A blockchain is a distributed digital ledger — a record of transactions copied and shared across many computers simultaneously. Because each record links to the one before it using cryptographic hashing, altering any past entry without detection is computationally infeasible, making the ledger highly tamper-resistant.
What is a ledger and why distribute it?
A ledger is a record of transactions — who paid whom, how much, and when. Banks keep a private, centralised ledger: one company controls the database, and everyone must trust that company not to alter it.
A distributed ledger spreads that record across thousands of computers simultaneously. No single party controls it; every participant holds a copy. If one copy is tampered with, the others disagree and the tampering is detected.
| Centralised ledger | Distributed ledger (blockchain) |
|---|---|
| Single authoritative copy | Thousands of identical copies |
| Controlled by one organisation | No single controller |
| If hacked or corrupted, single point of failure | Altering one copy is immediately detectable |
| Fast, efficient | Slower, but more resilient |
| Examples: bank database, company accounts | Examples: Bitcoin, Ethereum, supply-chain trackers |
What is a block and what does it contain?
A blockchain is built from a chain of blocks. Each block contains:
┌─────────────────────────────────────┐
│ Block 42 │
│ Timestamp: 2026-09-24 10:15:07 │
│ Data: [Transaction 1] │
│ [Transaction 2] │
│ [Transaction 3] │
│ Previous hash: 00ab4f7c... │
│ This block's hash: 0078e3d1... │
└─────────────────────────────────────┘
- Data — the actual content (e.g. "Alice sent Bob £50")
- Timestamp — when the block was created
- Previous hash — the cryptographic fingerprint of the block before this one
- Hash — a fingerprint of this block's entire contents
The hash of each block is computed from its own data and the previous block's hash. This creates the chain — each block is mathematically linked to its predecessor.
How does hashing make the chain tamper-evident?
Recall that a cryptographic hash function converts any input into a fixed-length output (the hash), and even the tiniest change to the input produces a completely different hash. This property — called the avalanche effect — is what makes blockchains tamper-resistant.
If an attacker changes Block 42's data:
- Block 42's hash changes completely.
- Block 43 stores the old hash of Block 42 as its "previous hash" — so Block 43's own contents are now wrong.
- Block 43's hash changes, invalidating Block 44, and so on.
The entire chain from Block 42 onwards is now inconsistent. Since thousands of other participants hold the correct original chain, the tampering is immediately visible. To successfully alter a historical block, an attacker would need to recompute every subsequent block faster than the rest of the network — a task that requires more computing power than all honest participants combined.
What is proof of work?
Adding a new block to the chain is deliberately made difficult through a process called proof of work. The new block must have a hash that begins with a certain number of zeros. Finding an input that produces such a hash requires enormous trial-and-error computation — effectively a guessing game that only powerful computers can play quickly.
The first computer to find a valid hash broadcasts the new block to the network; all participants verify it and add it to their copy of the chain. This competition is called mining in the context of cryptocurrencies.
Proof of work consumes large amounts of electrical energy — one of the main criticisms of blockchain systems based on it.
What are the applications of blockchain beyond cryptocurrency?
Cryptocurrency (Bitcoin, Ether)
→ Peer-to-peer payments without a central bank
Supply chain tracking
→ Record the journey of food, medicine, or luxury goods
from origin to shelf — impossible to falsify
Medical records
→ Patient data accessible to authorised providers,
with an auditable access log
Voting systems
→ Tamper-evident digital ballot recording
Smart contracts
→ Self-executing agreements coded directly on the blockchain;
payment triggers automatically when conditions are met
Academic credentials
→ Degrees and certificates verified without contacting the issuer
What are the limitations of blockchain?
- Energy consumption — proof-of-work blockchains use enormous amounts of electricity.
- Speed — a typical blockchain adds about 7 transactions per second (Bitcoin); Visa processes around 1,700 per second.
- Irreversibility — mistakes cannot be corrected. If you send cryptocurrency to the wrong address, there is no undo.
- Complexity — the underlying cryptography is difficult to audit, making bugs hard to spot.
- Not truly anonymous — every transaction is public; sophisticated analysis can often identify participants.
Frequently asked questions
What is a blockchain in simple terms for KS3?
A blockchain is a shared record book maintained simultaneously by thousands of computers. Each page (block) contains transactions and a cryptographic fingerprint of the previous page. Because every copy would disagree if one were altered, and because changing one page requires recomputing all subsequent pages faster than the entire network — it is practically impossible to alter the history. This makes it useful for recording anything where trust and tamper-resistance matter.
What is the difference between a blockchain and a database?
A traditional database is centralised — one organisation stores and controls it. A blockchain is decentralised — thousands of participants each hold a full copy, and no single party controls it. A database is fast and easily updated or corrected. A blockchain is slower and records are permanent — once confirmed, entries cannot be changed. Databases are better for most applications; blockchains add value specifically where you need a trustworthy shared record without a trusted central authority.
Is Bitcoin the same as blockchain?
No. Bitcoin is a cryptocurrency — a digital currency — that runs on a blockchain. Blockchain is the underlying technology: a method of creating a distributed, tamper-evident ledger. Many other things use blockchain technology that have nothing to do with cryptocurrency: supply-chain tracking, digital identity systems, and smart contracts all use blockchain without involving Bitcoin.
What is a smart contract?
A smart contract is a piece of code stored on a blockchain that executes automatically when predefined conditions are met. For example: "Release payment to the supplier automatically when the tracking data confirms delivery." Because the contract lives on the blockchain, neither party can alter the terms after agreement, and execution is automatic — no intermediary is required. Smart contracts are associated with the Ethereum blockchain.
Discuss blockchain, cryptography, and digital trust with Professor Turing at aitutors.me — ideas explored through questions, not lectures.