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?

a) 2^{n}

b) 2^{-n}

c) 2^{2n}

d) 2^{-2n}

View Answer

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?

a) 2^{-64}

b) 2^{64}

c) 2^{-112}

d) 2^{-128}

View Answer

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).

a) True

b) False

View Answer

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

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

View Answer

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

View Answer

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.

a) True

b) False

View Answer

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.

a) 2^{m}

b) 2^{(m-1)}

c) 2^{(m/2)}

d) (2^{m}) – 1

View Answer

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.

a) 2^{m}

b) 2^{(m-1)}

c) 2^{(m/2)}

d) (2^{m}) – 1

View Answer

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?

a) Pre-image

b) Second Pre-image

c) Collision

d) All required the same effort

View Answer

Explanation: Due to the birthday paradox it requires 2

^{(m/2)}computations only.

**Sanfoundry Global Education & Learning Series – Cryptography and Network Security.**

To practice all areas of Cryptography and Network Security, __here is complete set of 1000+ Multiple Choice Questions and Answers__.