Blockchain Technical Round Questions & Answers

With ethers v6, a wallet signs using EIP-191 personal sign, and verifyMessage recovers the address. import { Wallet, verifyMessage } from 'ethers'; const wallet = Wallet.createRandom(); const message = 'Login nonce: 48213'; const signature = await wallet.signMessage(message); const recovered = verifyMessage(message, signature); console.log(recovered === wallet.address); // true This pattern powers Sign-In with Ethereum; the server issues a fresh nonce each time to stop replay.

Hash leaves, then hash pairs until one remains, duplicating the last hash when a level is odd (Bitcoin style). import hashlib def h(b): return hashlib.sha256(b).digest() def merkle_root(txs): level = [h(t.encode()) for t in txs] while len(level) > 1: if len(level) % 2: level.append(level[-1]) level = [h(level[i] + level[i+1]) for i in range(0, len(level), 2)] return level[0].hex() print(merkle_root(['tx1', 'tx2', 'tx3']))

import {AggregatorV3Interface} from '@chainlink/contracts/src/v0.8/shared/interfaces/AggregatorV3Interface.sol'; AggregatorV3Interface public feed; uint256 constant MAX_DELAY = 1 hours; function getPrice() public view returns (uint256) { (, int256 answer, , uint256 updatedAt, ) = feed.latestRoundData(); require(answer > 0, 'bad price'); require(block.timestamp - updatedAt <= MAX_DELAY, 'stale price'); return uint256(answer); // scale using feed.decimals(), often 8 } Always check staleness, sign and decimals; on L2s also check the sequencer uptime feed.

pragma solidity ^0.8.24; contract Escrow { address public immutable buyer; address payable public immutable seller; bool public released; constructor(address payable _seller) payable { buyer = msg.sender; seller = _seller; } function release() external { require(msg.sender == buyer, 'only buyer'); require(!released, 'done'); released = true; (bool ok, ) = seller.call{value: address(this).balance}(''); require(ok, 'transfer failed'); } } State is updated before the external call to follow checks-effects-interactions.

Foundry tests are Solidity contracts; functions starting with test run as tests, and parameters make them fuzz tests. import {Test} from 'forge-std/Test.sol'; import {Vault} from '../src/Vault.sol'; contract VaultTest is Test { Vault v; function setUp() public { v = new Vault(); } function testDeposit() public { v.deposit{value: 1 ether}(); assertEq(v.balanceOf(address(this)), 1 ether); } function testFuzzDeposit(uint96 amt) public { vm.assume(amt > 0); vm.deal(address(this), amt); v.deposit{value: amt}(); assertEq(address(v).balance, amt); } } Run with forge test -vvv.

Each block stores the previous hash, so changing any earlier data breaks the chain. import hashlib, json def block_hash(block): data = json.dumps(block, sort_keys=True).encode() return hashlib.sha256(data).hexdigest() chain = [{'index': 0, 'data': 'genesis', 'prev': '0'}] for i, tx in enumerate(['A pays B 5', 'B pays C 2'], 1): chain.append({'index': i, 'data': tx, 'prev': block_hash(chain[-1])}) valid = all(chain[i]['prev'] == block_hash(chain[i-1]) for i in range(1, len(chain))) print(valid)

Use contract.on for live events and queryFilter for history. const abi = ['event Transfer(address indexed from, address indexed to, uint256 value)']; const token = new Contract(TOKEN, abi, provider); token.on('Transfer', (from, to, value, event) => { console.log(from, to, value.toString(), event.log.blockNumber); }); const filter = token.filters.Transfer(null, MY_ADDRESS); const past = await token.queryFilter(filter, -5000); // last 5000 blocks console.log(past.length); Use a WebSocket provider for live events, and page large ranges because providers limit block spans.

cast handles ABI work from the command line. # Function selector cast sig 'transfer(address,uint256)' # 0xa9059cbb # Encode calldata cast calldata 'transfer(address,uint256)' 0xAbc... 1000000 # Decode calldata from a transaction cast 4byte-decode 0xa9059cbb000000... # Decode with a known signature cast decode-calldata 'transfer(address,uint256)' 0xa9059cbb... This is useful when checking what a wallet is really asking you to sign.

Check chainId after connecting and listen for chainChanged. Then ask the wallet to switch, and add the chain if it is unknown. const target = '0xaa36a7'; // Sepolia 11155111 try { await window.ethereum.request({ method: 'wallet_switchEthereumChain', params: [{ chainId: target }] }); } catch (e) { if (e.code === 4902) { await window.ethereum.request({ method: 'wallet_addEthereumChain', params: [{ chainId: target, chainName: 'Sepolia', rpcUrls: [RPC], nativeCurrency: { name: 'ETH', symbol: 'ETH', decimals: 18 } }] }); } } Disable write buttons until the network is correct.