Digital Currency & Blockchain Lab
Money that only exists electronically. A ledger nobody controls. And a chain of blocks that shatters the moment anyone tries to cheat. Learn what a digital currency actually is, why cryptocurrency is decentralised, and how blockchain uses a time-stamped digital ledger to make transactions tamper-evident. Then try to break the chain yourself.
5.4 Path
Mistakes
Stats
Bookmarks
Book Notes
Knowledge Vault
Add entry
Entries
What do I need to know before I start?
Section 5.4 is about money that exists only electronically. A digital currency is real money that is never printed — sent between computers, usually encrypted with HTTPS. A cryptocurrency (e.g. Bitcoin) is a special kind that runs on a decentralised system: no bank, no central authority. Trust comes from blockchain — every transaction is added to a public digital ledger, time-stamped, and cryptographically linked to the block before it. Because each block's fingerprint depends on the previous one, records cannot be altered, only added to. Learn the exact mark-scheme words: digital ledger, time-stamp, linked to previous, cannot be altered, traced.
By the end of 5.4 you can…
| You can… | Why it matters |
|---|---|
| Define a digital currency and explain how it is used | A 1-mark definition and a 2-mark "features" question appear regularly |
| Distinguish centralised (bank-controlled) from decentralised systems | The classic trap — examiners penalise "run by a company" |
| Describe how blockchain uses a time-stamped, linked digital ledger | The 2024 Paper 1 process/diagram question rewards the right order |
| Explain why blockchain records cannot be altered, only added | Tests that you understand the hash-link, not just the buzzword |
| Name what each record in the ledger stores | Lets you answer "what is in a block" precisely |
Cambridge-approved terms
Money that exists only electronically / virtually, not physically; sent between computers.
A digital currency that uses encryption and a decentralised system (e.g. Bitcoin).
A central authority (usually a bank) manages and confirms transactions.
No central authority; the network of users verifies transactions.
The process that tracks transactions using a digital ledger of linked blocks.
A database recording every transaction, time-stamped and immutable.
The exact date and time a transaction was added; fixes its order.
A fingerprint of a block's contents; changes if any bit changes, linking the chain.
Cryptographic proof a transaction was signed by the sender; stops forgery.
The tables you must know cold
Centralised vs decentralised
| Feature | Centralised | Decentralised |
|---|---|---|
| Authority | One central authority (bank) | None — the network verifies |
| Example | Traditional money, bank transfers | Cryptocurrency (Bitcoin) |
| If something goes wrong | Someone to complain to; account can be frozen | Nobody to complain to; nobody can freeze it |
What each ledger record stores
| Field | What it stores | Why it matters |
|---|---|---|
| Transaction data | Who paid whom, and how much | The payment itself — the reason the block exists |
| Digital signature | Proof it was signed by the sender's key | Stops someone forging a payment from your wallet |
| Time-stamp | Exact date/time added | Establishes order; you can't spend the same coin twice |
| Previous hash | A fingerprint of the previous block | Links blocks into a chain — change one, every one after breaks |
| Hash of this block | A fingerprint of everything above | Becomes the next block's "previous hash" — this is the chain |
How a digital-currency payment happens (2024 P1 Q8)
| Step | What happens |
|---|---|
| 1 | User sends a payment using a digital currency |
| 2 | The transaction data is encrypted before it is sent (HTTPS + digital signature) |
| 3 | The transaction is broadcast to the decentralised network |
| 4 | The network verifies it against the existing digital ledger |
| 5 | It is added as a new block — transaction data, time-stamp, and a link (hash) to the previous block |
| 6 | The updated ledger is copied to every computer; the payment is traceable and cannot be altered |
Traps that cost marks every session
Check yourself — tap to reveal
Say the answer out loud in exam wording, then tap to check.
✅ Ready for Learn?
You've met the terms and the traps. Open the Learn tab to try the Signature Lab — edit a block and watch the chain break — then test yourself in Activities and Practice.
📚 From the Textbook
Every time you pay for something on the internet, you're probably using a digital currency — money that exists electronically, not physically. It's still real money; it's just never printed. When the payment is sent between computers, it's encrypted using HTTPS (the same protocol you met in 5.2), and your card details are often stored in a persistent cookie (5.3) so the site can auto-fill them next time. Credit cards, mobile phones, smart watches — all digital-currency payment tools. But there's a second, more radical kind of digital currency: cryptocurrency, like Bitcoin. Traditional money uses a centralised system — a bank sits in the middle and confirms every transaction. Cryptocurrency uses a decentralised system — no bank, no central authority. So how do you trust it? That's what blockchain solves. Every transaction is added to a public digital ledger, time-stamped, and cryptographically linked to the previous one. Change one transaction and every block after it visibly breaks. That's the demo you'll play with below.
💡 Getting Started
Have you ever paid for something using a card, phone or watch? That was a digital-currency transaction. See if you can list five things you (or your family) have bought this month using digital currency instead of physical cash. Then ask: who confirmed each payment? Usually a bank. Now imagine a payment system where nobody confirms it — but everyone can see it. That's cryptocurrency.
🔬 Computer Science in Context: Satoshi & the 2008 whitepaper
In October 2008, someone using the name Satoshi Nakamoto published a 9-page paper titled "Bitcoin: A Peer-to-Peer Electronic Cash System". It described a way to run digital cash without a bank — using a chain of cryptographically-signed transactions that anyone could verify but nobody could edit. In January 2009 the first Bitcoin block ("Block 0", the genesis block) was mined. Fifteen years on, there are over 2,000 different cryptocurrencies, and central banks around the world are building their own Central Bank Digital Currencies (CBDCs). Blockchain has jumped well beyond money — it's used to track shipping containers, prove ownership of digital art, and record who signed medical records.
💬 Discussion: is decentralised better?
Centralised systems have a single authority you can complain to when things go wrong. Decentralised systems don't — if you lose your Bitcoin password, there's nobody to phone. What do you gain? What do you lose? Which would you rather use for your monthly rent — a bank or a cryptocurrency? And which for a payment you want nobody to be able to reverse?
⚠️ The Cambridge Exam Trap — Centralised vs Decentralised
Examiners see students describe cryptocurrency as "controlled by a company" or "managed by a bank". That loses the mark. The whole point of a cryptocurrency is that nobody is in control.
A central authority manages transactions. Usually a bank.
Traditional money, credit cards, bank transfers. Someone to complain to. Someone who can freeze the account.
No central authority. Every user's copy of the ledger acts as verification.
Cryptocurrency. Nobody to complain to. Nobody who can freeze the account. Blockchain replaces the middleman.
⛓️ Signature Lab — Try to Break the Blockchain
Four transactions have been added to the ledger. Each block stores data, a time-stamp, and a hash — a fingerprint calculated from the block's own contents PLUS the previous block's hash. Try editing block 2's data and watch what happens to blocks 3 and 4. Then hit Verify chain.
How a digital-currency payment happens
This is the answer to the 2024 Paper 1 Q8 diagram question — draw and annotate this.
- User sends payment using a digital currency (e.g. a cryptocurrency).
- The transaction data is encrypted before it is sent (HTTPS + digital signature).
- The transaction is broadcast to the decentralised network of computers.
- The network verifies the transaction against the existing digital ledger.
- The transaction is added as a new block to the blockchain: contains the transaction data, a time-stamp, and a link (hash) to the previous block.
- The updated ledger is copied to every computer on the network — the payment is now traceable and cannot be altered.
Digital ledger — what's in each record?
| Field | What it stores | Why it matters |
|---|---|---|
| Transaction data | Who paid whom, and how much | The payment itself — the whole reason the block exists |
| Digital signature | Cryptographic proof it was signed by the sender's key | Stops someone forging a payment from your wallet |
| Time-stamp | The exact date and time the transaction was added | Establishes order — you can't spend the same coin twice |
| Previous hash | A fingerprint of the previous block | Links the blocks into a chain. Change one, every one after breaks. |
| Hash of this block | A fingerprint of everything above, plus the previous hash | Becomes the "previous hash" for the next block. This is the chain. |
Exam-ready wordings from Cambridge mark schemes
State what is meant by a digital currency [1]
"A currency that exists only electronically / virtually, not physically."
Give two features of a digital currency [2]
Any two from: exists electronically / not physically · decentralised (for cryptocurrencies) · uses encryption · transactions tracked using a digital ledger · time-stamped · cannot be altered once recorded · payments made between computers over the internet.
Identify the process that uses a digital ledger to track digital-currency transactions [1]
"Blockchain."
Fill-in-blank paragraph (2024 P1) [4]
"A digital currency does not exist physically, it can only be accessed electronically. Some digital currencies have digital ledgers called blockchains. These are decentralised databases where each transaction is stored as a new set of data with a time-stamp and is linked to the previous set of data. This means transactions cannot be altered, only new transactions added, which allows the location of the data to be traced."
⭐ Activity 1 — Feature Identifier
Read the description. Name the concept.
Press New.
🎮 Activity 2 — Centralised or Decentralised?
The mix-up examiners see every year. Read the scenario, pick the system.
Press New.
🎮 Activity 3 — Feature of Digital Currency?
Is this a valid feature to write in an exam answer, or a trap?
Press New.
🎮 Activity 4 — Blockchain Detective
Three ledgers are shown. Only one is untampered. Spot the valid chain.
⏱️ Activity 5 — 60-Second Ledger Sprint
Rapid recall. Match each description to the correct term.
Adaptive practice
Press New.
Cambridge-style questions from 2023–2025 past papers
Press New.
Review quiz
Press New.
Exam traps
Memory triggers
Tap to reveal
Mastery grid
Track which concepts you have secured. Tap a badge to toggle.