The Three Factor Categories
Authentication factors fall into three categories: something you know (a password or PIN), something you have (a phone, hardware key, or smart card), and something you are (a fingerprint or face scan). True multi-factor authentication combines factors from at least two different categories — a password plus a security question is not MFA, because both are 'something you know.' OWASP and NIST SP 800-63B both emphasize that combining independent factor categories is what defeats an attacker who has compromised just one, since stealing a password doesn't also grant physical possession of a hardware key or biometric data.
Cricket analogy: It's like a stadium's VIP box requiring both a printed ticket (something you have) and a signature match against ID (something you are/know) — a forged ticket alone, or a guessed name alone, isn't enough to get past both checks.
TOTP, SMS, and Push Notifications
Time-based One-Time Password (TOTP), defined in RFC 6238, generates a 6-digit code from a shared secret and the current Unix time, refreshed every 30 seconds — apps like Google Authenticator and Authy implement this standard, and it works entirely offline once the secret is provisioned via QR code. SMS-based MFA is widely deployed but NIST SP 800-63B has deprecated it as a 'restricted' authenticator because SMS is vulnerable to SIM-swapping attacks, where a social-engineered carrier employee ports a victim's number to an attacker's SIM. Push notifications (approve/deny prompts sent to a registered app) improve usability over TOTP but introduced a new risk called 'MFA fatigue' or 'prompt bombing,' where attackers spam approval requests hoping the victim eventually taps 'approve' out of annoyance or confusion — mitigated by requiring number-matching (the app shows a number the user must enter on the login screen) rather than a simple approve/deny button.
Cricket analogy: TOTP is like a physical stopwatch-synced signal flare system between the scorers and the third umpire that changes every 30 seconds — SMS OTP is like relaying that signal through a courier who could be intercepted (SIM swap) before delivery.
# Python: generating and verifying a TOTP code (RFC 6238) using pyotp
import pyotp
# Provisioning: generate a secret and enrollment URI for the user's authenticator app
secret = pyotp.random_base32()
totp = pyotp.TOTP(secret)
provisioning_uri = totp.provisioning_uri(
name="[email protected]",
issuer_name="SkillVeris"
) # encode this as a QR code for the user to scan
# Verification: during login, check the code the user typed
user_submitted_code = "123456"
if totp.verify(user_submitted_code, valid_window=1): # allow 1 step (30s) of clock drift
print("MFA verified")
else:
print("Invalid or expired code")NIST SP 800-63B classifies SMS/voice as a 'restricted' authenticator, not fully deprecated, but agencies and security-conscious platforms are encouraged to migrate users toward app-based TOTP, push with number-matching, or FIDO2 hardware keys wherever possible.
WebAuthn and FIDO2 Hardware Keys
WebAuthn, standardized by the W3C and FIDO Alliance, replaces shared secrets entirely with public-key cryptography: during registration, the authenticator (a YubiKey, or a platform authenticator like Touch ID) generates a key pair, keeps the private key on-device (never transmitted), and sends the public key to the server. During login, the server sends a random challenge that the authenticator signs with the private key, proving possession without ever exposing a secret that could be phished, replayed, or database-leaked. This makes WebAuthn phishing-resistant in a way TOTP is not — a fake login page can still trick a user into typing a TOTP code into it (real-time phishing/relay), but it cannot extract or replay a WebAuthn signature because the challenge is bound to the origin domain.
Cricket analogy: It's like a player's identity being verified by a unique, non-copyable action — bowling a specific documented action recognized biomechanically by the ICC — rather than a memorized password that could be stolen and repeated by an impostor.
OWASP's Multifactor Authentication Cheat Sheet recommends offering users a choice of MFA methods where possible, with FIDO2/WebAuthn as the strongest phishing-resistant option, TOTP as a solid fallback, and SMS reserved as a last resort for users without smartphone access.
- True MFA combines factors from at least two of: something you know, something you have, something you are.
- TOTP (RFC 6238) generates time-synced 6-digit codes offline from a shared secret.
- SMS-based MFA is vulnerable to SIM-swapping and is classified as 'restricted' by NIST SP 800-63B.
- Push notifications improve usability but are vulnerable to 'MFA fatigue' / prompt-bombing attacks.
- Number-matching in push MFA mitigates prompt bombing by requiring active user input, not just a tap.
- WebAuthn/FIDO2 uses public-key cryptography bound to the origin domain, making it phishing-resistant.
- OWASP recommends offering a choice of MFA methods, prioritizing WebAuthn, then TOTP, with SMS as a last resort.
Practice what you learned
1. Why is a password combined with a security question not considered true multi-factor authentication?
2. What vulnerability has led NIST to classify SMS-based MFA as a 'restricted' authenticator?
3. What is 'MFA fatigue' or 'prompt bombing'?
4. Why is WebAuthn considered phishing-resistant in a way TOTP is not?
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