# Symmetric Ciphers Questions and Answers – DES Modes of Operation – II

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This set of Cryptography Questions and Answers for experienced people focuses on “DES Modes of Operation”.

1. Which mode of operation has the worst “error propagation” among the following?
a) OFB
b) CFB
c) CBC
d) ECB

Explanation: The ECB or electronic code book mode of operation propagates the most errors. A single bit error is carried onto the next block and so on.

2. Which block mode limits the maximum throughput of the algorithm to the reciprocal of the time for one execution?
a) OFB
b) CTR
c) CBC
d) ECB

Explanation: The CTR mode of operation limits the maximum throughput of the algorithm to the reciprocal of the time for one execution.

3. Which mode requires the implementation of only the encryption algorithm?
a) ECB
b) CBC
c) CTR
d) OFB

Explanation: The CTR mode only requires the implementation of either the encryption or decryption phase. Both the phases are somewhat similar.

4. Which of the following modes of operation does not involve feedback?
a) ECB
b) CBC
c) CTR
d) OFB

Explanation: Electronic code book does not involve feedback.

5. Which of the following is a natural candidates for stream ciphers?
a) OFB
b) CFB
c) CBC
d) ECB

Explanation: OFB and CTR both produce outputs that are independent of both PT and CT. Thus they are ideal candidates for stream ciphers.

6. The XTS-AES mode was approved by NIST in
a) 1999
b) 2010
c) 2006
d) 2002

Explanation: The XTS-AES mode of operation was approved by NIST in 2010. It is the most recent mode of operation for block ciphers.

7. The XTS-AES mode is based on the concept of tweakable block cipher.
a) True
b) False

Explanation: The XTS-AES mode uses a tweak thus the name.

8. The purpose of a ‘tweak’ in XTS-AES mode is to
a) secure the public key
b) provide security
c) provide variability
d) all of the mentioned

Explanation: The purpose of the ‘tweak’ in the XTS-AES mode is to provide variability in each round.

9. A tweak is used in XTS-AES mode to provide a different output for the same input PT and same key.
a) True
b) False

Explanation: The statement is true for XTS-AES mode of operation.

10. XTS-AES mode of operation is a better version of
a) OFB
b) CFB
c) CTR
d) ECB

Explanation: XTS-AES mode overcomes the principle security weakness of ECB, which is that two encryptions of the same CT yeild the same PT.

11. What is the size of the XTS-AES key?
a) 1024 bits
b) 64 bits
c) 512 bits
d) 128 bits

Explanation: The key size can be either 256 bits or 512 bits. There are 2 keys of the same size K1 and K2.

12. Which of the following represent the tweak?
a) j
b) i
c) alpha
d) alphaj

Explanation: ‘i’ represents the tweak value.

13. Which of the following is true for the tweak in XTS-AES mode?
a) they are non-negative integers
b) they are of size 128 bits
c) they are assigned consecutively
d) all of the mentioned

Explanation: All of the statements are true in relation to the tweak ‘i’ from the XTS-AES mode of operation.

14. 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.

15. 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.

Sanfoundry Global Education & Learning Series – Cryptography and Network Security.

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