Blockchain and Hash-Linked Records

Lesson plan

Level

Beginner

Time

About 50 minutes; the steps below add up to 51

Learners design a trip register in which a change to an old entry would be noticed, and ask whether one keeper could rewrite it, before the blockchain is named. Predictions then precede which blocks break, what happens when one copy is rewritten, whether public means anonymous, when a shared ledger is worth it and what a false record proves; checks follow what a block stores, how many blocks to redo and central-bank money. The habit to build: separate tamper-evident from true, and ask who else holds a copy.

Objectives

• Describe a blockchain as a hash-linked ledger
• Explain why changing an old block is evident
• Explain why distribution and consensus are also needed
• Distinguish open public ledgers from permissioned blockchains
• Judge when a blockchain is useful, and its limits

Before the lesson

• Open this page on your own screen and use “Open the lesson window” to open the lesson in a second window. Share only that second window.
• Learners need the Hashing and Integrity lesson first.
• The hash-chain panel works best if learners predict before you press the button.
• The lesson takes about 51 minutes.
• In presentation mode every step is shown in turns: Next reveals one moment, part or answer at a time, and the instructor view names what is on screen and what comes next.

Steps

1. Introduction Opening

   On screen: the lesson’s title, module and number; Next adds its objectives and key terms, as the lesson page lists them; Next again begins the first step.

   Objectives on screen

   • Describe a blockchain as a hash-linked ledger
   • Explain why changing an old block is evident
   • Explain why distribution and consensus are also needed
   • Distinguish open public ledgers from permissioned blockchains
   • Judge when a blockchain is useful, and its limits

   Purpose: Learners design a trip register in which a change to an old entry would be noticed, and ask whether one keeper could rewrite it, before the blockchain is named. Predictions then precede which blocks break, what happens when one copy is rewritten, whether public means anonymous, when a shared ledger is worth it and what a false record proves; checks follow what a block stores, how many blocks to redo and central-bank money. The habit to build: separate tamper-evident from true, and ask who else holds a copy.

2. 1. A trip register Think 5 min

   Purpose: Design tamper evidence and distribution from a paper register.

   Ask

   • How would a change be noticed?
   • Could one keeper rewrite it?

   Responses to expect

   • The fingerprint idea comes with one hint; copies come later.
   • On screen in turns: the opening question, its answer; then the question on one keeper, and its answer.

   Discussion pause: Let learners sketch their register design before revealing.

   In the lesson window

   • Think it through: A class keeps a paper register of who has paid for a trip. Someone could quietly change an old entry. How could you design the register so that any change to an old entry would be noticed?
   • Think it through: If only one person kept the register, could they rewrite it without anyone noticing?

3. 2. The idea Explain 4 min

   Purpose: Define a ledger, a block and the hash link.

   Ask

   • What does each block store?

   Responses to expect

   • The previous block’s hash.
   • On screen in turns: the opening; then a blockchain, starting with the block; the previous block’s hash; the block’s own hash; the check question on what a block stores, and its answer; chained to every earlier block.

   In the lesson window

   • Check your understanding: What does each block store besides its records?

4. 3. See a change break the chain Experiment 4 min

   Purpose: Use the panel to watch a change break the chain.

   Ask

   • Which blocks will break?

   Responses to expect

   • Most predict only Block 2.
   • On screen in turns: the opening question, its answer.

   In the lesson window

   • Think it through: Before pressing the button: if Block 2 is changed, which blocks will show a problem, and why?

5. 4. Why hash-linking makes a change evident Explain 5 min

   Purpose: Explain why a change runs forward to the newest block.

   Ask

   • How many blocks must be redone?

   Responses to expect

   • Blocks 2 to 10.
   • Reveal Alice’s rewrite one step at a time; before each, ask what she must change next to hide the edit.
   • On screen in turns: the opening; then Alice rewrites Block 2, starting with its hash; Block 3 no longer matches; hiding it changes Block 3; every later block; the check question on how many blocks to redo, and its answer; an edit forces a rewrite.

   In the lesson window

   • Check your understanding: Alice changes Block 2 of a ten-block chain. How many blocks’ hashes must she recompute to hide it?

6. 5. What hash-linking alone does not do Explain 3 min

   Purpose: Show that one keeper could still rewrite the chain.

   Ask

   • What does hash-linking need to be useful?

   Responses to expect

   • Something to compare with.
   • On screen in turns: the opening; then evident only with something to compare.

   Misconceptions to challenge

   • A learner may believe: “Hash-linking alone makes a blockchain impossible to change.” Respond: Only evident; copies and consensus make it impractical.

7. 6. Many copies and consensus Predict 6 min

   Purpose: Explain distribution, consensus and signatures.

   Ask

   • What happens to Alice’s rewritten copy?

   Responses to expect

   • Her copy loses to the majority.
   • On screen in turns: the opening question, its answer; then a distributed ledger; Alice’s new problem; evident, then impractical; signatures, the third ingredient; how participants agree.

   To go further

   • Why does proof of work make rewriting history expensive?

   In the lesson window

   • Think it through: Many independent people each hold a full copy of the ledger. Alice rewrites hers perfectly. What happens?

8. 7. Public and permissioned blockchains Predict 4 min

   Purpose: Distinguish open public ledgers from permissioned ones.

   Ask

   • Ask whether a public ledger is anonymous.

   Responses to expect

   • Most say yes.
   • On screen in turns: the opening; then who may take part, starting with an open public ledger; a permissioned blockchain; open and permissioned, side by side; the question on whether a public ledger is anonymous, and its answer; public is not anonymous.

   Misconceptions to challenge

   • A learner may believe: “A public blockchain is anonymous.” Respond: Transactions are visible for ever; addresses can be traced.

   In the lesson window

   • Think it through: Is a public ledger anonymous?

9. 8. Where the idea is used, and where it is not needed Predict 6 min

   Purpose: Judge when a blockchain is useful, including the central bank case.

   Ask

   • When is it worth the trouble?
   • Does a central bank need one?

   Responses to expect

   • Blockchain is often seen as needed for anything digital.
   • On screen in turns: the opening question, its answer; then where blockchains suit, starting with cryptocurrencies; supply chains; shared registers; when an ordinary database is enough; a central bank digital currency; the check question on central bank money, and its answer; a central bank is the trusted party.

   If the explanation does not land: Ask who would keep the record if nobody trusted anyone else.

   In the lesson window

   • Think it through: When would a shared, tamper-evident ledger be worth the trouble, and when would an ordinary database do?
   • Check your understanding: A central bank issues digital money. Does it need a blockchain?

10. 9. Limits worth knowing Predict 4 min

    Purpose: Set out the limits: truth, confidentiality, permanence, keys.

    Ask

    • What does a false record prove?

    Responses to expect

    • Only that it is unchanged.
    • On screen in turns: the opening question, its answer; then limits worth knowing, starting with recorded and true; not confidential by design; permanence cuts both ways; keys still matter.

    In the lesson window

    • Think it through: Someone records a false delivery on a supply-chain blockchain. What does the blockchain prove about it?

11. 10. Security+ terminology Explain 1 min

    Purpose: Link the lesson to the Security+ wording.

    Ask

    • Which two bullets are listed?

    Responses to expect

    • Blockchain and open public ledger.
    • On screen in turns: the opening; then what the objectives do not list.

12. 11. Apply it in a new situation Apply 4 min

    Purpose: Set up independent application, ideally on a later day.

    Ask

    • Who will try these now, and who later in the week?

    Responses to expect

    • Names test permanence; the lost key tests control; lunch payments test need.
    • On screen in turns: the opening; then the question on names on the chain, and its answer; the question on a lost key, and its answer; the question on lunch payments, and its answer.

    In the lesson window

    • Apply it: A team plans to put customers’ names and addresses on a permanent blockchain. What problem should they consider?
    • Apply it: Someone loses the private key that controls their entry on a public ledger. Who can reset it?
    • Apply it: A school wants a tamper-evident record of its own lunch payments, which only its office keeps. What fits best?

13. 12. Check your understanding Check 3 min

    Purpose: Check the lesson against the approved revision-bank questions.

    Ask

    • Answer individually, then compare before revealing.

    Responses to expect

    • The tamper questions link to the panel step.
    • On screen in turns: the opening; then the revision questions one at a time, each followed by its answer; where to practise next.

14. 13. What to remember Consolidate 2 min

    Purpose: Summarise the five points, the key terms and the misunderstandings.

    Ask

    • Without looking: what does hash-linking alone not do?

    Responses to expect

    • Make the ledger hard to rewrite.
    • On screen in turns: the opening; then the summary; the key terms; the common misunderstandings.

15. Lesson recap Closing

    On screen: the lesson’s objectives again; Next adds the closing question, which asks the class to explain each one in their own words without looking back. It says nothing about what anyone has learnt: use the answers to decide what to revisit.

    Points to listen for

    • Each block holds the last one’s hash.
    • Change one, break the rest.
    • Copies and consensus make it stick.
    • Public is not anonymous.
    • Shared distrust, or a database.

Consolidation

• Each block holds the last one’s hash.
• Change one, break the rest.
• Copies and consensus make it stick.
• Public is not anonymous.
• Shared distrust, or a database.