# ECE 404 Homework #4 Advanced Encryption Standard solution

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## Description

In this homework, you will get a deeper understanding of finite fields of the form GF(2n
) and
Theory Problems
(a) (9x
5 + 4x
4 + 8x
3 + 2x
2 + 3x + 4) + (6x
5 + 2x
4 + 9x
3 + 7x
2 + 5x + 7)
(b) (8x
3 + 6x
2 + 8x + 1) × (3x
3 + 9x
2 + 7x + 5)
(c) 3x
3−5x
2+10x−3
3x+1
2. For the finite field GF(23
), calculate the following for the modulus polynomial x
3 + x + 1,
(a) (x
2 + x + 1) × (x
2 + x)
(b) (x
2
) − (x
2 + x + 1)
(c) x
2+x+1
x2+1
Programming Problem
Write a script in Python to implement the AES algorithm with a 256-bit key.
Instructions:
The following points may aid you in your implementation and are worth noting:
1. Each round of AES involves four steps:
(a) Single-byte based substitution
(b) Row-wise permutation
(c) Column-wise mixing
(d) Addition of the round key
2. The order in which these four steps are executed is different for encryption and
decryption. The last round for encryption does not involve the ‘Mix columns’
step. Similarly, the last round for decryption does not involve the ‘Inverse mix
columns’ step.
1
3. As you know, AES has variable key-length, and the number of rounds of processing depend
upon the key-length. The lecture assumes a 128-bit key length and all subsequent explanation
is based upon that assumption. But the key provided to you is 256 bits long, hence, there
will be a slight variation in how you generate the key schedule. The following explanation
will be helpful in that regard:
(a) For the key expansion algorithm, note that irrespective of the key-length, each round
still uses only 4 words from the key schedule. Just as we organised the 128-bit key in
4 words for key-expansion, we organise the 256-bit key in 8 words.
(b) Each step of the key-expansion algorithm takes us from 8 words in one round to 8
words in the next round. Hence, 8 such steps will give us a 64-word key schedule. The
implementation of the g(·) function remains the same. The logic of obtaining the 8
words from the j
th step of key expansion to the (j + 1)th step also remains the same.
(c) Note that since the key is 256-bits long, there will be 14 rounds of processing in the
AES, plus the initial processing. Because each round of processing uses only 4 words
from the key schedule, you will require only a 60-word key schedule. However, the
previous step generates a 64-word schedule, so you can ignore the last 4 words in the
schedule.
Program Requirements
Similar to your DES implementation in Homework 2, your program should have the following call
syntax:
python AES.py -e message.txt key.txt encrypted.txt
python AES.py -d encrypted.txt key.txt decrypted.txt
Explanation of the syntax:
• For Encryption (denoted by -e):
– AES.py reads the input plaintext (in this case message.txt) and the key (in this case
key.txt) using the specified file names in the second and third command-line arguments.
– The encrypted output is then written in hexstring form to the file specified by the
fourth command-line argument (in this case encrypted.txt ).
• For Decryption (denoted by -d):
– Read in the ciphertext from the second command-line argument (in this case encrypted.txt)
in hexstring format, decrypt it using the specified key, and save the decrypted output
to the file specified by the fourth command-line argument (in this case decrypted.txt).
Your implementation should not include any external modules (e.g. numpy) other than BitVector
and the standard modules already included in the Python Standard Library.
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Useful Notes
• This should go without saying, but start on this homework early. This homework is a
bit more involved than the previous homeworks.
• On the Brightspace webpage, we have provided the AES results for the plaintext from HW02
after each of the four steps in the first round of processing for the first block. The key
used is to get these results is: scuderiaferraritheprancinghorse!
• Make sure that when reading the ciphertext during decryption, you need to convert it to
a hex-string format and not use the ASCII characters themselves (i.e. do not simply use
BitV ector(f ilename = ′
encrypted.txt′
) without any more processing). You may want to
use BitV ector(hexstring = …) instead.
• Keep in mind that the block size is still 128 bits despite the key size being 256 bits.
• You can verify your encryption using the following website (make sure to change the key size
and select Hex output) :
https://www.devglan.com/online-tools/aes-encryption-decryption
Due to different padding methods for blocks that are not a multiple of the block size, the
final block of encryption generated from this website will not match your final encrypted
block depending on whether you are padding zeros from the right. However, the remaining
blocks should match.
Submission Instructions
• Make sure that the program requirements and submission instructions are followed. Failure
to follow these instructions may result in loss of points!.
• For this homework you will be submitting a zip file to Brightspace containing:
– A PDF titled hw04 .pdf containing:
∗ Your answers to the theory problems. You are allowed to include scans or photos
of handwritten work in the PDF, but your work must be clearly legible.
∗ a brief explanation of your code, and the encrypted and decrypted output for the
text mentioned above using the key provided.
– The file AES.py containing your code for the programming problem.
– If you decide to create any additional code files (e.g. files containing helper functions)
for your implementation, you must also include those files in your submission.
• In your program file, include a header as described on the ECE 404 Website.