# Foundations of Cryptography | NPTEL | Week 3 Assignment Solutions

This set of MCQ(multiple choice questions) focuses on the Foundations of Cryptography NPTEL Week 3 Assignment Solutions.

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### Foundations of Cryptography NPTEL Week 3 Assignment Solutions

Q1. Select the incorrect option(s) from the following:

a) Stream ciphers are deterministic
b) Stream ciphers are randomized
c) Same key can be used for encrypting multiple messages in stream ciphers
d) Stream ciphers are single-message CPA-secure

Q2. Which of the following is/are false for One-Way Functions (OWF)?

a) They are randomized functions
b) They are difficult to invert for all elements from the co-domain
c) They are always one-to-one function
d) They are deterministic functions

Q3. Which of the following candidates is(are) not necessarily secure PRF(s)? In all the following options, Fs (x) is a secure PRF, where x,s∈{0,1}n and output is also an n-bit string.

a) Gs(x) = Fs(x) Fs(x̄)
b) Gs(x) = 1 || Fs(x), where || denotes the concatenation operation
c) Gs(x) = F Fs(x) (x)

Q4.

a) Algorithm Enck is single-message COA-secure
b) Algorithm Enck is single-message CPA-secure
c) Algorithm Enck is multi-message CPA-secure
d) None of these

Q5. Let II = (Gen, Enc, Dec) be a symmetric-key cipher. Then consider a variant II’ = (Gen, Enc’, Dec’) of II, where Enc’k(m) = Enck (reverse(m)). The steps of Dec’ are performed accordingly. Here reverse(m) denotes the bit string obtained by reversing the order of the bits of m.

a) If II is single-message COA-secure, then II’ is also single-message COA-secure
b) If II is multi-message COA-secure, then II’ is also multi-message COA-secure
c) If II is CPA-secure, then II’ is also single-message CPA-secure
d) None of these