# Secure and Privacy-Aware Blockchain Design: Requirements, Challenges and Solutions
## 5 Evaluation and Discussion
### Comparative Analysis  
In order to compare existing work, we deem it appropriate to examine the following criteria: complexity, scalability, size overhead, memory cost, efficiency, computation overhead, communication overhead, and latency. Table 2 gives a global overview of existing technologies with respect to these criteria. We analyze in detail each privacy and security mechanism of Table 2, including their main advantages and limitations, in the following. We mentioned \"N/A\" in the table if we did not find detailed literature for specific criteria. Complexity is a general indicator that helps us assess how costly a solution will be when adopted, based on its mathematical complexity. Scalability gives an insight into how the compared solutions are suitable to large-scale contexts. Size overhead reflects the cost to apply the solution in terms of storage space. Memory cost shows an amount of memory that is required to apply the solution, which helps us to measure solution performance. Efficiency determines the speed and average execution time taken by a mechanism to complete the given task, which affects the solution performance. Computation overhead shows the cost of privacy and security mechanism operations, which facilitate us to measure the number of steps compulsory to solve the problem. Communication overhead is measured by the number of bytes in every message sent during communication over the network and latency refers to how much time it takes to transfer information to its destination. We follow with Table 3 that provides a detailed discussion on the advantages and drawbacks of the previously mentioned privacy and security techniques to guide further research. The tables show that ring signature [37], is relatively efficient and works with very low latency. Security properties (e.g., confidentiality and authentication) and data transaction protection [4; 39] are also provided without reducing signature size overhead. However, key management and long signature size are drawbacks as the number of key couples is linear to the number of ring participants. Zero-knowledge proof [118], is a promising solution to validate transactions or to ensure privacy of the data in the blockchain. It is good to provide anonymous authentication and is secure against man-in-middle attack [119]. However, zero-knowledge proof is not efficient for large-scale systems because it requires high computation time and memory resources to create and validate proofs [5]. Mix networks, as proposed in [1; 41], achieve both efficiency and scalability. It has a low size overhead because the size of input data is linear to the number of mix servers. The proposed scheme is unable to reduce complexity, latency, memory, and computation overhead due to the high cost of generating an input cipher text and cost of encryption keys used by mix servers. In [43; 44], the Tor mechanism is presented to improve the identified limitations of mix network and achieves low memory cost, data size, computation, and communication overhead. Their motivation was to reduce latency in the anonymity system. However, Tor is not scalable because it cannot handle a large number of nodes or servers. Using Tor entails reducing network speed as compared to normal internet and it does not ensure data security outside of the network. Along with this, onion routing protocol [42], enables sending end-to-end encrypted messages with minimal disclosure of user metadata. The focus is put on scalability and efficiency in communication. However, memory cost and communication overhead are high. From the data privacy perspective, k-anonymity technique is scalable and works without any latency [120]. It is easy to implement and risk of re-identification is reduced when the diverse values are high. However, it is not efficient because of the long processing time [121, 122]. It is prone to the background knowledge attack and homogeneity attack. In [123], the authors proposed the l-diversity mechanism that has a low size overhead.