Implementing Multi-Factor Authentication (MFA)

1. Understanding MFA Factors

Factor	Type	Example
Knowledge	Something you know	Password, PIN, security question
Possession	Something you have	Phone, hardware key, smart card
Inherence	Something you are	Fingerprint, face, voice
Location	Somewhere you are	IP geo, GPS, network
Behavior	Something you do	Typing pattern, gait

Note: True MFA requires factors from different categories (e.g., password + phone, not password + security question).

2. Implementing SMS-Based OTP

Aspect	Detail
Code length	6 digits
Validity	5–10 min
Rate limit	1 send / 60 sec, 5 / hour
Verification attempts	5 max per code
Provider	Twilio, AWS SNS, Vonage
Risk	SIM swap — NIST deprecated SMS for high-assurance

3. Implementing Email-Based OTP

Aspect	Detail
Code length	6–8 digits or token link
Validity	10–15 min
Risk	Only as strong as user's email account
Deliverability	SPF/DKIM/DMARC configured

4. Using Time-Based OTP

Example: TOTP (RFC 6238) generation

import { authenticator } from "otplib";
const secret = authenticator.generateSecret();  // base32, 160 bits
const uri = authenticator.keyuri(user.email, "Acme", secret);
// Show QR code of uri to user

// Verify
const ok = authenticator.verify({ token: userInput, secret });

Parameter	Default
Algorithm	HMAC-SHA1
Digits	6
Period	30 sec
Window	±1 step (clock skew)

5. Implementing Authenticator Apps

App	Notes
Google Authenticator	Cloud-sync since 2023
Microsoft Authenticator	Push notifications + TOTP
Authy	Multi-device, encrypted backup
1Password / Bitwarden	Built into password managers
Provisioning URI	`otpauth://totp/Issuer:user?secret=...&issuer=...`

6. Using Hardware Tokens

Token	Standard	Phish-Resistant?
YubiKey	FIDO2 / WebAuthn / U2F / OTP	Yes (FIDO2)
Titan Key	FIDO2 / U2F	Yes
RSA SecurID	OTP	No
Smart Card (PIV)	X.509	Yes

7. Implementing Biometric Authentication

Platform	API
iOS	LocalAuthentication / Face ID, Touch ID
Android	BiometricPrompt
Web	WebAuthn (platform authenticator)
Windows	Windows Hello

8. Implementing Push Notifications

Push MFA Flow

User submits password → backend issues challenge ID
Push notification sent to registered device
User taps Approve / Deny in app (biometric optional)
App signs challenge with device key → posts to server
Server verifies signature → completes auth
Display number-matching code to prevent fatigue attacks

Warning: Plain "approve" push prompts are vulnerable to MFA fatigue. Use number matching (user enters/confirms 2-digit code shown on login screen).

9. Setting Up Backup Codes

Aspect	Detail
Count	8–10 codes
Format	10-char alphanumeric (e.g., `x7k9-p4m2-q1n8`)
Storage	Hashed (bcrypt/argon2) — not retrievable
Use	Single-use, mark consumed on success
Regenerate	Invalidate all on regen

10. Enforcing MFA Policies

Policy	Trigger
Required	All users (highest assurance)
Risk-based	New device, suspicious IP
Role-based	Admins always, others optional
Resource-based	Step-up for sensitive endpoints
Grace period	14 days to enroll after signup

11. Implementing Step-Up Authentication

Example: ACR-based step-up (OIDC)

// Sensitive endpoint requires stronger AuthN
app.delete("/account", (req, res, next) => {
  if (req.user.acr !== "urn:mfa:phishing-resistant") {
    return res.status(401).json({
      error: "insufficient_authentication",
      acr_values: "urn:mfa:phishing-resistant",
      max_age: 300                  // require recent auth
    });
  }
  next();
});

12. Handling MFA Enrollment Flow

TOTP Enrollment

User initiates "Enable 2FA"
Server generates secret + QR code (provisional, NOT yet saved as enrolled)
User scans QR in authenticator app
User enters current TOTP code to confirm
Server verifies code → marks MFA enabled, generates backup codes
Display backup codes ONCE → user must save/print
Send confirmation email

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