Which of the following is a hash-based digital signature scheme included in the post-quantum candidate list?

• A. SPHINCS+
• B. CRYSTALS-Kyber
• C. CRYSTALS-Dilithium
• D. AES

Answer: (A)

Hash-based digital signatures rely on cryptographic hash functions to produce and verify signatures, rather than on number-theoretic hard problems. SPHINCS+ is a leading example in this class, designed to be resistant to quantum attacks by using hash chains and Merkle trees, which depend on the security of the hash function rather than on factoring or lattice problems. This makes it a natural fit for post-quantum candidate lists, since its security rests on hashing instead of quantum-exposed math problems. The other options come from different cryptographic families: CRYSTALS-Kyber is a lattice-based key encapsulation method (not a signature); CRYSTALS-Dilithium is a lattice-based signature scheme (not hash-based); AES is a symmetric cipher, not a public-key signature scheme. Therefore SPHINCS+ is the hash-based option included in the post-quantum candidate list.