CAS-004 Security Engineering Practice Question
A security architect is designing a new authentication system for a cloud-based application that requires strong multi-factor authentication. The solution must be resistant to phishing attacks and not rely on shared secrets. Which of the following is the BEST choice?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
FIDO2/WebAuthn
FIDO2/WebAuthn is a passwordless authentication protocol that uses public key cryptography and is resistant to phishing because the private key never leaves the user's device.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
HOTP with a hardware token
Why it's wrong here
HOTP counters are derived from a shared symmetric seed, and entered codes can be relayed by an attacker in real time, so it fails the no-shared-secret and phishing-resistance requirements. It is tempting because hardware tokens provide possession-based factors, suiting environments where replay-resistant public-key protocols are unavailable.
- ✓
FIDO2/WebAuthn
Why this is correct
FIDO2/WebAuthn satisfies both constraints by using public-key cryptography: the authenticator holds a private key, while the server stores only the public key, so no shared secret crosses the wire. Origin binding ties each credential to the legitimate domain, defeating phishing proxies that replay credentials against lookalike sites.
- ✗
TOTP via a mobile authenticator app
Why it's wrong here
TOTP derives codes from a shared seed secret, and codes are replayable through real-time phishing proxies, so it fails both the no-shared-secret and phishing-resistance requirements. It is tempting because authenticator apps are widely deployed and avoid SMS interception, making TOTP suitable where phishing resistance is not mandated.
- ✗
SMS one-time passcodes
Why it's wrong here
SMS one-time passcodes are bearer codes delivered over telecom networks, vulnerable to SIM swapping, interception and real-time relay phishing, and they rely on a shared secret. They are tempting because they need no app or token, suiting low-assurance consumer scenarios where phishing resistance is not required.
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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