Learning Objectives – after this chapter you will be able to:
- Define blockchain and distributed ledger technology (DLT).
- Describe what a block contains and how blocks are chained.
- Explain the core properties: decentralization, immutability, transparency and consensus.
- Identify where blockchain is useful – and where a normal database is the better choice.
Blockchain
A blockchain is a digital ledger that is shared and replicated across the computers (nodes) of a peer-to-peer network. Records are grouped into blocks, each block is linked to the one before it by a cryptographic hash, and new blocks are added only when the network agrees through a consensus mechanism.
- Because every participant keeps a copy, there is no central owner who can quietly change the records.
- Blockchains became famous as the technology behind cryptocurrencies such as Bitcoin, where they keep a secure, decentralized record of who owns what. Their use, however, is not limited to cryptocurrency.
- Any industry that needs a shared, immutable record – one that cannot be changed after it is written – can use a blockchain.
- Since a confirmed block cannot practically be changed, trust is required mainly at the moment data is entered. This reduces the need for trusted third parties such as clearing houses or auditors, who add cost and can make mistakes.
- A blockchain is one type of Distributed Ledger Technology (DLT). Every blockchain is a DLT, but not every DLT stores data as a chain of blocks.
What is Inside a Block?
| Part | What it stores |
|---|---|
| Block header | Metadata that identifies and links the block. |
| • Previous block hash | The hash of the parent block – the “link” of the chain. |
| • Timestamp | Approximate time the block was created. |
| • Merkle root | One hash that summarises every transaction in the block (see Chapter 21). |
| • Nonce / difficulty | In Proof-of-Work chains, the values miners use to meet the difficulty target (see Chapter 15). |
| Block body | The list of transactions or records included in the block. |
How a Blockchain Works – Step by Step
- Create: a user creates a transaction and signs it with a private key.
- Broadcast: the transaction is sent to the peer-to-peer network and waits in a pool of pending transactions.
- Validate: nodes check the signature, the balance and the protocol rules.
- Build a block: a miner (PoW) or validator (PoS) packages valid transactions into a new block that references the latest block’s hash.
- Agree: the network accepts the block through the consensus mechanism.
- Append: every node adds the block to its own copy of the chain. As more blocks are added on top, the transaction becomes practically irreversible.
Key Properties of a Blockchain
| Property | Meaning |
|---|---|
| Decentralization | Control is spread across many independent nodes; there is no single point of control or failure. |
| Immutability | Confirmed records cannot practically be altered because blocks are hash-linked and replicated. |
| Transparency | On public blockchains anyone can read and verify every transaction (for example through a block explorer). |
| Pseudonymity | Users are identified by addresses, not by real names – but all activity of an address is public. |
| Consensus | Nodes follow common rules to agree on which transactions are valid and in what order. |
| Security | Hash functions protect data integrity; digital signatures prove who authorised each transaction. |
Where is Blockchain Used?
Since Bitcoin was launched in 2009, blockchain use has grown through the creation of:
- Cryptocurrencies – digital money such as Bitcoin and Ether.
- Decentralized Finance (DeFi) applications – lending, trading and payments without banks (Chapter 29).
- Non-Fungible Tokens (NFTs) – unique digital ownership records (Chapter 27).
- Smart Contracts – programs that run automatically on the blockchain (Chapter 7).
A blockchain can immutably record many kinds of data points, for example:
- Financial transactions
- Votes in an election
- Product inventories and supply-chain movements
- State identifications and educational certificates
- Deeds to homes and land records
- …and much more (Chapter 31).
Do you really need a blockchain? A blockchain is most useful when several parties who do not fully trust each other must share and update the same records. If a single trusted organisation controls all the data, an ordinary database is usually faster, cheaper and simpler.
Key Takeaways
- A blockchain is a replicated, append-only ledger of hash-linked blocks maintained by a peer-to-peer network.
- Each block has a header (previous hash, timestamp, Merkle root …) and a body (transactions).
- Its value comes from removing the need for a single trusted intermediary.
Question / Answer (15 questions)
Answer : A blockchain is a shared digital ledger replicated across the nodes of a peer-to-peer network. Records are grouped into blocks, each block contains the hash of the previous block, and new blocks are added only after the network reaches consensus.
Answer : DLT is any system in which a ledger is copied and synchronised across multiple participants without a central administrator. A blockchain is a specific kind of DLT that stores the ledger as a chain of hash-linked blocks. Every blockchain is a DLT, but not every DLT uses blocks.
Answer : Typical fields are the previous block hash, a timestamp, the Merkle root of the block’s transactions and, in Proof-of-Work chains, the nonce and difficulty target. Many chains also include a version number.
Answer : Because each block contains the hash of the previous block and the chain is copied across many nodes. Changing a confirmed block would change its hash, break all later links and be rejected by honest nodes, so changes are practically impossible.
Answer : The network itself validates and records transactions using cryptography and consensus. Since confirmed records cannot be altered, participants can rely on the ledger instead of an intermediary such as a bank or auditor; trust is needed mainly at the point where data is entered.
Answer : Transparency means every transaction is publicly visible and verifiable. Pseudonymity means users appear only as addresses rather than real names. So activity is public, but the identity behind an address is not directly revealed.
Answer : The user signs a transaction, broadcasts it to the network, nodes validate it, a miner or validator includes it in a new block, the network agrees on the block through consensus, and every node appends the block to its copy of the chain.
Answer : It is the set of rules that lets independent nodes agree on which transactions are valid and in which order blocks are added, so that all honest nodes end up with the same ledger even when some participants are faulty or dishonest.
Answer : Votes in an election, product inventory and supply-chain records, identity documents or educational certificates, and land or property deeds.
Answer : A block explorer is a website or tool that reads blockchain data and lets anyone search blocks, transactions and addresses. It demonstrates the transparency of public blockchains.
Answer : When a single trusted organisation controls all the data, when data must be frequently edited or deleted, or when very high speed and low cost are essential. In such cases a traditional database is simpler and more efficient.
Answer : So that any node can independently verify the history, so that the network has no single point of failure, and so that no single party can censor or rewrite the records.
Answer : A transaction is confirmed once it has been included in a block that the network has accepted. Each additional block added on top increases confidence that it will not be reversed.
Answer : Cryptographic hash functions protect integrity and link blocks; public-key cryptography and digital signatures prove ownership and authorisation; and consensus mechanisms make it expensive to rewrite history.
Answer : A centralized database has one owner who can create, update and delete records at will. A blockchain is append-only, replicated across many independent nodes and updated only through consensus, so no single party can change it alone.
MCQ Test : Check Your Understanding (5 questions)
Select an option and press Check Answer, or answer all questions and press Submit Test to see your score. Each answer comes with an explanation.
Bibliography/References
The notes, questions and MCQs on this page are original explanations written for this course. The works listed below were consulted and are cited for attribution and further reading. Product names, logos and trademarks belong to their respective owners. Short quotations, where used, are marked and attributed.
- Yaga, D., Mell, P., Roby, N., & Scarfone, K. (2018). Blockchain Technology Overview (NISTIR 8202). National Institute of Standards and Technology. https://doi.org/10.6028/NIST.IR.8202
- Nakamoto, S. (2008). Bitcoin: A Peer-to-Peer Electronic Cash System. https://bitcoin.org/bitcoin.pdf
- Narayanan, A., Bonneau, J., Felten, E., Miller, A., & Goldfeder, S. (2016). Bitcoin and Cryptocurrency Technologies: A Comprehensive Introduction. Princeton University Press. ISBN 978-0-691-17169-2.
- Investopedia. (n.d.). Blockchain Facts: What Is It, How It Works, and How It Can Be Used. https://www.investopedia.com/terms/b/blockchain.asp
- ethereum.org. (n.d.). Blocks. Ethereum Developer Documentation. https://ethereum.org/en/developers/docs/blocks/