1. Which of the following is/are not perfectly secure encryption scheme(s):

• Playfair Cipher
• Vernam Cipher
• Affine Cipher
• Vigenère Cipher

Answer :- a, b, d

2. Which of the following is/are valid condition(s) for a perfectly-secure encryption scheme?

• Pr[M=m|C=c]=Pr[C=c]
• Pr[C=c|M=m0]=Pr[C=c|M=m1]
• Pr[M=m|C=c]=Pr[M=m]
• Pr[C=c|M=m]=Pr[M=m]

Answer :- b, c

3. Assume you are given a 500 -character encrypted message, encrypted in Vigenère cryptosystem, in which you know the plaintext word CRYPTOGRAPHY occurs exactly two times, and we know that the ciphertext sequence PTFDLSTTHDZC is the encryption of CRYPTOGRAPHY. The first occurrence starts at character position 24 and second at character position 198 (we start counting from 1
). What is the length of the key used for encryption?

• 5
• 6
• 8
• 9

Answer :- b

4. Which of the following statement(s) is/are correct?

(I)  Any encryption scheme with key length less than message length can not be a perfectly-secure scheme.
(II) A scheme is COA-secure if and only if it is KPA-secure.
(III) Given the one-time pad scheme, the scheme is still perfectly-secure if on any subsequent invocation of the key generation algorithm, an earlier key gets re-generated.
(IV) A scheme is CPA-secure if and only if it is CCA-secure.

• I and III
• I, II and III
• II and IV
• Neither of I, II, III or IV

Answer :- a

5.

Answer :- a, d