Authentication

Identity and security

Authentication verifies who you are. Authorization verifies what you can do.

Authentication methods

Method	When to use
OAuth	Login with Google, GitHub
Email/Password	Own users
Magic Links	Passwordless
JWT	Stateless APIs

OAuth 2.0 Flow

User → Your app → Provider (Google)
                        ↓
User ← Your app ← Token + Info

JWT (JSON Web Tokens)

// Structure: header.payload.signature
const token = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..."

// Content (payload)
{
  "sub": "user123",
  "email": "user@email.com",
  "exp": 1699999999
}

Recommended services

Service	Type	Ideal for
Firebase Auth	BaaS	Mobile, web apps
Supabase Auth	BaaS	Full-stack
Auth0	SaaS	Enterprise
NextAuth.js	Library	Next.js apps

Basic security

Practice	Why
HTTPS always	Encrypts traffic
Short tokens	Limits damage if stolen
Refresh tokens	Renew without re-login
Rate limiting	Prevents brute force

🏦 Security for Financial Apps

If your app handles money or financial data, you need additional measures.

Required Checklist

Requirement	Why	Standard
✅ Mandatory MFA	PSD2/Open Banking regulation	NIST 800-63B
✅ Immutable logs	Regulatory audits	SOC 2
✅ Encryption at rest	Sensitive data protection	PCI DSS
✅ Session timeout	Fraud prevention	OWASP
✅ Aggressive rate limiting	Anti-scraping	-

What is PCI DSS?

If your app processes, stores or transmits payment card data, you must comply with PCI DSS.

Level	Transactions/year	Requirements
4	< 20,000	SAQ (Self-Assessment)
3	20,000 - 1M	SAQ + quarterly scan
2	1M - 6M	SAQ + audit
1	> 6M	Full annual audit

💡 Tip: Use Stripe, PayPal or similar to avoid handling card data directly. They assume the compliance burden.

MFA Implementation

// Verify second factor before sensitive operations
async function requireMFA(userId: string, action: string) {
  const user = await getUser(userId)

  if (SENSITIVE_ACTIONS.includes(action) && !user.mfaVerifiedAt) {
    throw new Error('MFA_REQUIRED')
  }

  // Audit log
  await auditLog({
    userId,
    action,
    mfaVerified: true,
    timestamp: new Date().toISOString(),
    ip: request.ip
  })
}

const SENSITIVE_ACTIONS = [
  'TRANSFER_FUNDS',
  'CHANGE_PASSWORD',
  'ADD_BENEFICIARY',
  'EXPORT_DATA'
]

Practice

→ Auth with Firebase Google