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Cryptography and Network Security MCQs

Cryptography and Network Security Questions and Answers Part-2

1. Reduce the following big-O natations:
O[ ax7 + 3 x3 + sin(x)] =
a) O[ax7].
b) O[sin(x)].
c) O[x7].
d) O[x7 + x3].

Answer: c
Explanation: O[ ax7 + 3 x3 + sin(x)] = O(ax7) = O(x7)

2. Reduce the following big-O natations:
O[ en + an10] =
a) O[ an10 ].
b) O[ n10 ].
c) O[ en ].
d) O[ en + n10 ].

Answer: c
Explanation: O[ en + an10] = O[ en ].

3. Reduce the following big-O natations:
O [ n! + n50 ] =
a) O [ n! + n50 ].
b) O [ n! ].
c) O [ n50].
d) None of the Mentioned

Answer: b
Explanation: O [ n! + n50 ] = O [ n! ].

4. Use Caesar’s Cipher to decipher the following
HQFUBSWHG WHAW
a) ABANDONED LOCK
b) ENCRYPTED TEXT
c) ABANDONED TEXT
d) ENCRYPTED LOCK

Answer: b
Explanation: Caesar Cipher uses C =(p+3) mod 26 to encrypt.

5. Caesar Cipher is an example of
a) Poly-alphabetic Cipher
b) Mono-alphabetic Cipher
c) Multi-alphabetic Cipher
d) Bi-alphabetic Cipher

Answer: b
Explanation: Caesar Cipher is an example of Mono-alphabetic cipher, as single alphabets are encrypted or decrypted at a time.

6. Monoalphabetic ciphers are stronger than Polyalphabetic ciphers because frequency analysis is tougher on the former.
a) True
b) False

Answer: b
Explanation: Monoalphabetic ciphers are easier to break because they reflect the frequency of the original alphabet.

7. Which are the most frequently found letters in the English language ?
a) e,a
b) e,o
c) e,t
d) e,i

Answer: c
Explanation: The relativity frequency of these letters in percent : e-12.702, a-8.167, t-9.056, i-6.996, o-7.507.

8. Choose from among the following cipher systems, from best to the worst, with respect to ease of decryption using frequency analysis.
a) Random Polyalphabetic, Plaintext, Playfair
b) Random Polyalphabetic, Playfair, Vignere
c) Random Polyalphabetic, Vignere, Playfair, Plaintext
d) Random Polyalphabetic, Plaintext, Beaufort, Playfair

Answer: c
Explanation: Random Polyalphabetic is the most resistant to frequency analysis, followed by Vignere, Playfair and then Plaintext.

9. On Encrypting “thepepsiisintherefrigerator” using Vignere Cipher System using the keyword “HUMOR” we get cipher text-
a) abqdnwewuwjphfvrrtrfznsdokvl
b) abqdvmwuwjphfvvyyrfznydokvl
c) tbqyrvmwuwjphfvvyyrfznydokvl
d) baiuvmwuwjphfoeiyrfznydokvl

Answer: b
Explanation: Cipher text:= Ci = Pi + ki mod m (mod 26).

10. On Encrypting “cryptography” using Vignere Cipher System using the keyword “LUCKY” we get cipher text
a) nlazeiibljji
b) nlazeiibljii
c) olaaeiibljki
d) mlaaeiibljki

Answer: a
Explanation: Cipher text:= Ci = Pi + ki mod m (mod 26).

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