Copies of the blockchain shared across many computers

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

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

2. Who controls a blockchain ledger?

A The bank that created it

B No single person or company

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

A Each computer keeps a different version

B Every copy matches

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

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

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1decentralised ledger		AA blockchain is not held by one organisation. Many computers each keep a copy, so no single company controls it.
2copy1 == copy2 == copy3		BEvery copy of the blockchain matches the others when no one has cheated
3A changed copy is spotted by comparing it with the other copies		Ccomputers = 5000 copies = 5000
4No single owner: many computers each keep a copy of the ledger		DEditing one copy does not change the others. The edited copy no longer matches and is spotted when compared.
5Only the bank keeps the blockchain ledger		Eif myCopy != otherCopies then reject
6A hacker edits one copy, so all copies change		FA blockchain ledger copied on many computers, so no one person or company controls it

Definitions

decentralised ledger

Fill in the Blanks

Complete each line of code by filling in the missing parts.

IF myCopy __ majorityCopy THEN OUTPUT "Changed copy spotted" ENDIF

FOR node ← 1 __ 5 OUTPUT ledger[node] NEXT node

DECLARE copies : _____[1:1000] OF STRING

IF myCopy = otherCopy THEN matches ← matches _ 1 ENDIF

IF matches _ total / 2 THEN OUTPUT "Copy accepted" ENDIF

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A computers ← 1

B FOR c ← 1 TO computers

C copy[c] ← ledger

D NEXT c

Question 2

A copies ← 1000

B allMatch ← TRUE

C OUTPUT 'Copies match'

D owner ← 'One company'

Question 3

A mine ← hash(myCopy)

B other ← hash(theirCopy)

C IF mine = other THEN

D OUTPUT 'Changed copy'

E ENDIF

Question 4

A matching ← 4

B total ← 5

C valid ← matching < total / 2

D OUTPUT valid

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

Answer Key

Copies of the blockchain shared across many computers

Section A: Quick Questions

A — A copy is kept on many computers

B — No single person or company

B — Every copy matches

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

A — A cheat would have to change the copies on many computers

Section B: 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.

Section C: Definitions

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.

Section D: Fill in the Blanks

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

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

DECLARE copies : ARRAY[1:1000] OF STRING

IF myCopy = otherCopy THEN
   matches ← matches + 1
ENDIF

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

Section E: Spot the Error

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.

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

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.

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

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.