1. The inverse transformation matrix times the forward transformation matrix equals the identity matrix.
a) True
b) False
Explanation: The statement is true. The inverse transformation matrix times the forward transformation matrix does equal the identity matrix.
2. Which of the following are true
i) In the AES key expansion algorithm, the function g() operates on w i+3
ii) Perform a one-byte right circular rotation on the argument 4-byte word
iii) The round constant follows the formula RC[j] = RC[j − 1]
a) i) ii)
b) ii) only
c) i) only
d) All
Explanation: Only the first option is true. There is a left circular rotation in AES key expansion. RC[j] = 2 × RC [j − 1] is the formula for the round constant.
3. How many round keys are generated in the AES algorithm?
a) 11
b) 10
c) 8
d) 12
Explanation: 11 round keys are generated. One for each of the 10 rounds and one of the initial permutations (Round 0).
4. Which function can be used in AES multiplication
a) m(x)=x7+x4+x3
b) m(x)=x8+x4+x3+x+1
c) m(x)=x8+x3+x2+x+1
d) m(x)=x8+x5+x3+x
Explanation: m(x)=x8+x4+x3+x+1 stands for 100011011 which is an irreducible polynomial. Others are not irreducible polynomials.
5. How many modes of operation are there in in DES and AES?
a) 4
b) 3
c) 2
d) 5
Explanation: DES has 5 modes of operation.
6. 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.
7. 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)
Explanation: Block size is 64 bits. The same Key is used for each block.
8. 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.
9. 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
Explanation: The range of the output of each stage of the cipher system is 8-32 bits for a 64 bit system.
10. In OFB Transmission errors do not propagate: only the current ciphertext is affected, since keys are generated “locally”.
a) True
b) False
Explanation: Yes, transmission errors do not propagate in OFB mode because of the locally generated key.