The correct action is to compute the Pairwise Master Key Identifier (PMKID) from the PSK and AP MAC. After cracking the WPA2 Pre-Shared Key (PSK), you derive the Pairwise Master Key (PMK) using PBKDF2 with the SSID, but to decrypt WPA2 traffic after cracking PSK, you still need the Pairwise Transient Key (PTK), which requires the nonces from the captured handshake. The PMKID, calculated from the PMK, AP MAC, and STA MAC, serves as a verification token that tools like aircrack-ng use to confirm the correct PMK before deriving the PTK and decrypting the traffic. On the Certified Ethical Hacker CEH exam, this tests your understanding of the WPA2 4-way handshake mechanics and the distinction between cracking the PSK and actually decrypting data—a common trap is assuming the PSK alone is sufficient. Remember: PSK gets you the PMK, but the PMKID unlocks the decryption. Memory tip: “PSK to PMK, PMKID to decrypt.”
CEH Wireless, IoT and Cloud Security Practice Question
This CEH practice question tests your understanding of wireless, iot and cloud security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
During a penetration test, an ethical hacker captures a WPA2 handshake and successfully cracks the PSK. Which additional action must be taken to decrypt previously captured traffic?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Compute the Pairwise Master Key Identifier (PMKID) from the PSK and AP MAC.
Option A is correct because after cracking the WPA2 Pre-Shared Key (PSK), the Pairwise Master Key (PMK) is derived from the PSK, SSID, and hashing iterations (PBKDF2). To decrypt previously captured traffic, you need the Pairwise Transient Key (PTK), which is computed from the PMK, the client nonce (SNonce), the AP nonce (ANonce), and both MAC addresses. However, the nonces are exchanged during the 4-way handshake and are present in the captured handshake; the missing piece is the PMKID, which is derived from the PMK, AP MAC, and STA MAC. Computing the PMKID from the cracked PSK and AP MAC allows tools like aircrack-ng to verify the correct PMK and then derive the PTK to decrypt the traffic.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Compute the Pairwise Master Key Identifier (PMKID) from the PSK and AP MAC.
Why this is correct
The PMKID is used to derive the PTK, enabling decryption.
Related concept
Read the scenario before looking for a memorised answer.
✗
Capture the nonce values from the access point.
Why it's wrong here
Nonces are already in the handshake capture.
✗
Use the PSK directly to compute the Michael Integrity Check (MIC).
Why it's wrong here
The MIC is for integrity, not decryption.
✗
Obtain the Group Temporal Key (GTK) from the access point.
Why it's wrong here
GTK is derived after PTK; not the first step.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often think the PSK alone is sufficient to decrypt traffic, but the PSK only gives the PMK; the PTK requires the nonces and MACs from the handshake, and the PMKID is the verification step that ensures the correct PMK is used for decryption.
Detailed technical explanation
How to think about this question
In WPA2, the 4-way handshake generates the PTK from the PMK, ANonce, SNonce, AP MAC, and STA MAC. The PMKID is computed as HMAC-SHA1(PMK, "PMK Name" || AP_MAC || STA_MAC) and is included in the first message of the handshake (or optionally in the second). By computing the PMKID from the cracked PSK and comparing it to the captured PMKID, tools confirm the correct PMK and then derive the PTK to decrypt all subsequent data frames. A real-world scenario is when a pentester captures a handshake offline and later cracks the PSK; without the PMKID verification, they cannot be certain the PSK is correct for that specific handshake, and decryption may fail.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the CEH exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CEH question in full detail.
Wireless, IoT and Cloud Security — This question tests Wireless, IoT and Cloud Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Compute the Pairwise Master Key Identifier (PMKID) from the PSK and AP MAC. — Option A is correct because after cracking the WPA2 Pre-Shared Key (PSK), the Pairwise Master Key (PMK) is derived from the PSK, SSID, and hashing iterations (PBKDF2). To decrypt previously captured traffic, you need the Pairwise Transient Key (PTK), which is computed from the PMK, the client nonce (SNonce), the AP nonce (ANonce), and both MAC addresses. However, the nonces are exchanged during the 4-way handshake and are present in the captured handshake; the missing piece is the PMKID, which is derived from the PMK, AP MAC, and STA MAC. Computing the PMKID from the cracked PSK and AP MAC allows tools like aircrack-ng to verify the correct PMK and then derive the PTK to decrypt the traffic.
What should I do if I get this CEH question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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