Blockchain and Smart Contracts Explained

A blockchain is a shared record of transactions that many computers keep and update together, without needing a single trusted party such as a bank to maintain it. Records are grouped into blocks that are linked by cryptography, so altering past entries is extremely difficult. The idea first appeared in practice with Bitcoin (Nakamoto, 2008). Later platforms added smart contracts — programs stored on the blockchain that run automatically when conditions are met.

The problem a blockchain solves

Digital information can be copied, so how can digital money be prevented from being spent twice? Traditionally, a central party — a bank or payment system — keeps the authoritative ledger. A blockchain instead lets a network of participants agree on one shared history of transactions, even if some participants are unreliable or dishonest. This is called solving the double-spending problem without a central intermediary.

How it works

How a transaction is added to a blockchain

1. 1SignThe owner signs a transaction with a private key
2. 2BroadcastThe transaction is shared with network nodes
3. 3ValidateNodes check signatures and balances
4. 4Form a blockValid transactions are grouped together
5. 5ConsensusThe network agrees which block comes next
6. 6ConfirmThe block is linked to the chain

Hashing

A hash function turns any data into a fixed-length "fingerprint". Changing even one character of the input produces a completely different hash. Each block contains the hash of the previous block, so blocks form a chain: altering an old transaction changes its block’s hash, which breaks the link to every later block and is immediately visible.

Keys and digital signatures

Users hold a private key (kept secret) and a public key or address (shared). Signing a transaction with the private key proves ownership without revealing the key. Losing the private key generally means losing access to the assets permanently — a crucial practical risk.

Consensus mechanisms

Mechanism	How agreement is reached	Notes
Proof of work	Participants ("miners") compete to solve a computational puzzle; the winner adds the block	Very secure for large networks but uses a lot of energy (used by Bitcoin)
Proof of stake	Validators lock up ("stake") tokens and are selected to propose and confirm blocks; dishonesty can be penalised	Much lower energy use (Ethereum moved to it in 2022)
Permissioned consensus	Known, approved participants vote on blocks	Common in private networks between financial institutions

Public and private blockchains

• Public (permissionless) blockchains are open to anyone to read, transact and validate — Bitcoin and Ethereum are examples.
• Private or consortium (permissioned) blockchains limit participation to approved members, such as a group of banks, offering more control, privacy and speed but less decentralisation.

Smart contracts

A smart contract is code stored on a blockchain that executes automatically. For example: "if payment of 1,000 tokens is received by 30 June, transfer ownership of the digital asset; otherwise return the payment". Ethereum (described in its white paper) popularised general-purpose smart contracts.

Uses include:

• Escrow and conditional payments without a third party holding funds.
• Decentralised finance (DeFi) — lending, borrowing and trading through code rather than institutions.
• Automated settlement of trades when both sides of a transaction are on the same ledger ("delivery versus payment").
• Trade finance — sharing documents and triggering payments on shipment events, potentially reducing paperwork in letters of credit (see trade finance).

Smart contracts do exactly what their code says — including any errors. Bugs have led to large losses, and contracts cannot by themselves verify real-world events; they rely on data feeds called oracles, which can themselves fail or be manipulated.

Tokenisation

Tokenisation means representing an asset — money, a bond, a share, real estate or a commodity — as a digital token on a ledger. Potential benefits include faster settlement, fractional ownership and programmability. Central banks and financial institutions have run pilots of tokenised deposits, bonds and wholesale central bank digital currencies, while legal questions about ownership, insolvency and regulation are still being worked through.

Benefits and limitations

Potential benefits	Limitations and risks
Shared, tamper-evident record	Limited transaction capacity on some public chains
Reduced need for reconciliation between parties	Energy use (proof of work)
Faster settlement and 24-hour operation	Smart contract bugs and oracle risk
Programmable assets and payments	Irreversible errors and lost keys
Transparency of transactions	Privacy concerns on public ledgers
	Legal and regulatory uncertainty
	Data on the chain is only as accurate as what was entered

A blockchain is not always the best solution. If one trusted organisation already maintains a record, a conventional database is usually faster and cheaper. Blockchains add most value where several parties who do not fully trust each other need to share and update the same record.

Security

The core ledger of a large public blockchain has proved hard to tamper with, but most losses happen around it: stolen keys, hacked exchanges, phishing, fraudulent projects and smart contract exploits — see cybersecurity in finance.

Common mistakes

• Assuming that "on the blockchain" means the information is true.
• Treating all blockchains as the same — public and permissioned networks differ greatly.
• Believing smart contracts are legally binding contracts in every jurisdiction.
• Using a blockchain where a simple shared database would do.