# Blockchain-Based Multimedia Content Protection: Review and Open Challenges
## 3 State-of-The-Art of Blockchain-Based Content Protection Systems
The cryptographic hash can be used to retrieve the information of multimedia content (e.g., ownership and modification history) that is stored on the multimedia blockchain, and the image hash can be used to identify the tampered regions. The CS samples can be utilized for reconstructing the original image and locating the tampered regions. In the blockchain, a transaction is composed of the transaction information of the image containing transaction ID and the information of CS samples. Upon approval of the transaction by the validating nodes, the image is distributed and is then stored on a media database server. Though storing image verification information on the blockchain is a good strategy, the image is still stored in centralized manner or kept by the owner, which affects the availability of image management. In Reference [45], an automated penalization of breach (APB) contract is proposed that consists of four main components: a claim-or-refund smart contract, a robust watermarking scheme, an oblivious-transfer scheme and a non-interactive zero knowledge (NIZK) proof for mutually distrusting parties. In this scheme, the sender and the receiver create a claim-or-refund transaction on Bitcoin, where an amount is deposited that can be spent at any time with a jointly signed transaction or spent after a period of time by a sender-only signed transaction. At the receiver's end, the received document consists of the receiver's secret key, which is embedded into it with a robust binary watermarking scheme. A two-party computation protocol is jointly performed by the parties to embed and ensure that the receiver's embedded key is retrievable for the sender in case of a content leakage. Reference [46] proposes a blockchain-based data hiding method for digital video protection, which improves the integrity authentication of confidential data and videos. The proposed method consists of the following three parts: (1) on-chain data protection method that focuses on the integrity check and the security of the video by registering the signature of the video content on the blockchain; (2) off-chain data protection based on a data hiding algorithm that can achieve a good balance between visual distortion, embedding capacity and robustness; and (3) data protection management agreement based on a smart contract that consists of registration, inquiry and transfer contract models. However, in the proposed scheme, the users need to request data extraction from data hiding servers so as to enable multimedia playback. In [47], the blockchain technology is used to store the watermark securely and to provide timestamp authentication for multiple watermarks. The proposed system uses the perceptual hash function for calculating a hash value of an image, the blockchain technology for recording metadata related to the copyright information, the QR code for generating a watermark, the digital watermarking algorithm for embedding the copyright information, the cryptographic hash function for calculating the hash values of both original and watermarked images, and the IPFS network for saving, managing and distributing the watermarked image and its related copyright information. The proposed scheme, however, provides the proof-of-concept design for copyright management of digital images only. Moreover, it can be observed that the perceptual hash values of modified/edited images (such as rotated or cropped) considerably differ from those of the original and the original image hash values recorded on the blockchain. In [48], [PERSON] proposes BDRM, a blockchain-based DRM system with the property of a fine-grained usage control. BDRM utilizes a smart contract to achieve copyright management related operations, such as copyright registration and copyright transactions. Moreover, a novel authorization tree is designed in the blockchain. Each time a user conducts a rights transaction, a usable digital watermark is embedded, and digital content distribution is performed under the encryption domain.