Cryptography Questions experienced people 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
Answer: d
Clarification: 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
Answer: b
Clarification: 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
Answer: c
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: b
Clarification: 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
Answer: a
Clarification: 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
Answer: c
Clarification: 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
Answer: a
Clarification: 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
Answer: d
Clarification: 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
Answer: c
Clarification: 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
Answer: b
Clarification: ‘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
Answer: d
Clarification: 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
Answer: d
Clarification: 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 –
a) padded as 10*
b) encrypted/ decrypted using ciphertext-stealing
c) padded as 10*1
d) padded and then swapped after encryption/ decryption
Answer: b
Clarification: 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.