# Blockchain-Based Multimedia Content Protection: Review and Open Challenges
## 3 State-of-The-Art of Blockchain-Based Content Protection Systems
The deployed smart contract is responsible for sharing the copyright parameters of the music owners and automatic royalty payments distribution to the wallet addresses of the different copyright owners. However, BMCProtector provides the proof-of-concept design for a copyright management of audio files only. Moreover, it cannot redrovide copyright redrotection of music files in other formats, e.g., an audio file recorded during playing and then uploaded illegally. In Reference [41], the Blockchain as a Service (BaaS) model is proposed for building a DRM platform that provides high-level credit and security to the content provider, the service provider, and the customers. The DRM platform provides core content rights information storage in the blockchain for tamper-resistant protection to prevent copyright from being violated or misused. The content consumers can use blockchain-based digital currency for content consumption payment. A cryptocurrency digital rights coin based on multi-signatures is proposed as a payment mechanism on the platform. Dynamic key agreement and session data encryption are used to ensure secure communications and data transfer. This scheme uses many modulo operations that significantly limit the cost-effectiveness of the generating a temporary shared key. Moreover, the scheme is based on the alliance chain, and thus has a centralized authority that prevents direct transactions between the content owner and the customer. [PERSON] et al. [42] proposed a Ethereum-based scheme, DRMChain, which ensures the correct usage of digital content by the authenticated users, and provides flexible external storage of decentralized digital content using IPFS. DRMchain employs two isolated blockchain application interfaces (BAI): BAI plain interface that stores the original content with its cipher summary, and BAI cipher interface, which stores the DRM-protected content service, such as content watermark, encryption, license and redivolation tracing, among others. DRMChain provides efficient and secure authentication, privacy protection, multi signature-based conditional traceability, and trusted and high-level credible content protection. However, DRMChain does not prevent the offline spread of the divulged copies. Moreover, the system lacks the diversified copyright management functions, such as copyright transaction. In addition, it lacks an effective punishment and reward mechanism. Reference [43] proposed a blockchain-based DRM scheme for copyright protection of design works. The proposed system is categorized into two methods: copyright protection and trading. The copyright protection method performs copyright registration, information query and correlation verification, while the trading process encompasses design content protection and a proof-of-delivery method to guarantee fair trade. The enrolled buyer can purchase registered works from the content providers (sellers) through smart contracts. During the content delivery, the content is encrypted with the buyer's public key, and it is then delivered to the buyer through the application. Before receiving the content, the buyer needs to input his/her secret key to the application first, that performs decryption and makes it available to the buyer. However, the proposed scheme does not guarantee the security of the user's secret key submitted to the blockchain application for signing for the delivered content and the content decryption. In Reference [44], a watermarking-based tamper-proof multimedia blockchain framework is proposed that provides security and integrity to the distributed image. The proposed blockchain model is based on a compressed sensing (CS)-based self-embedding watermarking algorithm in which the unique watermark information consists of a cryptographic hash and an image hash. The cryptographic hash comprises of transaction histories for retrieving the metadata of multimedia content from the multimedia blockchain, while the image hash is used for preserving retrievable original multimedia content.