1. For the AES-128 algorithm there are __________ similar rounds and _________ round is different.
a) 2 pair of 5 similar rounds ; every alternate
b) 9 ; the last
c) 8 ; the first and last
d) 10 ; no
Explanation: In the AES-128 there are 9 similar rounds and the last round is different.
2. Which of the 4 operations are false for each round in the AES algorithm
i) Substitute Bytes
ii) Shift Columns
iii) Mix Rows
iv) XOR Round Key
a) i) only
b) ii) iii) and iv)
c) ii) and iii)
d) only iv)
Explanation: AES rounds involve substitute bytes, shift rows, mix columns and addition of round key.
3. There is an addition of round key before the start of the AES round algorithms.
a) True
b) False
Explanation: In AES the final round contains only three transformations, and there is an initial single transformation (Add Round Key) before the first round which can be considered Round 0. Each transformation takes 4×4 matrixes as input and produces a 4×4 matrix as output.
4. How many computation rounds does the simplified AES consists of?
a) 5
b) 2
c) 8
d) 10
Explanation: The simplified AES has only 2 rounds of computation.
5. 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.
Explanation: The statment is true and can be checked as it is similar to the matrix forms of mixed columns and inverse mixed columns.
6. 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}
Explanation: 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}.
7. 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
Explanation: There is no equivalent to swapping of halves in the AES algorithm.
8. 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}
Explanation: Perform the mix columns transformation to obtain the output {08 55 FF 3A}.
9. 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}
Explanation: Perform the mix columns transformation to obtain the output {28 45 EF 0A}.
10. 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
Explanation: On multiplying this matrix with the mix columns matrix we get the identity matrix, this proves that it is an inverse matrix.