**2. Taxonomy of Blockchain-Based Multimedia Content Protection Systems**

This section presents the proposed taxonomy that allows the decomposition and comparison of blockchain-based multimedia content applications in the literature in a systematic manner. This taxonomy identifies the common dimensions and requirements of blockchain-based copyright protection systems. The proposed taxonomy defines seven categories that are further divided into sub-categories. A comprehensive and in-depth classification of the defined categories is graphically represented in Figure 1.

**Figure 1.** Taxonomy of Blockchain-based Copyright Protection Systems.

## *2.1. Types of Blockchain*

The three possible types of blockchain are listed below:


#### *2.2. Types of Transaction*

A transaction implies a state transition that changes data in the blockchain from one value to another. A blockchain transaction can involve cryptocurrency, smart contract, record, or data storage. The three different types of transactions on the blockchain are listed below:

• On-chain transactions: These are available on the distributed ledger and are visible to all participants on the network. Different details of this transaction are recorded on the block and distributed to the entire blockchain, thus making the transaction irreversible as it cannot be altered. For the transaction to be complete, there has to be an agreed number of confirmations by miners. Moreover, the completion depends on network congestion. Consequently, sometimes transactions are delayed in case of a large volume of transactions waiting to be confirmed.


#### *2.3. Data Automation*

A self-automated code, also known as a smart contract, consists of a program code, a storage file and an account balance. It is executed by miners that use consensus protocols to agree upon the sequence of actions resulting from the contract's code. Any user can create a contract by posting a transaction to the blockchain. The program code of a contract is fixed when the contract is created and cannot be changed, thus providing immutability. A contract's storage file is stored on the public blockchain. A smart contract can be invoked by entities within (other smart contracts) and outside (external data sources) the blockchain. While executing its code, the contract may read from or write to its storage file. Moreover, it can receive money into its account balance, and send money to other contracts or users. A smart contract is identified by a hash of 160 bits (a hexadecimal address used by many cryptocurrencies, e.g., Ethereum or LiteCoin, among others), and is operated within the environment that supports the use of a public-key cryptography. Smart contracts are widely used in most of the currently existing cryptocurrency networks and are the prominent features of Ethereum. A smart contract can be used to perform one type of transaction, while a distributed application (dApp) can bundle a set of smart contracts together to perform complex transactions. This dApp is a user-friendly interface (similar to a traditional website) that allows users to interact with the smart contracts stored on the blockchain.

Smart contracts could offer a number of benefits, such as accuracy (less prone to manual error), lower execution risk (automatic execution by the network), fewer intermediaries (reduced reliance on third-party intermediaries), lower cost (less human intervention and fewer intermediaries), speed and real-time updates (automated tasks), and new business models (from DRM and watermarking/fingerprinting of multimedia content to automated access to storage units).

#### *2.4. 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:

