# Blockchain-Based Multimedia Content Protection: Review and Open Challenges
## 3 State-of-The-Art of Blockchain-Based Content Protection Systems
The authorization tree is then updated and the transaction is recorded on the blockchain. The content is encrypted with the secret key of the content owner and is stored in the distributed file system (IPFS). However, BDRM is only applicable to copyright registration of a single content owner. Reference [49] presents Y-DWMS, a digital watermark management system, based on a public smart contract platform to prevent digital rights infringement. The proposed system adopts non-repudiation of smart contracts and non-tampering of blockchain to implement a DRM mechanism that prevents users from sharing encryption keys or their accounts. The smart contract is designed to perform verification of watermarks in the disclosed copy, authentication of the informer's report, traceability of infringement, an act of rewarding informers and punishing infringers, and recovery of losses suffered by the copyright holders. However, Y-DWMS is still in an early stage of development and suffers from some security issues, such as account security and privacy. [PERSON] et al. [50] proposed a blockchain and smart contract-based data trading system with data tracking and illegal behavior detecting functions. It enables two trading scenarios with privacy protection against any unauthorized party, including the trading platform. An effective fingerprint method is designed to detect the manipulated image, thus protecting data copyright. A data fingerprint generator is designed to generate a fingerprint by concatenating multiple feature vectors extracted from the data. Upon finding an illegally distributed copy, the data fingerprint generator extracts an identifiable vector, which is then compared with the fingerprints recorded in all existing contracts. The generated fingerprint is resistant to minor data modifications, such as cropping, adding noise and changing brightness. However, the system does not satisfy the privacy and security properties of an anonymous fingerprinting protocol in a decentralized environment. In Reference [51], the authors propose a P2P content distribution system based on the blockchain technology. The proposed system uses collusion-resistant fingerprinting (to provide collusion resistance), homomorphic and symmetric encryption schemes (to ensure content protection and data confidentiality), a perceptual hash function (to provide content authentication), an Ethereum-based smart contract (to execute atomic payment and provide proof-of-delivery) and the IPFS network (to store multimedia content). While the privacy and security properties of an anonymous fingerprinting protocol in a distributed environment are addressed by the proposed system, it is a proof-of-concept that has not been implemented and evaluated in the real world. In Reference [52], [PERSON] proposes a blockchain-based novel fingerprint-related chaotic image encryption scheme that provides authentication, traceability, and resistance against security attacks (e.g., chosen plaintext attack or tampering). In this scheme, the content distributors' fingerprints embedded in the encrypted images are encoded with Tardos's collusion-resistant codes to record multiple fingerprints with fixed length of data and to provide traceability. Before content distribution, the original image is embedded with the signature of the sender and the fingerprints of all system distributors using a reversible watermarking scheme and a chaotic map. This fingerprinted image is then encrypted using Fridrich's structure, which consists of substitution, permutation and diffusion. The fingerprint, the data hiding key, and the encryption key are recorded on the blockchain. At the receiver's end, upon decryption, the fingerprinted image is obtained, and it contains the signature of the sender and all the fingerprints of the superior distributors (merged fingerprint), which can be extracted individually, and then compared with the recorded information on the blockchain for verification.