Quick Answer
The correct answer is the birthday attack. This attack exploits the birthday paradox from probability theory to find two different inputs that produce the same MD5 hash value, known as a collision, with far less computational effort than a brute-force preimage attack. For MD5’s 128-bit output, the birthday attack requires only about 2^64 operations, a threshold that modern hardware can reach, making MD5 fundamentally broken for security purposes. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of hash vulnerabilities and often appears in questions about cryptographic weaknesses or digital signature forgery. A common trap is confusing the birthday attack with a preimage attack—remember that the birthday attack seeks any collision, not a specific target hash. Memory tip: “Birthday collisions happen twice as fast—half the bits, half the work.”
This CEH practice question tests your understanding of advanced topics: wireless, cloud, iot, cryptography. 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.
Which of the following is a well-known attack against the MD5 hash function that allows two different inputs to produce the same hash value?
Birthday attack
Correct: The birthday attack exploits the birthday paradox to find collisions in hash functions like MD5.
Replay attack
Why wrong: Replay attacks capture and retransmit data, not produce hash collisions.
Downgrade attack
Why wrong: Downgrade attacks force weaker protocol versions.
Dictionary attack
Why wrong: Dictionary attacks try many passwords, not hash collisions.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Birthday attack
The birthday attack exploits the birthday paradox in probability theory to find two different inputs that produce the same MD5 hash value (a collision) with significantly less effort than a brute-force preimage attack. For an n-bit hash, the birthday attack requires only about 2^(n/2) operations, making MD5's 128-bit output vulnerable to collisions in roughly 2^64 attempts, which is computationally feasible today.
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.
Birthday attack
Why this is correct
Correct: The birthday attack exploits the birthday paradox to find collisions in hash functions like MD5.
Related concept
Read the scenario before looking for a memorised answer.
Replay attack
Why it's wrong here
Replay attacks capture and retransmit data, not produce hash collisions.
Downgrade attack
Why it's wrong here
Downgrade attacks force weaker protocol versions.
Dictionary attack
Why it's wrong here
Dictionary attacks try many passwords, not hash collisions.
Common exam traps
The trap here is that candidates often confuse the birthday attack with a dictionary attack because both involve generating many inputs, but the birthday attack specifically targets collision resistance (two different inputs, same hash) while a dictionary attack targets preimage resistance (finding an input that matches a given hash).
Detailed technical explanation
The birthday attack works by generating many inputs, computing their MD5 hashes, and storing them in a hash table; when two inputs produce the same hash, a collision is found. For MD5, practical collision attacks (like the 2004 Wang et al. attack) can find collisions in seconds on modern hardware, which is why MD5 is considered broken for security-sensitive applications such as digital signatures or certificate validation. Real-world examples include the Flame malware using an MD5 collision to forge a Microsoft code-signing certificate.
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
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FAQ
Advanced Topics: Wireless, Cloud, IoT, Cryptography — This question tests Advanced Topics: Wireless, Cloud, IoT, Cryptography — Read the scenario before looking for a memorised answer..
The correct answer is: Birthday attack — The birthday attack exploits the birthday paradox in probability theory to find two different inputs that produce the same MD5 hash value (a collision) with significantly less effort than a brute-force preimage attack. For an n-bit hash, the birthday attack requires only about 2^(n/2) operations, making MD5's 128-bit output vulnerable to collisions in roughly 2^64 attempts, which is computationally feasible today.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 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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