CAS-004 Security Engineering Practice Question
A security engineer is reviewing how a Transport Layer Security session derives its keys and protects data. The engineer wants to identify the mechanisms that provide confidentiality and integrity for application data in TLS 1.3. (Choose two.)
⚠ Common exam trap
The trap here is crediting the certificate's signature or handshake randoms with protecting application data, when only the AEAD layer and its derived keys do that.
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
✓
HKDF-based key schedule using the transcript hash
In TLS 1.3, confidentiality and integrity for application data come from AEAD ciphers such as AES-GCM and ChaCha20-Poly1305, whose keys are produced by the HKDF-based key schedule bound to the handshake transcript. The certificate signature authenticates the peer, while randoms, session IDs, and compression do not protect record data.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
HKDF-based key schedule using the transcript hash
Why this is correct
The TLS 1.3 key schedule uses HKDF to derive secrets from the shared ECDHE value and the handshake transcript hash, producing traffic keys for each direction. Those derived keys are what the AEAD ciphers use, so the key schedule underpins confidentiality and integrity by ensuring unique, context-bound keys per session and epoch.
- ✗
The server certificate's RSA signature over the handshake
Why it's wrong here
The certificate signature authenticates the server's identity and proves possession of the private key, but it does not encrypt application data or provide integrity for records. Confidentiality and integrity come from the symmetric AEAD layer, not from the asymmetric signature that only binds the handshake to the certificate.
- ✗
The ClientHello random value and session ID
Why it's wrong here
The random value and session ID contribute entropy and session identification during the handshake. They are inputs that help derive keys but do not themselves encrypt or authenticate application records, so they do not provide the confidentiality and integrity guarantees the question asks about.
- ✓
AEAD ciphers such as AES-GCM and ChaCha20-Poly1305
Why this is correct
TLS 1.3 requires authenticated encryption with associated data for record protection. AES-GCM and ChaCha20-Poly1305 combine encryption with an authentication tag in a single operation, so they deliver both confidentiality of the plaintext and integrity of the ciphertext and associated header data in one pass, which is exactly the protection the engineer is looking for.
- ✗
Compression of the record payload before encryption
Why it's wrong here
Compression is removed in TLS 1.3 precisely because it enabled attacks such as CRIME and BREACH that infer plaintext from ciphertext length. It provides no confidentiality or integrity guarantee and would weaken the session, so it is not a mechanism that protects application data.
Quick reference
Symmetric Encryption Algorithm Comparison
| Algorithm | Key Size | Block Size | Status | Notes |
|---|---|---|---|---|
| AES-128 | 128-bit | 128-bit | Current standard | NIST approved; WPA3, TLS |
| AES-256 | 256-bit | 128-bit | Current standard | Preferred for sensitive / govt data |
| 3DES | 112-bit effective | 64-bit | Deprecated (2023) | Replaced by AES |
| DES | 56-bit | 64-bit | Broken | Cracked in < 24 h; never deploy |
| ChaCha20 | 256-bit | Stream cipher | Current | TLS 1.3, WireGuard |
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official CompTIA exam blueprint
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