# Secure and Privacy-Aware Blockchain Design: Requirements, Challenges and Solutions
## 5 Evaluation and Discussion
### Current Recommendations and Research Directions
#### 5.2.2 Research Directions  
As an emerging technology, blockchain is facing some challenging issues. In the following, we summarize privacy and security directions for blockchain environments. * **Key management**: As it is the case with ring signature, most solutions today rely on asymmetric cryptography. Therefore, a pair of public/private is required for each involved actor. Key management is an issue that needs to be mitigated as a large-scale adoption of developed solutions would need to handle very large numbers of key pairs, including the important constraint that the private key must not be disclosed. * **Energy consumption of consensus protocols**: Blockchain consensus protocols consume a lot of computing resources, especially the ones based on proof-of-work, which leads to low-system throughput and high system latency. Designing a better consensus mechanism to improve system throughput is challenging due to the algorithmic complexity involved. * **Security and consensus protocols**: As detailed in Section 4 a good part of security issues related to consensus protocols, such as proof of work, proof of stake, and delegated proof of stake. While consensus mechanisms also have their own challenges in terms of computational cost, they are also central from a security perspective. One possible option in this direction is to constrain the longest chain rule to alleviate majority attacks, which remains an open problem [97]. * **Privacy**: On most public blockchains, transaction data is verified and stored on every node due to its decentralized nature and security concerns. It increases the chances of misuses of the user identities and transaction data. As stated in [34], blockchain does not guarantee transaction and user's identity privacy since all data on the blockchain is publicly available. A lot of work as been ongoing to integrate solutions for privacy protection into the blockchain, a good example of implementation being Monero. Further work is required to protect anonymity at the level of transaction contents, blockchain, history of added blocks, and even blockchain access, so that data read and write accesses becomes totally protected from monitoring attacks. It is noticeable that this problem spreads across the OSI model layers, as identifiers also relate to the network and data link layers (IP and MAC addresses), therefore a well-designed solution should make sure that all layers are covered. The ability to perform computation on encrypted data will arguably have enormous implications on privacy, security, and scalability. Therefore, future research on homomorphic encryption should be aimed not only at mathematical constructions but also audited implementations. The ability to verify, and process transactions without revealing the data will be the key enabler technology for the future development of DLT. * **Storage cost**: Due to the decentralized nature of blockchain, data is copied on every node in the network. Therefore, it imposes a huge cost due to the ever-increasing size of the chain. Building storage-wise lightweight blockchain solutions still remains an open challenge. One direction to explore in this domain is the study of multiple subsets with data intersections, where replication would occur on relevant data subsets while maintaining the security properties that make the blockchain interesting. Local network structures could serve as filters in such setup, guaranteeing that majority attacks could not occur within one particular network by preventing external participants to contribute. At the higher level, a decentralized marketplace of micro-currencies would guarantee global communication, maybe using a central blockchain as we know them now to store the history of the different subsets. * **Blockchain vulnerabilities**: Consensus mechanisms are arguably the biggest security risk. Different consensus mechanisms are available, each with different properties and trade-offs between scalability, security, and decentralization.