**1. Introduction**

Content distribution is a process of digital distribution or delivery of multimedia content such as audio, text, animation and video. Traditionally, multimedia content was distributed through physical exchange of papers, compact discs, or DVDs. With the technological evolution and growth of the Internet, multimedia content in the form of digital formats can be published online through digital distribution channels, such as the Internet-based delivery platforms [1] or peer-to-peer (P2P) file distribution and sharing systems [2], among others. These online distribution platforms have become the de facto standards for content delivery ensuring great performance, wide availability, and cost efficiency. According to the 2019 Global Internet Phenomena Report [3], media streaming applications, and on-demand video [1,4] and audio [5] delivery platforms, constitute a large portion of Internet traffic. However, with the widespread use of these delivery platforms, the safety of the multimedia content, the preservation of copyright, the traceability of copyright violators, and the secure distribution of content have become increasingly ubiquitous problems for content owners, multimedia producers and distributors.

To prevent data from being illegally downloaded and shared, and to track and punish copyright violators, it is important that the multimedia content owners can prove their copyrights over the contents upon copyright infringement. Many traditional content protection technologies, such as encryption [6], Digital Rights Management (DRM) [7], watermarking [8,9], and forensic watermarking (digital fingerprinting) [10–14] have been designed to protect data copyright or content ownership. Though a few of the distribution systems [6–14] address the problems of copyright protection, traceability, and secure content distribution, there are several open issues to be addressed: (1) there does not exist an effective proof-of-delivery mechanism; (2) a deposit is required to place an order for the content before its delivery, which involves a certain risk that the customer may receive

**Citation:** Qureshi, A.; Megías, D. Blockchain-Based Multimedia Content Protection: Review and Open Challenges. *Appl. Sci.* **2021**, *11*, 1. https:// dx.doi.org/10.3390/app11010001

Received: 28 November 2020 Accepted: 17 December 2020 Published: 22 December 2020

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tampered data; (3) the specific information of copyright and transaction is not public to the clients; (4) clients' identities need to be verified through multiple interactions; and (5) often these systems are dependent on centralized trusted third parties for payment, management of the licenses and keys, and generation and verification of the watermark or fingerprint. These trusted third parties are vulnerable to single point failures, compromise and hacking attacks.

The blockchain technology, which is widely known as a mechanism to provide transaction verification, can be used to design a decentralized and transparent multimedia distribution system. The blockchain [15] is a distributed digital ledger of cryptographically signed transactions that are grouped into blocks. Each block is cryptographically linked to the previous one after validation and undergoing a consensus decision. As new blocks are added, older blocks become more difficult to modify (i.e., creating resistance against tampering).

In recent years, the blockchain technology has become a source of new hope with its broad spectrum of applications, e.g., finance, health care, supply-chain management or intrusion detection, to name a few. Recently, its footprint can be observed in intellectual property or copyright protection applications. The main attributes of the blockchain technology—i.e., transparency, decentralization, reliable database, collective maintenance, trackability, security and credibility, digital cryptocurrency, and programmable contracts provide innovative ideas for protecting digital intellectual property and ensuring traceability. In recent years, a rapid development of decentralized applications based on blockchain technology has been observed, but the combination of content protection and blockchain technologies has not received much attention from researchers. Apart from a few commercially available blockchain-based copyright protection platforms [16,17], one can find only a handful of blockchain-based copyright protection schemes in the literature. Recently, the authors in [18] investigated the use of blockchain technology in diverse online multimedia applications, such as music and advertising industries, healthcare, social media, and content delivery networks. Though the descriptive study analyzes the characteristics (target market, underlying platform technologies, consensus protocols and reward system) of the existing blockchain-based online media platforms, the research direction is not focused on addressing the problems related to integration of copyright protection mechanisms with the blockchain technology. Through this research work, we attempt to investigate how copyright infringement-related problems can be resolved using the blockchain technology.

The key contribution of this paper is to provide a holistic survey of multimedia content protection applications that use the blockchain technology. A taxonomy is developed to classify these applications based on technical blockchain characteristics, content protection mechanisms, and performance criteria. To the best of our knowledge, no systematic taxonomy has been defined in the literature to characterize the state-of-the-art of blockchainbased copyright protection schemes. The proposed taxonomy integrates technical aspects and application knowledge to address the challenges with feasible solutions and identify the possible research gaps in blockchain-based copyright protection applications. We believe that the current moment is opportune to present this survey due to recent emergence of copyright protection systems based on blockchain.

The rest of the paper is organized as follows. Section 2 presents the taxonomy of blockchain-based multimedia content protection applications. Section 3 surveys recent work on copyright protection schemes based on the blockchain technology. Additionally, we compared the schemes w.r.t. the attributes defined in the taxonomy. Section 4 provides an overall discussion and reveals a number of insights into the field of research. In addition, the future research directions are outlined. Finally, we present the conclusions of this research work in Section 5.
