## 3 Blockchain  
Blockchain is a type of database that is managed by a network of computers, rather than a central authority, making it virtually impossible to alter or tamper with the records. The blockchain ledger consists of blocks containing a set of transactions. Once a new transaction is made, it is verified by a network of computers (nodes) that use complex algorithms to confirm its validity. Once the transaction is verified, it is added to the latest block, and the block is added to the existing chain of blocks, forming a permanent and immutable record. Blockchain security is maintained through cryptography and each transaction is secured by using a hash function. Figure 1 is part of the official development documentation of BitCoin, freely available for download on GitHub.com, and illustrates the basic implementation of transactions using witnesses in a blockchain context. Figure 1: Transaction and witness hashing [8]. One of the key uses of blockchain is smart contracts. Smart contracts are self-executing computer programs that automatically enforce the rules and conditions of a contract. They make use of blockchain technology, because of its decentralization, immutability, and transparency in recording transactions. Once a smart contract is deployed on a blockchain network, it cannot be altered or tampered with, ensuring that all parties involved in the contract have access to the same information and that the terms of the contract are enforceable. Smart contracts can also be programmed to trigger automatic actions when certain conditions are met, such as releasing funds to a supplier once a shipment has been delivered. In the context of the metaverse, smart contracts can be used to automate and enforce the rules of various transactions that take place within the virtual world. For example, smart contracts can be used to facilitate the buying and selling of virtual assets, such as in-game items or virtual real estate. They can also be used to create decentralized autonomous organizations (DAOs) within the metaverse, where members can vote on important decisions using a transparent and secure system. Smart contracts can also be used to create unique non-fungible tokens (NFTs) that represent virtual assets within the metaverse. These NFTs can be bought, sold, and traded just like physical assets, and their ownership and transaction history are recorded on the blockchain. This creates a new economy within the metaverse, where users can create, own, and trade valuable virtual assets. Presently, there are three well-known smart contract platforms employing JVM languages. These libraries are commonly used alongside other types of programming languages when implementing smart contracts:  
* Corda is an open-source blockchain platform that supports the use of Java and Kotlin for developing smart contracts. Corda smart contracts are called \"CorDapps\" and can be written in Java or Kotlin using the Corda framework. CorDapps are designed to be highly modular, allowing developers to easily plug in their own custom code. * Ethereum is a popular blockchain platform that supports the use of Solidity, a programming language that is similar to JavaScript. However, it is also possible to use other languages, such as Java, with tools such as Ethereum) or the Web3 library. * Hyperledger Fabric is a permissioned blockchain platform that supports the use of multiple programming languages, including Java, JavaScript, and Go. Smart contracts on Hyperledger Fabric are called \"chaincode\" and can be written in any of these supported languages. These smart contract platforms, along with four others, were comparatively analyzed by [PERSON] and [PERSON], in an article published in 2021, and while the authors did not come to a clear conclusion as to which platform is the most fully featured overall, they did highlight each platform's unique features and capabilities. The analysis was not limited to only the programming languages supported, but also covered consensus mechanisms, scalability, interoperability, privacy, and security [9].