# Blockchain-Based Multimedia Content Protection: Review and Open Challenges
## 3 State-of-The-Art of Blockchain-Based Content Protection Systems  
Recently, both industry and academia have started to consider preservation of intellectual property rights using blockchain technologies. In the existing research, blockchain is considered to be a transparent and reliable ledger, which is used to solve the problems of copyright protection faced by the content owners and producers, e.g., the rights attribution certificate, data integrity, authenticity, piracy tracing and transparency, among others. In the remainder of this section, a brief overview of existing blockchain-based content protection systems is provided w.r.t. their main attributes and implementation details. In Reference [36], the authors propose a blockchain-based framework that guarantees copyright compliance of multimedia objects by means of smart contracts. The proposed system uses an off-chain centralized storage solution, data lake, to store the transaction details of all the data added on the blockchain. The information stored on the data lake is encrypted and digitally signed to ensure the privacy and authenticity of the information. The stored data can only be accessed by the authorized users after verification of their digital signatures and access rights by the consent of the majority nodes. Although the decentralized data management framework ensures user data privacy and control, it is a proof-of-concept that has not been implemented and evaluated in the real world. [PERSON] et al. [37] propose an Ethereum-based digital copyright management system that enables content owners and customers to deal directly without the need of a centralized authority. In the proposed system, digital watermarking, the ElGamal cryptosystem, a perceptual hash function, the smart contract, and IPFS are used. However, the scheme incurs high overhead (memory and CPU time) due to use of ElGamal encryption for encrypting the whole multimedia content. [PERSON] et al. [38] introduce a secure and reliable blockchain-based real time eBook market system that allows users to publish themselves and receive direct payments from readers without any trusted party involvement. The proposed trading platform uses blockchain for protecting copyright of paid content and securely managing direct payments. It provides eBook ownership verification, data protection and confidentiality, permission to read the purchased ebook, authentication of a legitimate purchaser, non-forgeability and verifiability of both eBook contents and direct payment transactions, and prevention of eBook piracy and illegal distribution. The published encrypted (using Elliptic Curve Cryptography) eBook contents along with the book key are stored in a book repository. [PERSON] et al. [39] proposed a high-definition video copyright management system based on a decentralized blockchain to assist the content creators' demand for an efficient way to manage DRM. In the proposed scheme, the right holders themselves can control the system, which is based on the PoW mechanism. In this technique, the headers of ultra-high resolution video content (i.e., 4K or 8K) are encrypted and decrypted to balance the cryptographic costs associated with encryption/decryption operations. However, the system does not have an incentive mechanism for mining computation power. In addition, it does not provide cross-platform rendering and access policy control of the media file. [PERSON] and [PERSON] [40] proposed BMCProtector, a prototype implementation based on an Ethereum blockchain and smart contract technologies, for effective protection of music copyright and rights of copyright owners. BMCProtector uses the AES algorithm to encrypt the audio file, vector quantization (a watermarking technique) to track ownership of the music file off-chain, and an off-chain access control mechanism (DRM) to control the copyright of music during its distribution and usage.