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1. What is the effectiveness of an n-bit hash value?
a) 2n
b) 2-n
c) 22n
d) 2-2n

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Answer: b [Reason:] 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?
a) 2-64
b) 264
c) 2-112
d) 2-128

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Answer: c [Reason:] 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).
a) True
b) False

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Answer: b [Reason:] 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
Efficiency
Pre image resistant
Second Pre image Resistant
Collision resistant
Pseudo randomness

A has function that satisfies the first _______ properties in the above table is referred to as a weak hash function.
a) 5
b) 4
c) 3
d) 2

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Answer: a [Reason:] 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

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Answer: b [Reason:] 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.
a) True
b) False

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Answer: b [Reason:]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.
a) 2m
b) 2(m-1)
c) 2(m/2)
d) (2m) – 1

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Answer: b [Reason:] 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.
a) 2m
b) 2(m-1)
c) 2(m/2)
d) (2m) – 1

View Answer

Answer: c [Reason:] 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?
a) Pre-image
b) Second Pre-image
c) Collision
d) All required the same effort

View Answer

Answer: c [Reason:] Due to the birthday paradox it requires 2(m/2) computations only.