CAS-004 Security Engineering Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✗

  Better hardware acceleration support

  Why it's wrong here

  ChaCha20-Poly1305 is a software-oriented cipher designed for CPUs without AES-NI; AES-256-GCM is the one with ubiquitous hardware acceleration via AES-NI and vector extensions. Its genuine advantage is constant-time software performance on low-power or ARM devices lacking dedicated AES circuitry, not acceleration support.

• ✗

  Higher security margin

  Why it's wrong here

  ChaCha20-Poly1305 offers no higher security margin than AES-256-GCM; both target roughly 256-bit and 128-bit security respectively. The margin argument is tempting because ChaCha20's design is newer, but its real advantage is performance on CPUs lacking AES-NI hardware acceleration, such as some mobile and IoT chips.

• ✗

  Smaller ciphertext size

  Why it's wrong here

  ChaCha20-Poly1305 and AES-256-GCM both produce ciphertext of identical length for the same plaintext, since both are stream-based AEAD constructions with a 128-bit tag. The temptation is real: ChaCha20's 256-bit key and 96-bit nonce look compact, but they do not shrink output. Its actual advantage is speed on CPUs lacking AES-NI.

• ✓

  Faster performance on devices without AES-NI

  Why this is correct

  ChaCha20-Poly1305 relies solely on ARX operations, which execute efficiently in software on CPUs lacking AES-NI hardware acceleration. AES-256-GCM depends on dedicated AES instructions for competitive throughput, so on such constrained devices ChaCha20-Poly1305 delivers markedly faster encryption, directly satisfying the stem's "certain environments" constraint.

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