CEH Practice Question: Advanced Topics: Wireless, Cloud, IoT, Cryptography
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?
⚠ Common exam trap
Watch out — 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).
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
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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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Birthday attack
Why this is correct
The Birthday attack leverages the mathematical 'birthday paradox' to significantly reduce the computational effort required to find two distinct inputs that produce the same hash output (a collision). For hash functions like MD5, which has a 128-bit output, finding a collision through brute force would theoretically require 2^64 attempts, making it practically feasible. This vulnerability undermines the integrity and collision resistance properties critical for cryptographic security, as demonstrated against MD5.
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Replay attack
Why it's wrong here
A replay attack involves an attacker intercepting valid data transmission, such as authentication credentials or session tokens, and then retransmitting it later to impersonate a legitimate user or repeat an authorized action. This attack does not aim to find mathematical collisions within a hash function itself, but rather exploits the lack of proper session management or nonce usage to reuse previously valid data, bypassing security controls without needing to break cryptographic primitives.
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Downgrade attack
Why it's wrong here
A downgrade attack forces a system to abandon a more secure, modern protocol version or cryptographic algorithm in favor of an older, weaker, and potentially vulnerable one. This typically occurs during the initial handshake phase of a communication session, where an attacker manipulates the negotiation to select a less robust option, making subsequent exploitation easier. It does not directly target the collision resistance of a hash function but rather exploits protocol negotiation weaknesses.
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Dictionary attack
Why it's wrong here
A dictionary attack attempts to crack passwords or passphrases by systematically trying a large list of commonly used words, phrases, and permutations, often pre-computed and stored in a 'dictionary' file. While it often involves hashing the dictionary entries to compare against a target hash, its objective is to discover the original plaintext input that produced a specific hash, not to find two different inputs that yield the same hash output, which is the goal of a collision attack.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.