# Foundations of Cryptography | NPTEL | Week 10 Assignment Solutions

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### Foundations of Cryptography NPTEL Week 10 Assignment Solutions

Q1.

Answer: a)

Q2. Suppose in RSA cryptosystem, we have an e, which has at least one 0 in its binary representation. Now let e’ be another RSA public key, obtained by flipping one of the 0 bit in the binary representation of e to 1. Then

a) e and e’ are coprime
b) e and e’ are not coprime
c) e’ will be even
d) None of the above

Answer: a)

Q3. Suppose the El Gamal encryption scheme parameter-generation algorithms outputs safe prime (p,q) and the public-key (g,h), where g is the generator of a prime-order subgroup G of Zpâ‹†, where the order of G is q.

a) For CPA-security, the plaintext space has to be necessarily G
b) For CPA-security, the plaintext space does not matter
c) The scheme will not be COA-secure, if the plaintext space is Zpâ‹†
d) It is sufficient to have plaintext space as Zpâ‹† for COA-security

Answer: a), c)

Q4.

a) From the view point of the adversary, the output key is uniformly random and unknown
b) The adversary will completely know the underlying output key
c) From the view point of the adversary, the output key will be pseudorandom
d) None of the these

Answer: b)

Q5.

a) Function f is collision-resistant, provided CDH assumption holds in G
b) Function f is not collision-resistant
c) Function f is collision-resistant, provided DDH assumption holds in G
d) Function f is collision-resistant, provided DLog assumption holds in G

Answer: b)

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