tough Cryptography Questions on “The AES Algorthm”.
1. How many computation rounds does the simplified AES consists of?
a) 5
b) 2
c) 8
d) 10
Answer: b
Clarification: The simplified AES has only 2 rounds of computation.
2. For the case of Mixed Columns and Inverse Mixed Columns, is it true that b(x) = a-1(x)mod(x4 + 1)
where a(x) = {03}x3 + {01}x2 + {01}x + {02} and b(x) = {0B}x3 + {0D}x2 + {09}x + {0E}
a) True
b) False. The expression for a(x) is wrong.
c) False. The expression for b(x) is wrong.
d) False. Both a(x) and b(x) are faulty.
Answer: a
Clarification: The statment is true and can be checked as it is similar to the matrix forms of mixed columns and inverse mixed columns.
3. For an inputs key of size 128 bits constituting of all zeros, what is w(7) ?
a) {62 63 63 63}
b) {62 62 62 62}
c) {00 00 00 00}
d) {63 63 63 62}
Answer: a
Clarification: Applying the key algorithm we get,
w(0) = {00 00 00 00}; w(1) = {00 00 00 00}; w(2) = {00 00 00 00}; w(3) = {00 00 00 00};
w(4) = {62 63 63 63}; w(5) = {62 63 63 63}; w(6) = {62 63 63 63}; w(7) = {62 63 63 63}.
4. On comparing AES with DES, which of the following functions from DES does not have an equivalent AES function?
a) f function
b) permutation p
c) swapping of halves
d) xor of subkey with function f
Answer: c
Clarification: There is no equivalent to swapping of halves in the AES algorithm.
5. On perform the Mix Columns transformation for the sequence of bytes “77 89 AB CD” we get output
a) {01 55 EE 4A}
b) {0A 44 EF 4A}
c) {08 55 FF 3A}
d) {09 44 DD 4A}
Answer: c
Clarification: Perform the mix columns transformation to obtain the output {08 55 FF 3A}.
6.On perform the Mix Columns transformation for the sequence of bytes “67 89 AB CD” we get output
a) {08 55 FF 18}
b) {28 45 EF 08}
c) {28 45 FF 18}
d) {25 35 EF 08}
Answer: b
Clarification: Perform the mix columns transformation to obtain the output {28 45 EF 0A}.
7. Is the following matrix the inverse matrix of the matrix used in the mix columns step?
x3 + 1 x
x x3 + 1
a) Yes
b) No
c) Can’t say
d) Insufficient Information
Answer: a
Clarification: On multiplying this matrix with the mix columns matrix we get the identity matrix, this proves that it is an inverse matrix.
8. For the cipher text 0000 0111 0011 1000 and Key 0110 1111 0110 1011, apply the Simplified AES to obtain the plaintext. The plain text is
a) 0110 1001 0111 0001
b) 0110 1111 0110 1011
c) 0010 1001 0110 1011
d) 1111 0101 0111 1111
Answer: b
Clarification: On applying the simplified AES we would obtain 0110 1111 0110 1011 as the plain text.
9. What is the block size in the Simplified AES algorithm?
a) 8 bits
b) 40 bits
c) 16 bits
d) 36 bits
Answer: b
Clarification: The block size for the AES algorithm is 16 bits.
10. What is the key size in the S-AES algorithm?
a) 16 bits
b) 32 bits
c) 24 bits
d) None of the mentioned
Answer: a
Clarification: The key size in the S-AES algorithm is 16 bits.
11. S-AES and S-DES were both developed by the same person as an educational cryptography system to teach students
a) True
b) False
Answer: a
Clarification: None.
12. Which of the following is a faulty S-AES step function?
a) Add round key
b) Byte substitution
c) Shift rows
d) Mix Columns
Answer: b
Clarification: The correct version in S-AES would be nibble substitution as 4 bits are taken at a time.