Blockchain Exercises — Set 2 (Chain Validation & Proof-of-Work)

Let's think about this for a second

This lesson is a self-test practice set that combines the consensus, PoW/PoS, wallets, and keys concepts you learned in the Intermediate/Advanced chapters. It steps up from Exercise Set 1 with tasks like writing functions from scratch and checking security properties. There's no new teaching here — you'll be referencing the codebase you built in the Mini Blockchain Project and rewriting it yourself.

Exercises

Task 1 — Say you're given a chain array (mock data) with three blocks; write an isChainValid(chain) function from scratch that checks both the hash and the previousHash link. Task 2 — Tamper with only chain[1]'s data (leave the hash field alone) and check whether isChainValid() returns false. Task 3 — Modify the mineBlock(difficulty) function to accept difficulty as a parameter, and use console.time() to measure the difference in mining time between difficulty 2 and 4. Task 4 — Generate a key pair with crypto.generateKeyPairSync(), sign a message string with the private key, and verify it back with the public key.

Can you write the isChainValid() function yourself from scratch?
Have you proven that tamper detection catches a broken hash link?
Have you observed how much mining time grows exponentially as difficulty increases?

Code Example

javascript

const crypto = require('crypto');

// Mock chain (Task 1-2 အတွက်)
const chain = [
  { index: 0, data: 'Genesis Block', previousHash: '0', hash: 'abc0...' },
  { index: 1, data: 'Tx: A->B 10', previousHash: 'abc0...', hash: 'abc1...' },
  { index: 2, data: 'Tx: B->C 5', previousHash: 'abc1...', hash: 'abc2...' },
];

function calculateHash(block) {
  return crypto
    .createHash('sha256')
    .update(block.index + block.previousHash + block.data)
    .digest('hex');
}

// Task 1: isChainValid() ကို scratch ကနေ ကိုယ်တိုင်ရေးပါ
function isChainValid(chain) {
  // TODO: for loop နဲ့ hash + previousHash link နှစ်ခုလုံး စစ်ပါ
}

// Task 2: chain[1].data ကို tamper လုပ်ပြီး isChainValid() ပြန်ခေါ်ကြည့်ပါ

// Task 3: mineBlock(block, difficulty) function ကို difficulty parameter ပါအောင် ရေးပြီး
// console.time('mine-2') / console.timeEnd('mine-2') နဲ့ difficulty 2 vs 4 timing ယှဉ်ပါ

// Task 4: key pair generate လုပ်ပြီး message sign & verify practice
const { generateKeyPairSync, sign, verify } = crypto;
// TODO: generateKeyPairSync('ec', { namedCurve: 'secp256k1', ... }) နဲ့ sign/verify ကို ကိုယ်တိုင်ပြီးအောင်ရေးပါ

You should see

After Tasks 1-2, you'll see two booleans in the terminal — true before tampering, false after tampering. After Task 3, two mining times (ms) for different difficulty levels. After Task 4, a signature verify result of true.

5-Minute Try-It

Push Task 3's difficulty up to 5 and spend 5 minutes watching how sharply the mining time spikes, paying attention to the exponential growth pattern.

A Quick Word of Caution

When doing Task 2, only change the data field — don't manually rewrite the hash field. If you edit the hash field too, you won't actually be testing the tamper detection logic.