# Empirical Evaluation of Privacy Efficiency in Blockchain Networks: Review and Open Challenges
## 7 Open Research Challenges and Proposed Future Directions  
In our study, we found out some open research challenges that must be considered for wider adoption of the blockchain technology in privacy-centric applications. These research challenges include: \begin{table}
\begin{tabular}{c c} \hline \hline
**Grading** & **Precision Value** \\ \hline Poor & \(0\leq\text{Precision}\leq 0.3\) \\ \hline Good & \(0.31\leq\text{Precision}\leq 0.6\) \\ \hline Excellent & \(0.61\leq\text{Precision}\leq 1\) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Privacy precision grading model. Challenge 1: Varying linkability requirements. Although we proposed the existing solutions to be evaluated based on linkability analysis, however, the linking techniques and heuristics vary from solution to solution based on their design. Different techniques have different heuristics and methods of linkability and deanonymization of users. This means different techniques can be assessed differently and may yield different results. They do not have a uniform form of evaluation. Proposed Future Direction: A comprehensive literature survey and extensive research on uniform characteristics of blockchain based privacy solutions should be carried out for design of a uniform linkability attack. Common heuristics and similar data will enable justified comparison on the basis of transaction linking. Challenge: Consensus Based Privacy  
We classified the existing solutions based on the fundamental blockchain component associated with enhancing the privacy. Consensus protocols are an integral part of blockchain networks as they are responsible for maintaining integrity, validity, and authenticity of the blockchain network. However, protecting privacy using consensus is yet an underexplored area. Proposed Future Direction: Research and analysis in consensus-based privacy protection is much needed to protect data eavesdropping. The effect of strengthening consensus to preserve transaction anonymity and confidentiality should be explored. Design of some consensus protocols that will secure privacy in blockchain combined with cryptography and/or smart contracts is expected to yield promising results in future. Challenge: Smart Contracts Exploration  
Major portion of blockchain deployment and functionality is achieved through self-executing smart contracts, hence utilizing them in an efficient way will add a layer of privacy protection in blockchain systems. The findings of our survey as presented in Section 3, depict that although smart contracts are widely being used for various application scenarios, still they are comparatively underexplored in comparison to cryptographic primitives for privacy preservation. Proposed Future Direction: Investigating the constructs of smart contracts and using appropriate encryption schemes will add an additional layer of privacy in blockchain transactions. Hence, it is suggested to conduct further research and experiments using solidity smart contracts as Ethereum and Solidity are privacy-friendly blockchain platforms. Challenge 4: Designing Scalable Privacy Protocols  
Various privacy preserving solutions, such as ZKSNARKS and other variants of zero-knowledge proofs provide good privacy protection, however, it comes with a cost of higher consumption of computational resources. Since verifying proof to approve a transaction requires advanced mathematics, it takes longer to verify the transaction. For applications that require a high number of transactions per minute, such as finance and banking systems, the ZKPs tend to produce the problem of transaction scalability. A proper balance between greater privacy preservation and provision of appropriate scalability requirements remains an unsolved challenge to the date. Proposed Future Direction: It is suggested to design a scalable privacy protocol that not only preserves privacy but also does not create scalability issues in the network.