This set of Cryptography Multiple Choice Questions & Answers (MCQs) focuses on “Hash Functions Based on Cipher Block Chaining”.
1. What is the effectiveness of an n-bit hash value?
Explanation: When an n-bit hash value is used its effectiveness is 2^-n, that is, the probability that a data error will result in an unchanged hash value is 2-n.
2. What is the effectiveness of a 128 bit hash value?
Explanation: In most normal text files, the high order bit of each octet is always zero. So if a 128 bit hash value is used, instead of an effectiveness of 2-128, the hash function will have an effectiveness of 2-112.
3. We define collision as: A collision occurs if we have x=y and H(x) = H(y).
Explanation: A collision occurs if we have x not equal to y and H(x) = H(y).
4. Consider the following properties
Variable Input size
Fixed Output size
Pre image resistant
Second Pre image Resistant
A has function that satisfies the first _______ properties in the above table is referred to as a weak hash function.
Explanation: If the sixth property is also satisfied it is referred to as a strong hash function.
5. The second pre-image resistant property is
a) It is computationally infeasible to find any pair (x, y) such that H(x) = H(y)
b) For any given block x, it is computationally infeasible to find y not equal to x, with H(y) = H(x)
c) For any given hash value h it is computationally infeasible to find y such that H(y) = h
d) None of the mentioned
Explanation: The second pre-image property is defined by: For any given block x, it is computationally infeasible to find y not equal to x, with H(y) = H(x).
6. A function that is second pre-image resistant is also collision resistant.
Explanation:The statement is false. A function that is collision resistant is also second image resistant.
7. For an m-bit value, the adversary would have to try ____________ values to generates a given hash value h.
d) (2m) – 1
Explanation: The adversary would have to try 2(m-1) values to generates a given hash value h.
8. For an m bit hash value, if we pick data blocks at random we can expect to find two data blocks with the same hash value within ____ attempts.
d) (2m) – 1
Explanation: This is known as the birthday paradox. If we choose random variables from a uniform distribution in the range 0 through N-1, then the probability that a repeated element is encountered exceeds 0.5 after root (N) choices have been made.
9. Which attack requires the least effort/computations?
b) Second Pre-image
d) All required the same effort
Explanation: Due to the birthday paradox it requires 2(m/2) computations only.
Sanfoundry Global Education & Learning Series – Cryptography and Network Security.
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