Symmetric Ciphers Questions and Answers – DES Modes of Operation – I

This set of Cryptography Multiple Choice Questions & Answers (MCQs) focuses on “DES Modes of Operation”.

1. How many modes of operation are there in in DES and AES?
a) 4
b) 3
c) 2
d) 5
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2. 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)
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3. Which of the following is false for ECB mode of operation
i) The Plain text is broken into blocks of size 128 bytes
ii) Blocks can be swapped, repeated, replaced without recipient noticing
iii) Good for short data
iv) Encryption of each block is done separately using a randomly generated key for each block

a) i) only
b) ii) and iii)
c) i) and iv)
d) i) ii) and iv)
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4. 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)
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5. There is a dependency on the previous ‘s’ bits in every stage in CFB mode. Here ‘s’ can range from ___
a) 8-16 bits
b) 8-32 bits
c) 4-16 bits
d) 8-48 bits
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6. What is the fault in the above figure?

a) The output of the XOR is sent to the next stage
b) The “Encrypt” Box should be replaced by “Decrypt Box
c) b-s bits are selected for the XOR operation
d) No fault
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7. Which of the following can be classified under advantages and disadvantages of OFB mode?
i) Transmission errors
ii) A bit error in a ciphertext segment
iii) Cannot recover from lost ciphertext segments
iv) Ciphertext or segment loss

a) Advantages: None; Disadvantages: All
b) Advantages: All; Disadvantages: None
c) Advantages: i); Disadvantages: ii) iii) iv)
d) Advantages: i); ii) Disadvantages: iii) iv)
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8. In OFB Transmission errors do not propagate: only the current ciphertext is affected, since keys are generated “locally”.
a) True
b) False
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9. 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
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10. The counter value in CTR modes repeats are a regular interval.
a) True
b) False
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Sanfoundry Global Education & Learning Series – Cryptography and Network Security.

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