Quick Answer
The answer is ECB mode, because it produces ciphertext that is the same length as the plaintext when the plaintext aligns with the block size. In Electronic Codebook mode, each 16-byte block is encrypted independently with the same hardcoded key, so no initialization vector or authentication tag is added, meaning the output length matches the input exactly. On the Certified Ethical Hacker CEH exam, this scenario tests your understanding of how ECB differs from modes like CBC or GCM, which add extra data and thus increase ciphertext length. A common trap is assuming any block cipher mode changes length, but ECB’s lack of chaining or IV is the key giveaway. Remember the mnemonic: ECB Equals Ciphertext Bytes—no extra baggage.
This CEH practice question tests your understanding of cryptography and malware analysis. 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.
An ethical hacker is analyzing a piece of malware that uses a custom encryption algorithm. The malware sample contains a hardcoded key that is 16 bytes long. The analyst observes that the encrypted data is the same length as the plaintext. Which encryption mode is most likely being used?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
GCM
Why wrong: GCM adds an authentication tag, increasing length.
CFB
Why wrong: CFB mode produces ciphertext same length as plaintext but often used with feedback.
ECB
ECB encrypts each block independently; no IV, no expansion beyond padding.
CBC
Why wrong: CBC requires an IV and padding, potentially increasing length.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
ECB
ECB (Electronic Codebook) mode encrypts each block of plaintext independently using the same key, so the ciphertext length equals the plaintext length (assuming no padding is needed for exact block sizes). The hardcoded 16-byte key and identical input/output lengths strongly suggest ECB, as other modes typically add an IV or authentication tag, altering the output length.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
GCM
Why it's wrong here
GCM adds an authentication tag, increasing length.
CFB
Why it's wrong here
CFB mode produces ciphertext same length as plaintext but often used with feedback.
ECB
Why this is correct
ECB encrypts each block independently; no IV, no expansion beyond padding.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
CBC
Why it's wrong here
CBC requires an IV and padding, potentially increasing length.
Common exam traps
The trap here is that candidates often forget that ECB does not use an IV or authentication tag, so they incorrectly assume all block cipher modes add overhead, leading them to choose CBC or GCM despite the length constraint.
Detailed technical explanation
ECB mode works by dividing plaintext into 16-byte blocks (for AES) and encrypting each block independently with the same key. This deterministic behavior means identical plaintext blocks produce identical ciphertext blocks, a weakness that can leak patterns in the data. In malware analysis, ECB is sometimes used for simplicity, but its lack of diffusion makes it unsuitable for secure encryption; analysts can detect ECB by looking for repeated ciphertext blocks in the output.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Cryptography and Malware Analysis — This question tests Cryptography and Malware Analysis — Read the scenario before looking for a memorised answer..
The correct answer is: ECB — ECB (Electronic Codebook) mode encrypts each block of plaintext independently using the same key, so the ciphertext length equals the plaintext length (assuming no padding is needed for exact block sizes). The hardcoded 16-byte key and identical input/output lengths strongly suggest ECB, as other modes typically add an IV or authentication tag, altering the output length.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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.
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