Why records that cannot be altered matter

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Quick Questions

Shade in the A or B circle for each question.

1. Why does a ledger that cannot be altered build trust in a digital currency?

A It makes every transaction happen faster than before

B Nobody can secretly change past records, so everyone can rely on the ledger

2. Ava paid Ben 50 coins. Later Ava tries to edit the record to say 5 coins. What happens on a ledger that cannot be altered?

A The change is rejected and the record still says 50 coins

B The record changes to 5 coins and Ben loses money

3. Which cheat does an unalterable ledger stop?

A Choosing which digital wallet to use

B Deleting a payment you made so it looks like you never paid

4. What is the main danger if a ledger records CAN be altered?

A People could cheat by changing amounts or removing payments

B The ledger would use too many time-stamps

5. Zoe says: 'I will change the amount in an old transaction to get more money.' Will this work on an unalterable ledger?

A No, the old amount is fixed and cannot be changed afterwards

B Yes, as long as she changes it before anyone checks

Match the Pairs

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

Left column		Right column
1amount = 50 -> amount = 5		Atrust = records_unchanged
2delete payment #204		BA cheat that tries to change a saved payment amount afterwards, which an unalterable record stops
3Because saved records cannot be changed, everyone can trust the ledger and the currency		CIf records could be altered, no one could trust the ledger. Trust exists because records cannot be altered.
4A cheat fails when the changed record no longer matches the shared copies		DA cheat that tries to remove a payment from the ledger, which cannot work if records cannot be altered
5Ben edits his old payment from 20 to 2 and the ledger accepts it		EIn a ledger with unalterable records, the edit would be rejected. Saved amounts cannot be changed afterwards.
6Records can be altered, so everyone can trust the ledger		Fif edited_copy != shared_copies: reject

Definitions

immutable record

Fill in the Blanks

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

IF newAmount __ record.amount THEN OUTPUT "Change rejected" ENDIF

IF action = "DELETE" THEN ______ "Payments cannot be removed" ENDIF

ledgerChanged ← FALSE trusted ← ___ ledgerChanged

savedAmount ← 50 newAmount ← 500 cheatFailed ← newAmount != ___________

ledger.add(payment) payment.locked ← ____

Spot the Error

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

Question 1

A IF record IS EMPTY THEN

B REJECT delete request

C ENDIF

Question 2

A SAVE 20 TO ledger

B TRY TO CHANGE 20 TO 5

C OUTPUT "Change allowed"

Question 3

A IF payment NOT IN ledger THEN

B REJECT delete request

C ENDIF

Question 4

A FOR EACH payment IN ledger

B LOCK payment

C NEXT payment

D trusted ← FALSE

Question 5

A SAVE payment TO ledger

B UNLOCK payment

C TRY payment.amount ← 5

D OUTPUT "Edit failed"

Answer Key

Why records that cannot be altered matter

Section A: Quick Questions

B — Nobody can secretly change past records, so everyone can rely on the ledger

A — The change is rejected and the record still says 50 coins

B — Deleting a payment you made so it looks like you never paid

A — People could cheat by changing amounts or removing payments

A — No, the old amount is fixed and cannot be changed afterwards

Section B: Match the Pairs

amount = 50 -> amount = 5 → A cheat that tries to change a saved payment amount afterwards, which an unalterable record stops

delete payment #204 → A cheat that tries to remove a payment from the ledger, which cannot work if records cannot be altered

Because saved records cannot be changed, everyone can trust the ledger and the currency → trust = records_unchanged

A cheat fails when the changed record no longer matches the shared copies → if edited_copy != shared_copies: reject

Ben edits his old payment from 20 to 2 and the ledger accepts it → In a ledger with unalterable records, the edit would be rejected. Saved amounts cannot be changed afterwards.

Records can be altered, so everyone can trust the ledger → If records could be altered, no one could trust the ledger. Trust exists because records cannot be altered.

Section C: Definitions

immutable record — A record that cannot be changed once saved, so no one can cheat by editing an amount or deleting a payment, and everyone can trust the ledger.

Section D: Fill in the Blanks

IF newAmount != record.amount THEN
    OUTPUT "Change rejected"
ENDIF

IF action = "DELETE" THEN
    OUTPUT "Payments cannot be removed"
ENDIF

ledgerChanged ← FALSE
trusted ← NOT ledgerChanged

savedAmount ← 50
newAmount ← 500
cheatFailed ← newAmount != savedAmount

ledger.add(payment)
payment.locked ← TRUE

Section E: Spot the Error

A — The ledger should only reject a delete request when the record is locked (cannot be altered). Checking for an empty record does not stop anyone removing a payment. The line should be IF record IS LOCKED THEN.

C — The saved record cannot be altered, so the cheat of changing 20 to 5 fails. The message should say the change is blocked, not allowed.

A — A delete request must be rejected when the payment IS in the ledger. A saved payment cannot be removed afterwards. The condition should be IF payment IN ledger THEN.

D — Every payment has been locked so none can be changed or removed, which means everyone can trust the ledger. trusted should be set to TRUE.

B — The payment must be locked after saving so that its amount cannot be changed later. UNLOCK would let a cheat edit the amount. The line should be LOCK payment.