# Blockchain-Based Multimedia Content Protection: Review and Open Challenges
## 2 Taxonomy of Blockchain-Based Multimedia Content Protection Systems
### Cryptocurrency  
A cryptocurrency is a digital currency that uses cryptography to secure and verify transactions as well as to control the creation of new units of a particular cryptocurrency. The most attractive characteristic of cryptocurrency is its organic nature as it is not issued by any centralized authority (e.g., bank or financial intermediary). The main benefit of using cryptocurrency is that the funds are transferred more easily between any two parties in the transaction. All these transactions are facilitated through the use of public and private keys for security purposes, and are carried out with minimal processing costs. Bitcoin and Ethereum have been the two most popular cryptocurrencies since the emergence of the cryptocurrency phenomenon. Recently, Ripple has emerged as the third largest cryptocurrency in the trading market [19]. These three cryptocurrencies have a high market cap and a liquidity rate [20], and are the most traded cryptocurrencies on the mainstream trading platforms such as Plus500 [21]. A brief overview of top three cryptocurrencies is presented below:  
* Bitcoin is the first P2P payment network of electronic cash based on the blockchain technology. The Bitcoin network adopts a hash-based proof-of-work (PoW) distributed consensus protocol. Bitcoin is pseudonymous, i.e., the funds are delivered to the Bitcoin addresses instead of the real-world identities. The average block creation time is 10 min, and the block size is limited to 1 MB [22], which constrains the network throughput. Moreover, the scalability is limited to 3-7 transactions/second [23]. Moreover, it is vulnerable to hacking attacks and theft, while being completely encrypted.
* Ethereum is an open-source, public, blockchain-based distributed computing platform that uses a proof-of-stake (PoS) consensus mechanism and allows programming of various types of smart contracts within the system. The transferable amount in Ethereum is ether. Any action in Ethereum requires gas, which is used as a fee instead of ether for ease of computations. The average block creation time is 17 s [24], and blocks are limited by a 6.7 million gas [25]. A simple Ethereum transaction can cost around 21,000 units of gas. However, a complicated smart contract can cost a lot more. The scalability of Ethereum is limited to 15-20 transactions/second [26]. Moreover, it is susceptible to security violations due to Solidity Language, causing compromise of stored data.
* Ripple is a for-profit technology platform that allows banks, payment providers, and digital asset exchanges to provide faster payment settlements and offers lower currency exchange costs. XRP is the cryptocurrency used by the Ripple payment network to make cross-border payments. Unlike blockchain mining, the Ripple network uses a unique distributed consensus protocol to validate transactions. This enables faster transactions without any centralized authority. The transactions in the Ripple network are confirmed instantly by the XRP gateways and take roughly 4 s. The Ripple network can process 1500 transactions/second [27]. Since Ripple is pre-mined, there exist little or no incentive for common nodes to work in the network, which subsequently leaves the corporations to provide the validator nodes.