Blockchain

• a set of validated transactions,
• its own cryptographic fingerprint (hash),
• and the hash of the previous block, forming an immutable chain.

This design ensures that modifying a past piece of data would require altering all subsequent blocks and convincing most of the nodes, which is computationally unfeasible without controlling a large part of the network.

The operation of a blockchain is based on several key components:

• Consensus algorithms: mechanisms that allow nodes to agree on which blocks are valid (e.g. Proof of Work, Proof of Stake, BFT).
• Asymmetric cryptography: ensures that only the holder of a private key can sign transactions.
• Peer-to-peer network: each node keeps a copy of the history and propagates new transactions and blocks.
• Immutability and traceability: everything written on the blockchain is permanently accessible and cannot be erased or falsified.

There are different types of blockchain:

• Public (such as Bitcoin, Ethereum): open to all, resistant to censorship.
• Private or permissioned: controlled by specific entities (used in companies). In Spain, ISBE is a public permissioned infrastructure: anyone can query it, but only identified organizations can write to it.
• Consortiums: managed by a group of organizations that share access and validation.

In addition to registering cryptocurrency, blockchain makes it possible to execute smart contracts, create tokens, develop decentralized applications (dApps), implement traceability systems, manage digital identities or tokenize physical assets.

Blockchain is transforming sectors such as:

• Finance (DeFi, stablecoins)
• Arts and entertainment (NFTs)
• Logistics and traceability
• Real estate and tokenization
• Electronic voting
• Health and document management