y & N & N & N & N \\ R19 & N & N & Y & Y & Y & Y & Y & Y & Y \\ R20 & N & N & N & Y & Y & Y & N & Y & Y \\ R21 & N & N & Y & Y & Y & N & Y & Y & Y \\ R21 & N & Y & Y & Y & Y & N & Y & Y & Y \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Access control mechanisms and requirements included in the review, N: no, Y: yes, N/A: not applicable. The current review showed that some researchers have not evaluated their MCAs in accordance with all the essential requirements of the cloud environment. Some researchers, for example, evaluated privacy, while others focused on data protection without taking into account the elastic nature of the cloud (R9) or the protection of cloud metering data (R10). There are several possible reasons why an ACM has not been assessed according to all criteria.