CAS-004 Security Engineering Practice Question
A security analyst is reviewing cryptographic implementations for a new application. The application needs to support digital signatures that are quantum-resistant and provide high performance. Which TWO algorithms should the analyst consider? (Select TWO.)
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
✓
XMSS (eXtended Merkle Signature Scheme)
XMSS (eXtended Merkle Signature Scheme) (B) is correct because it is a hash-based, post-quantum digital signature scheme standardized in NIST SP 800-208, whose security relies only on hash function properties and is therefore resistant to Shor's algorithm attacks from quantum computers. CRYSTALS-Dilithium (E) is correct because it is a lattice-based post-quantum digital signature algorithm selected by NIST (FIPS 204) that offers strong quantum resistance with efficient signing and verification performance, making it well suited for high-performance applications. Ed25519 (A) is not correct because it is an elliptic-curve signature scheme (EdDSA over Curve25519) that is vulnerable to quantum attacks via Shor's algorithm. ECDSA P-384 (C) is not correct for the same reason: it is a classical elliptic-curve signature algorithm, not quantum-resistant. BLAKE3 (D) is not correct because it is a cryptographic hash function, not a digital signature algorithm, so it cannot fulfill the digital signature requirement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Ed25519
Why it's wrong here
Ed25519 is a fast classical Edwards-curve signature scheme, but its security rests on elliptic-curve discrete logarithms, which quantum computers solve. It is tempting for its speed and small keys, and would be correct for high-performance signing where quantum resistance is not required.
- ✓
XMSS (eXtended Merkle Signature Scheme)
Why this is correct
XMSS is a stateful hash-based signature scheme whose security rests solely on hash-function collision resistance, so it resists quantum attacks via Shor's algorithm. It satisfies the quantum-resistance requirement while signing and verifying quickly, meeting the high-performance constraint. Its stateful nature, however, demands careful key-state management to prevent reuse of a one-time key.
- ✗
ECDSA P-384
Why it's wrong here
ECDSA P-384 relies on elliptic-curve discrete logarithms, which Shor's algorithm breaks on a quantum computer, so it is not quantum-resistant. It is tempting as a high-performance classical signature, and would be correct where quantum resistance is not a requirement.
- ✗
BLAKE3
Why it's wrong here
BLAKE3 is a cryptographic hash function, producing digests for integrity and key derivation, not digital signatures. It is tempting because it is fast and modern, and it would be correct where high-throughput hashing or message authentication is required instead of signing.
- ✓
CRYSTALS-Dilithium
Why this is correct
CRYSTALS-Dilithium is a lattice-based post-quantum signature scheme, selected by NIST for standardisation, that resists quantum attacks while signing and verifying far faster than hash-based alternatives. It directly satisfies the stem's twin constraints of quantum-resistant digital signatures and high performance, making it a suitable choice for the new application.
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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