CEH Practice Question: Advanced Topics: Wireless, Cloud, IoT, Cryptography
Which cryptographic algorithm is vulnerable to a birthday attack on its hash output size of 128 bits, reducing the effective security to 64 bits against collision resistance?
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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MD5
MD5 produces 128-bit hashes. Birthday attack complexity for collision is 2^(n/2) = 2^64 for n=128. SHA-1 is 160-bit, SHA-256 is 256-bit, RC4 is a stream cipher not a hash.
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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SHA-256
Why it's wrong here
SHA-256, while theoretically susceptible to a birthday attack as a hash function, produces a 256-bit output. This large output size provides a collision resistance of approximately 2^128, meaning finding a collision using current computational capabilities is practically infeasible. Therefore, SHA-256 is considered robust against birthday attacks and is not deemed vulnerable in a practical sense.
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SHA-1
Why it's wrong here
SHA-1 generates a 160-bit hash output, which implies a theoretical birthday attack collision resistance of 2^80 operations. Although significantly weaker than SHA-256, achieving a practical collision for SHA-1 using a birthday attack still demands immense computational resources and specialized hardware. While theoretical weaknesses exist and practical attacks have been demonstrated, it is not the most vulnerable option presented, especially compared to MD5.
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MD5
Why this is correct
MD5 produces a 128-bit hash value, which means its collision resistance against a birthday attack is approximately 2^64 operations. This level of complexity is well within the reach of modern computing resources, allowing for practical collision finding within a reasonable timeframe. Consequently, MD5 is widely considered cryptographically broken and highly vulnerable to birthday attacks, making it unsuitable for security-critical applications requiring collision resistance.
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RC4
Why it's wrong here
RC4 is a stream cipher, a type of cryptographic algorithm designed for encrypting data streams by generating a pseudorandom keystream. The birthday attack specifically targets hash functions by exploiting the probability of finding two different inputs that produce the same fixed-size output hash. Since RC4 does not produce hash outputs and is not designed for collision resistance, the concept of a birthday attack does not apply to its functionality.
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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.