Symmetric Ciphers Questions and Answers – The AES Algorthm – IV

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This set of Cryptography Inteview Questions and Answers for freshers focuses on “The AES Algorithm”.

1. How many step function do Round 1 and 2 each have in S-AES?
a) 4 and 3
b) Both 4
c) 1 and 4
d) 3 and 4
View Answer

Answer: a
Explanation: Round 1 has four step functions whereas Round 2 has three step functions.
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2. For a key 25D5 and PT input A479 what is the output we obtain after the “add round key” function?
a) F34D
b) 81AC
c) 79DF
d) 327D
View Answer

Answer: b
Explanation: Simply apply XOR to the state matrix PT with the key matrix to obatain the output which in this case is 81AC.

3. The output of the previous question, on passing through “nibble substitution” gets us the output
a) 3267
b) 1344
c) 64C0
d) CA37
View Answer

Answer: c
Explanation: 81AC after passing through the “nibble substitution” round produces an output 64C0. A corresponding substitution is referred to in this step.

4. The output of the previous question on passing through the “shift row” step function gives us the output
a) C046
b) 0C64
c) 64C0
d) 640C
View Answer

Answer: b
Explanation: 64C0 after passing through the “shift row” round produces an output 640C. A one nibble shift is performed in the second row in this step.

5. The output of the previous question on passing through the “mix columns” step function gives us the output
a) 3252
b) 3743
c) 3425
d) 3473
View Answer

Answer: d
Explanation: 64C0 after passing through the “mix columns” round produces an output 3473. This is obtained by multiplying this matrix with the matrix 1441.
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6. On performing the mix column transformation on the matrix
cryptography-interview-questions-answers-freshers-q6a
(Verify for the first column only)
a) True
b) False
View Answer

Answer: a
Explanation:
cryptography-interview-questions-answers-freshers-q6

7. The inverse transformation matrix times the forward transformation matrix equals the identity matrix.
a) True
b) False
View Answer

Answer: c
Explanation: The statement is true. The inverse transformation matrix times the forward transformation matrix does equal the identity matrix.

8. 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
View Answer

Answer: c
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.

9. How many round keys are generated in the AES algorithm?
a) 11
b) 10
c) 8
d) 12
View Answer

Answer: a
Explanation: 11 round keys are generated. One for each of the 10 rounds and one of the initial permutations (Round 0).
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10. 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
View Answer

Answer: b
Explanation: m(x)=x8+x4+x3+x+1 stands for 100011011 which is an irreducible polynomial. Others are not irreducible polynomials.

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

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn