# Cryptography Questions and Answers – Overview – II

This set of Cryptography Multiple Choice Questions & Answers (MCQs) focuses on “Overview -2”.

1. The multiplicative Inverse of 550 mod 1769 is –
a) 434
b) 224
c) 550
d) Does not exist

Explanation: The multiplicative Inverse of 550 mod 1769 is 550.

2. It is desirable to revoke a certificate before it expires because –
a) the user is no longer certified by this CA
b) the CA’s certificate is assumed to be compromised
c) the user’s private key is assumed to be compromised
d) all of the mentioned

Explanation: All of the options are true with regard to revocation of a certificate.

3. Suppose that A has obtained a certificate from certification authority X1 and B has obtained certificate authority from CA X2. A can use a chain of certificates to obtain B’s public key. In notation of X.509, the correct order of this chain is represented as –
a) X2 X1 X1 B
b) X1 X1 X2 A
c) X1 X2 X2 B
d) X1 X2 X2 A

Explanation: The correct representation would be X1 X2 X2 B.

4. Certificate extensions fall into 3 categories. Which one of the following is not a Certificate extensions category?
a) Subject and Issuer attributes
b) Key and Policy information
c) Certification path constraints
d) All of the above are Certificate extensions categories

Explanation: Subject and Issuer attributes, Key and Policy information and Certification path constraints are the three categories of Certificate extensions.

5. A total of seven messages are required in the Public-Key distribution scenario. However, the initial five messages need to be used only infrequently because both A and B can save the other’s public key for future – a technique known as ____________
a) time stamping
b) polling
c) caching
d) squeezing

Explanation: This technique is known as caching.
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6. Which of the following slows the cryptographic algorithm –
1) Increase in Number of rounds
2) Decrease in Block size
3) Decrease in Key Size
4) Increase in Sub key Generation

a) 1 and 3
b) 2 and 3
c) 3 and 4
d) 2 and 4

Explanation: Increase in any of the above 4 leads to slowing of the cipher algorithm i.e. more computational time will be required.

7. Which of the following statements are true.
i) In the CBC mode, the plaintext block is XORed with previous ciphertext block before encryption.
ii) The CTR mode does not require an Initialization Vector
iii) The last block in the CBC mode uses an Initialization Vector
iv) In CBC mode repetitions in plaintext do not show up in ciphertext

a) iii)
b) ii) and iv)
c) All the Statements are true
d) i) ii) and iv)

Explanation: The first block in CBC mode uses an IV.

8. What is the number of possible 3 x 3 affine cipher transformations ?
a) 168
b) 840
c) 1024
d) 1344

Explanation: Since ‘A’ cannot have columns of ‘0’s. so there are ‘7’ choices i.e. 001/010/011/100/101/110/111. ‘a1’ is chosen for first column of ‘A. We have ‘6’ choices for second column, let ‘a2’ be chosen for second column. The final column can be any 3-tuple except 0, a1, a2, a1+a2. That means any one of the remaining ‘4’ 3-tuples may be chosen for the final column.
(Total number of@ possibilities for A)}=k=7×6×4=168
(Number of affine@ block cipher transformation)}=k×t=8×168 =1344.

9. The blowfish algorithm’s key expansion converts a key of at most 448 bits into several subkey arrays totaling ___ bytes.
a) 4096
b) 4608
c) 4168
d) 4864

Explanation: Key expansion converts a key of at most 448 bits into several sub key arrays totaling 4168 bytes i.e. eighteen 32-bit sub keys + four 8×32-bit s boxes = 4168.

10. For the group Sn of all permutations of n distinct symbols, what is the number of elements in Sn?
a) n
b) n-1
c) 2n
d) n!

Explanation: There there are n distinct symbols there will be n! elements.

11. Which of the following statements are true with respect to the blowfish algorithm?
i) Provided key is large enough, brute-force key search is not practical, especially given the high key schedule cost
ii) Key dependent S-boxes and sub keys make analysis very difficult
iii) Changing both halves in each round increases security
iv) Blowfish has improved avalanche effects

a) i) ii) and iv)
b) ii) and iii)
c) i)
d) All are true

Explanation: All of the above statements are true with respect to the blowfish algorithm.

12. Which one of the following modes of operation in DES is used for operating short data?
a) Cipher Feedback Mode (CFB)
b) Cipher Block chaining (CBC)
c) Electronic code book (ECB)
d) Output Feedback Modes (OFB)

Explanation: The Electronic code book mode is used for operating on short data as the same key is used for each block. Thus repetitions in Plain Text lead to repetitions in Cipher Text.

13. Which of the following is the correct encryption statement representation for the XTS-AES mode?
a) E(K1,P) XOR T
b) E(K2,T) XOR P
c) E(K1,T XOR P) XOR P
d) E(K1,P XOR T) XOR T

Explanation: The correct encryption representation would be : C = E(K1,P XOR T) XOR T.

14. The last two blocks of the XTS-AES mode are
b) encrypted/ decrypted using ciphertext-stealing
d) padded and then swapped after encryption/ decryption

Explanation: The correct term used to encrypt/ decrypt the last 2 blocks is ‘cipher-text stealing’ where C(m) and C(m-1) are interchanged with each other.

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