1. Which of the following modes does not implement chaining or “dependency on previous stage computations”?
a) CTR, ECB
b) CTR, CFB
c) CFB, OFB
d) ECB, OFB
Explanation: Only CTR and ECB do not implement chaining.
2. The counter value in CTR modes repeats are a regular interval.
a) True
b) False
Explanation: The Counter value in CTR mode should never be repeated, else it leads to vulnerability of the mode. We must ensure never reuse key/counter values; otherwise it could break (OFB).
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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.