# Blockchain-Based Multimedia Content Protection: Review and Open Challenges
## 4 Limitations, Open Challenges and Future Research Directions
### Future Research Directions  
In this section, we describe possible research directions that are identified during the fine-grained analysis of the 18 blockchain-based multimedia content protection systems. Future research works should address these research potentials to tackle the challenges identified in Sections 4.2 and 4.3, and to enhance the usability of the blockchain technology in copyright protection applications. 3.1 Designing an Efficient Blockchain-Based Framework to Satisfy the System Requirements of Copyright Protection Applications  
* Scalability: Off-chain mechanisms such as Lightning Network (LN) for Bitcoin [54] and Plasma for Ethereum [55], or on-chain schemes like SegWit for Bitcoin [56] and Sharding [57] for Ethereum, can solve the blockchain's scalability problem. However, for LN, the main challenges are security and transparency per-transaction basis and centralization (presence of hubs), while in Plasma, long waiting periods (7-14 days) for withdrawal of funds and security risks are the open challenges. Similarly, the main challenges of SegWit are complexity, increased storage space and network bandwidth, while communication and security are the main issues in Sharding. The studies on addressing security, complexity, decentralization, performance, and communication strategies are expected to achieve significant research interest in the future. * Validation of a framework: Many of the blockchain-based copyright protection applications solely focus on the technology's benefits, while leaving aside the details of their implementation. Therefore, it is important to design a practical blockchain-based framework that should address both technical and implementation details, e.g., assessing the advantages and disadvantages of permissioned and permissionless systems before opting for one of these solutions, the selection of the appropriate consensus mechanism depending on the requirements (e.g., transaction throughput, latency, minimum transaction fee, centralization/decentralization and security, among others), and the evaluation of the performance by implementing the proof-of-concepts to calculate computational overheads and the total response time. * Standardization: The recognized technological standards establish specifications and procedures that are beneficial in terms of ensuring efficiency, reliability, and enhanced security. Through our fine-grained analysis, it can be deduced that there is a need of a universal standard that the multimedia content providers, producers and involved companies could follow to share new blockchain-based copyright protection solutions as well as to integrate these with the existing systems. Similarly, this standard should allow automatic conversion between different cryptocurrencies to enhance user experience. * Privacy-aware design: Future research studies need to investigate possible privacy-aware solutions that could protect the privacy of the entities (content owner, buyer, and so on) involved in the transactions of the blockchain-based content protection applications. The privacy and security requirements should be defined at the initial stage of these schemes due to the fact that the data (e.g., information related to the content owner, the public keys of the participants, pseudonyms and copyright information, among others) is visible to everyone on the network. * Unlinkability: The privacy leakage issue in the blockchain needs to be addressed in order to prevent linkability and possible identification. Future research works could study the feasibility of incorporating anonymity mechanisms, such as CoinShuffle [58] (shuffle addresses), Zerocash [59] (hides the payment's origin, destination, and transferred amount) or differential privacy into the applications to meet the anonymity requirements. * Smart contract security: All possible security and privacy attacks (e.g., eavesdropping, DDoS attack or impersonation) on the smart contract should be analyzed through formal security analysis.