# Blockchain-Based Multimedia Content Protection: Review and Open Challenges
## 3 State-of-The-Art of Blockchain-Based Content Protection Systems
### Comparative Analysis
the attributes defined in the taxonomy:  
* Most of the selected schemes [36; 37; 39; 42; 45; 46; 49; 51] use a permissioned (private) blockchain, which implies a control layer on top of the blockchain that is governed by a trusted authority, who is responsible for permitting actions to be performed by the allowed system entities. Several other schemes [41; 43; 48] use consortium blockchain as a distributed ledger to retain control and privacy, while cutting down costs and increasing transaction speeds. * The majority of the schemes [36; 37; 38; 40; 42; 46; 47; 48; 49; 51] utilize hybrid transactions, which imply on-chain recording of data in a private or publicly accessible blockchain service, such as the content's metadata, copyright owners' or users' information, watermark or fingerprint of the user (in an encrypted form), DRM license and content obligation, among others, and off-chain mechanisms, such as generation and storage of copyrighted content, extraction of copyright information from the content or traceability protocol. The on-chain transactions are performed to achieve transparency, security, immutability and auditability, and are considered best for cryptocurrency transfers in a completely decentralized manner. The off-chain mechanisms allow the authorities to save on-chain costs as these do not have a transaction fee. In addition, these mechanisms are quite fast since these are not bound by the transactional speed limitations linked to on-chain transactions. * In many schemes [36; 37; 40; 41; 43; 49; 50], multiple smart contracts are bundled together to create a blockchain-enabled dApp to automate the desired functionality between the copyright owner and the buyers. These contracts include a series of functions, such as data processing, value transfer, copyright protection, content distribution, and traceability. In other systems [44; 45; 46; 48; 51; 53], a single smart contract is proposed to facilitate the purchase of the copyrighted content between the copyright owner and the clients (buyers). * In the majority of the schemes [36; 37; 40; 42; 44; 46; 48; 49; 51], Ethereum, which is the second largest cryptocurrency platform by market capitalization behind Bitcoin, is used as a cryptocurrency for transactions. The Ethereum crypto token (Ether) transactions tend to be confirmed faster by the blockchain, and are much cheaper than most transactions on Bitcoin. Moreover, Ethereum not only allows Ether transfers between people, but can be used to create all types of other cryptocurrencies, such as in [41]. * Many schemes [38; 39; 40; 41; 42] use the most prominent consensus protocol, i.e., PoW, to confirm transactions and produce new blocks in the chain. PoW is the dominant choice in existing digital currencies due to its security features, which use cryptography that deters denial-of-service (DoS) and sybil attacks. * The majority of the schemes guarantee content protection by using digital watermarking either as a standalone mechanism [44; 45; 46; 47] or in combination with DRM [40; 42; 48; 49] or encryption [37]. Only three schemes [50; 51; 52; 53] used digital fingerprinting to provide copyright protection, while only two schemes, proposed in [39; 41], used DRM techniques to ensure conditional access. * From Table 2, we can make the following observations w.r.t. to the performance of the systems:
* Since the majority of the schemes have utilized either Bitcoin or Ethereum for development or testing purposes, the average transaction throughput (3-7 tps for Bitcoin and 15-20 tps for Ethereum) and latency rates (10 min for Bitcoin and 17 s for Ethereum) have been considered. However, only a few schemes [37; 41; 42; 44; 46] consider the transaction fees per block per day. * Only a few schemes [41; 42; 45] address scalability by using the Schnorr's multi-signature technology, which also improves the privacy of the system and prevents spam attacks to a higher degree. In [49], a quite small number of miners
are pseudo-randomly selected to execute off-chain computationally intensive smart contracts.