Block n:
  index:     12345
  timestamp: 2026-04-19T10:00:00Z
  data:      [tx1: Alice→Bob 1BTC, tx2: Carol→Dave 0.5BTC]
  nonce:     48293
  prev_hash: 0000a8f3...
  hash:      0000c4d9...

Linked chain:
  Genesis Block → Block 1 → Block 2 → Block 3 → ...
  (each stores hash of previous block)

Private key: 5HueCGU8rMjxECyDialwujzT...  (KEEP SECRET)
Public key:  04bfcab8722991ae774db48f...  (share freely)
Address:     1PMycacnJaSqwwJqjawXBErnD...  (your 'account')

# Transaction signing
Tx = {from: address, to: bob, amount: 1 BTC}
signature = sign(private_key, hash(Tx))
# Network verifies: verify(public_key, signature, hash(Tx))

# Mining: find nonce where SHA256(block_data + nonce) < target
# Target: hash must start with many zeros (difficulty)

# Example: finding valid hash
for nonce in range(infinity):
    hash = SHA256(block_header + nonce)
    if hash.startswith('00000'):   # 5 zeros = simplified
        print(f'Valid block! nonce={nonce}')
        break  # broadcast to network

# Difficulty adjusts so block found every ~10 min
# Network hash rate now: ~600 EH/s (600 × 10^18 hashes/sec)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.19;

contract SimpleBank {
    mapping(address => uint256) private balances;

    event Deposit(address indexed user, uint256 amount);
    event Withdrawal(address indexed user, uint256 amount);

    function deposit() external payable {
        require(msg.value > 0, 'Amount must be > 0');
        balances[msg.sender] += msg.value;
        emit Deposit(msg.sender, msg.value);
    }

    function withdraw(uint256 amount) external {
        require(balances[msg.sender] >= amount, 'Insufficient balance');
        balances[msg.sender] -= amount;
        payable(msg.sender).transfer(amount);
        emit Withdrawal(msg.sender, amount);
    }

    function getBalance() external view returns (uint256) {
        return balances[msg.sender];
    }
}

# Libraries
ethers.js  — interact with Ethereum from JavaScript
web3.py    — Python Ethereum library
wagmi      — React hooks for Ethereum
Viem       — TypeScript Ethereum library

# Development environment
Hardhat    — smart contract development framework
Foundry    — fast Rust-based framework (Forge)
Remix IDE  — browser-based Solidity IDE (beginner)
Ganache    — local blockchain for testing

# ethers.js example
const { ethers } = require('ethers');
const provider = new ethers.JsonRpcProvider(ALCHEMY_URL);
const signer = new ethers.Wallet(PRIVATE_KEY, provider);

// Read contract
const contract = new ethers.Contract(ADDRESS, ABI, provider);
const balance = await contract.getBalance();

// Write transaction
const tx = await contract.connect(signer).deposit({ value: ethers.parseEther('0.1') });
await tx.wait();

# Testing with Hardhat
const [owner, addr1] = await ethers.getSigners();
const contract = await (await ethers.deployContract('MyContract')).waitForDeployment();
await expect(contract.connect(addr1).withdraw(100)).to.changeEtherBalance(addr1, 100);