🔒 Encryption vs Hashing

Encryption (2 chiều):
  "Hello" + Key → encrypt → "x7Kp9..." → decrypt + Key → "Hello"

Symmetric (cùng key): AES, ChaCha20
  encrypt(data, key) → ciphertext
  decrypt(ciphertext, key) → data

Asymmetric (2 keys): RSA, ECC
  encrypt(data, publicKey) → ciphertext
  decrypt(ciphertext, privateKey) → data

const crypto = require('crypto');

// === AES-256 Symmetric Encryption ===
const algorithm = 'aes-256-gcm';
const key = crypto.randomBytes(32); // 256-bit key

function encrypt(text) {
    const iv = crypto.randomBytes(16);
    const cipher = crypto.createCipheriv(algorithm, key, iv);
    let encrypted = cipher.update(text, 'utf8', 'hex');
    encrypted += cipher.final('hex');
    const authTag = cipher.getAuthTag();
    return { iv: iv.toString('hex'), encrypted, authTag: authTag.toString('hex') };
}

function decrypt({ iv, encrypted, authTag }) {
    const decipher = crypto.createDecipheriv(algorithm, key, Buffer.from(iv, 'hex'));
    decipher.setAuthTag(Buffer.from(authTag, 'hex'));
    let decrypted = decipher.update(encrypted, 'hex', 'utf8');
    decrypted += decipher.final('utf8');
    return decrypted;
}

const result = encrypt('Số thẻ: 4111-1111-1111-1111');
console.log('Encrypted:', result.encrypted);
console.log('Decrypted:', decrypt(result)); // → "Số thẻ: 4111-1111-1111-1111"

Hashing (1 chiều):
  "Hello" → hash → "185f8db3..." (KHÔNG thể giải ngược)
  "Hello" → hash → "185f8db3..." (cùng input = cùng output)
  "Hello!" → hash → "a8cfcd74..." (khác 1 ký tự = đổi hoàn toàn)

const crypto = require('crypto');
const bcrypt = require('bcrypt');

// === SHA-256 (Fast hash - cho file integrity) ===
const hash = crypto.createHash('sha256')
    .update('Hello World')
    .digest('hex');
console.log(hash); // "a591a6d4..." (luôn 64 ký tự)

// === bcrypt (Slow hash - cho passwords) ===
// Tại sao slow? Để chống brute-force!
async function hashPassword(password) {
    const saltRounds = 12; // chi phí tính toán cao
    return await bcrypt.hash(password, saltRounds);
}

async function verifyPassword(password, hash) {
    return await bcrypt.compare(password, hash);
}

// Sử dụng:
const hashed = await hashPassword('myPassword123');
// "$2b$12$LJ3m4ys..." (chứa salt + hash)

const isValid = await verifyPassword('myPassword123', hashed);
// true ✅ (KHÔNG cần giải mã, chỉ so sánh)