312-50 Question 470
Single answer▪ CryptanalysisDuring an authorized wireless security assessment, you capture a WPA2-PSK 4-way handshake from a client connecting to the company guest network. The client is already disconnected, and the rules of engagement prohibit deauthentication or other active attacks. The security manager wants to know which technique gives you the best chance of recovering the Wi-Fi passphrase from the captured data without interacting with the network further. What should you do next?
- A
Perform an offline dictionary or rule-based attack against the captured handshake using candidate passphrases
- B
Use a chosen-plaintext attack against the access point to force predictable encrypted traffic and derive the PSK
- C
Attempt a ciphertext-only attack directly against the handshake to mathematically recover the plaintext passphrase without guesses
- D
Exploit a collision attack against SHA-1 in the 4-way handshake to reveal the Pairwise Master Key
Show answer and explanation
Correct answer: A
Explanation
The most effective and realistic next step is an offline dictionary or rule-based attack against the captured WPA2-PSK 4-way handshake. In WPA/WPA2-Personal, the passphrase is converted into a 256-bit PMK using PBKDF2-HMAC-SHA1 with the SSID as salt and 4096 iterations, as documented in IEEE 802.11i and described in Wi-Fi security references. Because the handshake includes values needed to validate a guessed PMK, an ethical hacker can test candidate passwords offline without sending additional traffic. This is why weak or predictable passphrases remain a major risk in WPA2-PSK deployments. The other options misuse cryptanalytic terms: chosen-plaintext attacks require active influence, ciphertext-only attacks do not reflect the structure of WPA2-PSK cracking, and SHA-1 collision attacks are not a practical means of recovering the Wi-Fi passphrase from a captured handshake.
- A. Correct.
Correct. In WPA2-PSK, the captured 4-way handshake provides enough material to verify password guesses offline. A tester can use dictionary, mask, hybrid, or rule-based attacks against the passphrase by deriving the Pairwise Master Key (PMK) from each candidate using PBKDF2-HMAC-SHA1, then checking whether the derived values match the handshake data. This is the standard practical cryptanalytic approach when a valid handshake has been captured and no further interaction is allowed.
- B. Incorrect.
Incorrect. A chosen-plaintext attack requires the ability to influence plaintext input and observe resulting ciphertext under controlled conditions. In this scenario, the tester is explicitly prohibited from active interaction such as forcing reconnects or manipulating traffic. More importantly, WPA2-PSK recovery in practice is typically performed by offline password guessing against the handshake, not by deriving the PSK through a chosen-plaintext technique against the access point.
- C. Incorrect.
Incorrect. A ciphertext-only attack assumes the attacker has only ciphertext and attempts to recover plaintext or key material without additional structure or guesses. That does not reflect the practical attack path for WPA2-PSK. The handshake does not allow direct mathematical recovery of the passphrase from captured packets alone; instead, it enables validation of guessed passphrases offline.
- D. Incorrect.
Incorrect. Although PBKDF2-HMAC-SHA1 is involved in WPA/WPA2-PSK key derivation, collision attacks on SHA-1 are not a practical method for recovering a WPA2-PSK passphrase from a captured 4-way handshake. Collision resistance issues do not translate into a workable attack for deriving the PMK or PSK in this scenario. This option reflects a common misconception that any weakness associated with SHA-1 automatically breaks all protocols that use it.