CEH Practice Question: Advanced Topics: Wireless, Cloud, IoT, Cryptography
A penetration tester is analyzing a captured TLS 1.3 handshake between a client and a server. The tester notices that the server's certificate is signed with RSA-PSS and the key exchange uses X25519. Which of the following statements is TRUE regarding the security of this handshake?
⚠ Common exam trap
The trap here is assuming that RSA-PSS is vulnerable to Bleichenbacher attacks, which actually target PKCS#1 v1.5 padding, or confusing key exchange with authentication.
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
✓
The handshake provides forward secrecy because X25519 is used for key exchange.
X25519 is an ephemeral elliptic-curve Diffie-Hellman function that provides forward secrecy. In TLS 1.3, forward secrecy is mandatory, and the use of X25519 ensures that session keys are unique and not derivable from the server's long-term private key. The other statements are incorrect because TLS 1.3 supports RSA-PSS, mitigates Bleichenbacher attacks, and separates authentication from key exchange.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The handshake provides forward secrecy because X25519 is used for key exchange.
Why this is correct
X25519 is an ephemeral Diffie-Hellman key exchange, which provides forward secrecy. In TLS 1.3, forward secrecy is mandatory for all cipher suites. The use of X25519 ensures that session keys are not compromised even if the server's long-term private key is later compromised, making this statement true.
- ✗
The handshake is susceptible to a Bleichenbacher attack due to RSA-PSS padding.
Why it's wrong here
Bleichenbacher attacks target RSA PKCS#1 v1.5 padding, not RSA-PSS. RSA-PSS is designed to be resistant to such padding oracle attacks. TLS 1.3 further mitigates these attacks by removing RSA key exchange and using only ephemeral key exchanges. Therefore, this statement is false.
- ✗
The handshake does not provide authentication because X25519 is used for key exchange.
Why it's wrong here
X25519 is used for key exchange, but authentication is provided by the server's certificate and its signature (RSA-PSS in this case). The key exchange and authentication are separate processes. The certificate proves the server's identity, so authentication is present. This statement is false.
- ✗
The handshake is vulnerable to a downgrade attack because TLS 1.3 does not support RSA-PSS.
Why it's wrong here
TLS 1.3 supports RSA-PSS for signatures. The use of RSA-PSS is actually a security improvement over PKCS#1 v1.5. Downgrade attacks are mitigated in TLS 1.3 by the use of the supported_versions extension and the downgrade protection mechanism. This statement is false.
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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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 EC-Council exam blueprint
This CEH practice question is part of Courseiva's free EC-Council certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CEH exam.