Jordan University of Science and Technology
Networks Security – CPE 542
Homework # 4
Done By
Hanadi O'beidat
ID# 20010171100
Review Question
5.3
What is power analysis?
Power Analysis is a type of security attack. It is a way to extract
information from an electronic system by examining the power supply
current over time. By doing this it is possible to figure out exactly
what some computing systems are doing. It is even possible in some
cases to extract cryptographic keys or other secrets. Each time a computer
executes an instruction, or fetches or stores a data item, transistors switch on
and off. The current consumed by all of the transistors in the computer at any
one instant, is reflected in the total power supply current.
This depends on processing 0's and 1's so the sequence of bits can be
determined.
5.9
How many bytes in State are affected by ShiftRows?
The number of shifts for each row in the State:
Row 1: No shifts.
Row 2: 1 – byte shifts.
Row 3: 2 – bytes shifts.
Row 4: 3 – bytes shifts.
In each row there are 4 bytes so the total number of the affected bytes is 12
bytes.
5.13
What is the difference between SubBytes and SubWord?
SubBytes: Transformation in the Cipher that processes the State using a
nonlinear byte substitution table (S-box) that operates on each of the
State bytes independently.
SubWord: Function used in the Key Expansion routine that takes a four-byte
input word and applies an S-box to each of the four bytes to produce an output
word.
Problems
5.3
Show the first 8 words of the key expansion for 128 – bit key of all zeros?
The first 4 subkeys (w0 – w3) are all zeros shown below:
i
temp
4
5
6
7
00000000
62636363
62636363
62636363
w0 = 00000000
w4 = 62636363
After
RotWord()
After
SubWord()
Rcon[i/4]
After XOR
with
Rcon
00000000
63636363
01000000
63636362
w1 = 00000000
w5 = 62636363
w2 = 00000000
w6 = 62636363
w[i–4]
w[i]=
temp XOR
w[i-4]
00000000
00000000
00000000
00000000
62636363
62636363
62636363
62636363
w3 = 00000000
w7 = 62636363
5.6
Compare the AES to DES. For each of the following elementst of DES,
indicate the comparable element in AES or explain why it is not needed in
AES.
a. XOR of subkey material with the input to the function f function.
The similar element in AES for XOR of subkey with the input to the
function (that passes different stages before XORing) is the added round
key stage in all the 10 rounds.
b. XOR of the f function output with left side of the block.
There is no similar element in AES for XOR the f function output with left
half side of the block, this is because AES structure is not a feistel
structure. The entire block is processed in parallel (No two halves are
using one half to modify the other half).
c. The f function.
There is no single element that is similar to f function, but the four stages
(Substitution bytes, shift rows, mix columns, added roundly) in each round
do the same as f function.
d. Permutation P.
The similar element for P is the shift rows in each of the 10 rounds.
e. Swapping of halves of the block.
No similar element in AES this is because that AES structure not a feistel
structure and no need to swap halves since work in parallel (No half needs
to modify the other half).