# Blockchain Applications in Education: A Systematic Literature Review
## 1 Introduction  
Blockchain is the technology behind Bitcoin, the cryptocurrency initially described by [PERSON] [1]. A blockchain is a time-ordered set of blocks; that is, each block is cryptographically linked to the previous one forming a chain (chain of blocks). All blocks are stored in a decentralized and distributed ledger and become trustworthy digital records that are that are unmodifiable in practice, but easy to verify. There is no centralized or hierarchical structure in the blockchain network, and the information is shared by a network of peers. Each block contains a reliable register of one or more actually executed transactions that are created and exchanged by the participants (peers) of the blockchain network, which eventually modify its state. To add new information to the chain, a consensus about its truthfulness has to be reached among the peers in the network. The content of the transactions depends on the specific blockchain and its purpose. For example, in Bitcoin, the main information registered are exchanges of bitcoins between accounts. Some blockchains support the use of smart contracts [2], which are similar to computer programs but are deployed on the blockchain and executed autonomously by all the active participants in the network. These smart contracts can read and write information from the blockchain and other sources using oracles (oracles are hardware or software devices that feed the smart contract with external information and/or send data to the outside world) [3], process information, store transaction outcomes, and even trigger actions in systems off-chain. All of the operations are tamper-proof and are recorded in the blockchain to add confidence. There are three main blockchain types:
* Public: these are open to anyone who wants to be part of the peer-to-peer network (i.e., they are permissionless). Once connected, peers can download and access all of the information, generate and validate transactions and execute smart contracts. To reach a consensus about new transactions, a task must be executed by peers (typically, up to millions of participants). Public blockchains achieve maximum immutability, decentralization and transparency, but they are very inefficient, since a lot of processing power, storage and electricity are required to reach a consensus. The transaction volume and speed are very low. Bitcoin and Ethereum [4] are examples of this type. * Private: participants may only join if they are invited (i.e., permissioned). There are rules set by the organization that controls the network. In these highly centralized blockchains, immutability and transparency of the chain are limited. The number of peers is typically low, and the network tends to be much smaller and specialized. Therefore, the system is comparatively more efficient, and higher transaction volumes and speeds can be achieved, and, consequently, lower costs and resource usage are incurred. Hyperledger [5] is an example of this type. * Consortium: is a combination of the two previous types. Like in a private blockchain, participants may only join if they are invited, however, there is not a single organization that controls the network but instead a group of them. From a governance perspective, they maintain the decentralized nature of a public blockchain, although they are more controlled and regulated. Consequently, transaction volume, speed and resources usage are also better. There are four stages of blockchain maturity [6]:
* Blockchain 1.0 was focused on transactions and it was mainly used on the development of crypto currencies in cash-related applications. * Blockchain 2.0 adds privacy, smart contracts, and non-native tokens, among other features. * Blockchain 3.0 incorporates decentralized applications (dApps), back-end code that is executed on a decentralized peer-to-peer network, expanding the uses of blockchain to different markets as health, supply chain, government, education, etc.