## 2 Materials and Methods
### On-Chain and Off-Chain Data Management  
Since the initial deployment of blockchain infrastructure beyond cryptocurrencies, this technology emerged as a novel solution for data storage, characterised by its decentralised, immutable, and secure nature [41]. At its core, blockchain operates as a distributed ledger, where data are stored across a network of computers rather than in a single centralised location. Decentralisation enhances data security, as it reduces the risk of tampering or unauthorised access, and immutability is particularly valuable in applications where data integrity is required, such as supply chain management. However, there are relevant drawbacks to using blockchain for data storage. One major concern is scalability, because as the amount of data grows, the blockchain network can become bloated because of ledger replication, leading to slower transaction times and increased costs for storage and processing [42] (cf. Section 4.1). Moreover, the energy consumption associated with maintaining a blockchain network, particularly those using proof-of-work consensus mechanisms, raise environmental concerns. Additionally, while blockchain provides transparency, it can also lead to privacy issues, as sensitive information may be exposed on a public ledger permanently. Being a public and transparent digital information ledger, all data deposited on a blockchain are visible to all network participants and cannot be removed from the chain. This may raise privacy and confidentiality issues when data are stored in the clear, as it would be possible to exploit such data in the future using advanced data-analysis techniques. While individual data items may not be vulnerable by themselves, advanced data-analysis techniques may reveal patterns and correlations among different sets of seemingly irrelevant data [43]. These patterns could reveal sensitive or strategic information about organisations or other entities, or even individuals. In addition, processed data can be used for social engineering attacks, where attackers use the collected information to manipulate or deceive individuals or entities in order to gain unauthorised access to information or systems [44]. Finally, advanced data analysis of information stored on the blockchain may reveal hidden biases in the data, which could lead to discriminatory or unfair decisions in areas such as tendering or financial procedures (e.g., evidence of the long-term story of a company's partnerships may affect its future social or perception). To address data inflation in blockchain networks, alternate models were proposed to manage the multiple copies of the blockchain ledger, for example by keeping in the blockchain just the information to guarantee its properties (e.g., block hashes, nonces, timestamps and Merkle trees) and storing the transactions' payloads off-chain [45]. Managing only essential information on-chain while keeping transaction payloads off-chain offers several advantages. First and foremost, it significantly reduces the above-mentioned data bloated by limiting on-chain data to critical components and thus minimising blockchain size. This reduction helps maintain faster transaction speeds and lowers the storage requirements for nodes, making the network more efficient. Another key benefit is improved scalability because with off-chain storage the blockchain can accommodate a higher volume of transactions without becoming congested. This is particularly relevant for applications that require high throughput, as it allows the system to handle increased activity more effectively. Additionally, this model can lead to cost savings, as storing large amounts of data on-chain can incur substantial transaction fees. Blockchain platforms that incorporate a virtual machine such as those based on Ethereum offer a powerful means to balance on-chain and off-chain data storage by utilising the smart contracts introduced in Section 2.2.