*4.2. Limitations and Research Challenges of the Blockchain Technology*

In this section, we discuss the limitations and research challenges of the blockchain technology:


simple code is not Turing complete and does not support all possible programming structures, specifically loops.


### *4.3. 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.

4.3.1. Designing an Efficient Blockchain-Based Framework to Satisfy the System Requirements of Copyright Protection Applications


the content owner, the public keys of the participants, pseudonyms and copyright information, among others) is visible to everyone on the network.


4.3.2. Designing Multimedia Content Protection Systems to Support the Blockchain Technology


#### **5. Conclusions**

This study is aimed to present an overview of blockchain-based content protection applications. In this work, we have defined a taxonomy to classify the state-ofthe-art blockchain-based copyright protection schemes w.r.t. technical characteristics of the blockchain technology, the most commonly used content protection mechanisms, and performance criteria. The discussion begins with some background knowledge of the blockchain technology and four commonly used content protection techniques. Then, a detailed review of blockchain-based copyright protection applications is presented. Moreover, these schemes are compared w.r.t. the defined taxonomy. In addition, some significant research challenges of content protection schemes and the blockchain technology are discussed. Finally, several future research directions are outlined.

For researchers, blockchain has an excellent potential to be broadly applied in copyright protection and management applications. The blockchain-based copyright protection applications allow copyright owners and consumers to interact without costly intermediaries. These applications allow the content owners to upload copyrighted content, control licensing/copyright options, manage distribution, trace sources of piracy, and receive payments upon content usage. However, there are still many open issues that need to be further researched and analyzed in order to create workable copyright protection applications that can fully benefit from the use of the blockchain technology. The success of such applications is however dependent on different factors related to the blockchain technology, such as scalability, reliability or market adoption, that are difficult to foresee. Researchers must consider all these aspects while designing and implementing a new blockchain-based content protection scheme. We hope that this survey will be considered a primary reference that can facilitate the process of finding the most relevant information w.r.t. integration between the content protection mechanisms and the blockchain technology.

**Author Contributions:** A.Q. and D.M.J. commonly finished the manuscript. Both authors have read and approved the final manuscript. Original draft preparation: both authors; Writing—review & editing: both authors. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors acknowledge the financial support received from the Spanish Government through grant RTI2018-095094-B-C22 "CONSENT".

**Acknowledgments:** The authors thank Alice Keefer Riva for proofreading the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

