← IGCSE Computer Science Paper 1

Copies of the blockchain shared across many computers

A blockchain ledger copied on many computers. No one person or company controls it, and a changed copy is spotted by comparing it with the others.

Part of: IGCSE Computer Science 0478 — 5.2 Digital Currency — Module 21

Understanding Copies of the blockchain shared across many computers in IGCSE Computer Science Paper 1

What it does: The computers compare copies. A copy that does not match the others is rejected as false. Each computer in the network holds its own copy, so no one person or company owns or controls the ledger. A blockchain is shared. Many computers each hold a copy, so there is no single central store. Decentralised means control is spread out. A centralised ledger is held and controlled in one place. The shared copies should all be identical. Matching copies show the ledger is trustworthy. This describes a centralised ledger, not a shared blockchain ledger.

Common mistakes: A shared ledger needs many computers, each holding a copy. With only 1 computer there is a single copy, so it would not be decentralised. It should be a number greater than 1, such as 5. A decentralised ledger has no single owner. No one person or company controls it, so owner should be NONE.

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This module's key terms are also in a printable crossword (Modules 510–519) and word search (Modules 510–546).

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Learning

Duo

Where is a blockchain ledger stored?

A copy is kept on many computersOne copy is kept on a single central server

Matches

Only the bank keeps the blockchain ledger

↓

A blockchain is not held by one organisation. Many computers each keep a copy, so no single company controls it.

Applying

Meanings

Term

decentralised ledger

Meaning

A blockchain ledger copied on many computers. No one person or company controls it, and a changed copy is spotted by comparing it with the others.

Practice

FOR Computer ← 1 TO 5

Testing

Gaps

IF myCopy majorityCopy THEN

Check Answer

Spots

1computers ← 1

2FOR c ← 1 TO computers

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Quick Questions

1. Where is a blockchain ledger stored?

A. A copy is kept on many computers ✓

B. One copy is kept on a single central server

A blockchain is shared. Many computers each hold a copy, so there is no single central store.

2. Who controls a blockchain ledger?

A. The bank that created it

B. No single person or company ✓

Because the ledger is shared across many computers, no one owner is in charge of it.

3. Which statement about the copies of a blockchain is correct?

A. Each computer keeps a different version

B. Every copy matches ✓

All the computers hold the same records, so the copies match each other.

4. A hacker changes a transaction on their own copy of the blockchain. What happens?

A. The change is spotted by comparing it with the other copies ✓

B. All the other copies change to match it automatically

The altered copy no longer matches the many other copies, so it is spotted and rejected.

5. Why is a shared ledger hard to cheat?

A. A cheat would have to change the copies on many computers ✓

B. A cheat only needs to change one computer's copy

Changing one copy is not enough. It would not match the others, so the cheat would be noticed.

Match the Pairs

decentralised ledger → A blockchain ledger copied on many computers, so no one person or company controls it

copy1 == copy2 == copy3 → Every copy of the blockchain matches the others when no one has cheated

A changed copy is spotted by comparing it with the other copies → if myCopy != otherCopies then reject

No single owner: many computers each keep a copy of the ledger → computers = 5000 copies = 5000

Only the bank keeps the blockchain ledger → A blockchain is not held by one organisation. Many computers each keep a copy, so no single company controls it.

A hacker edits one copy, so all copies change → Editing one copy does not change the others. The edited copy no longer matches and is spotted when compared.

Key Term

decentralised ledger

A blockchain ledger copied on many computers. No one person or company controls it, and a changed copy is spotted by comparing it with the others.

Fill in the Blanks

1. IF myCopy <> majorityCopy THEN OUTPUT "Changed copy spotted" ENDIF

Hint: Type the pseudocode operator that means not equal to, so a changed copy is spotted.

2. FOR node ← 1 TO 5 OUTPUT ledger[node] NEXT node

Hint: Type the keyword that gives the end value of the loop, so every computer's copy is checked.

3. DECLARE copies : ARRAY[1:1000] OF STRING

Hint: Type the data structure keyword that stores many copies of the ledger under one name.

4. IF myCopy = otherCopy THEN matches ← matches + 1 ENDIF

Hint: Type the arithmetic operator that counts one more copy that matches.

5. IF matches > total / 2 THEN OUTPUT "Copy accepted" ENDIF

Hint: Type the comparison operator that checks whether more than half of the copies agree.

Spot the Error

Question 1

A computers ← 1

B FOR c ← 1 TO computers

C copy[c] ← ledger

D NEXT c

Line A — A shared ledger needs many computers, each holding a copy. With only 1 computer there is a single copy, so it would not be decentralised. It should be a number greater than 1, such as 5.

Question 2

A copies ← 1000

B allMatch ← TRUE

C OUTPUT 'Copies match'

D owner ← 'One company'

Line D — A decentralised ledger has no single owner. No one person or company controls it, so owner should be NONE.

Question 3

A mine ← hash(myCopy)

B other ← hash(theirCopy)

C IF mine = other THEN

D OUTPUT 'Changed copy'

E ENDIF

Line C — A changed copy is spotted by comparing it with another copy. The message 'Changed copy' should only show when the two hashes are different, so the test should be mine <> other, not mine = other.

Question 4

A matching ← 4

B total ← 5

C valid ← matching < total / 2

D OUTPUT valid

Line C — A copy is accepted as valid when MORE than half of the copies match it. The comparison should be matching > total / 2, not <.

Question 5

A copyA ← 'Ava pays Ben 5'

B copyB ← 'Ava pays Ben 5'

C copyC ← 'Ava pays Ben 50'

D bad ← copyA

E OUTPUT bad

Line D — copyC is the only copy that differs from the others, so it is the changed copy. It is spotted by comparing it with the other copies. It should be bad ← copyC.

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