AES Simplified

Encryption script based on a simplified version of the Advanced Encryption System (AES), implemented using python3.

Table of Contents

1. Description
2. Program Output
3. Execution
4. Function Headers
5. Resources

Description

AES Simplified start by asking the user for the filenames that contain the plaintext to be encrypted and the encryption key. The resulting encrypted text will be outputted and the user will have the option to save the encrypted text to a file.

NOTE:

When executing, ensure the files containing the plaintext and the encryption key are in the same directory.

Plaintext encryption uses various methods implemented by AES such as:

• Polyalphabetic Substitution via the Encryption Key

  A Vigenere table is used to encrypt the plaintext

• Padding

  Further encryption is done on 4x4 character blocks therefore, the character A is used for padding so that the length of the message is divisible by 16

• Row Shifts

  Shifting of the four rows are done in the following fashion: row one is unshifted, row two is shifted one position, row three is shifted two positions, and row four is shifted three positions. All shifts are done to the left

• Parity Bits

  Each character's binary representation will have either an even amount of 1s or an odd amount of 1s. If it is odd then its most significant bit set to 1, if it is even then no changes will occur

• Mix Columns

  This step diffuses the data by transformation. The transformation is performed by multiplying the circulant MDS matrix with each column of the blocks of characters

Program Output

Execution

To execute run the command python3 main.py

Function Headers

get_files()

• Description:

  Receives filenames from user

• Parameters:

  None

• Return:

  input_file String representing the name of the file that contains the plaintext to be encrypted

  key_file String representing the name of the file that contains the encryption key

  output_file String representing the name of the file that the cyphertext will be written to

---

get_text(input_file, key_file)

• Description:

  Reads in all contents of the input file and key file

• Parameters:

  input_file String representing filename containing plaintext

  key_file String representing filename containing encryption key

• Return:

  message List containing all text from the input file

  key String containing encryption key from the key file

---

parse_text(message)

• Description:

  Removes all whitespace and punctuation marks from message leaving only characters A-Z

• Parameters:

  message List containing plaintext

• Return:

  final_message String containing plaintext with only characters A-Z

---

output_data(title, data, key_size)

• Description:

  Outputs steps in encryption where data is output in 4x4 blocks

• Parameters:

  title String representing which step is being outputted

  data String representing the current data being encrypted

  key_size Int representing size of encryption key

• Return:

  None

---

Vcipher_encrypt(text, key)

• Description:

  Encryption via the Vigenere cipher using polyalphabetic substitution. Encrypts text using an encryption key

• Parameters:

  text String representing text to be encrypted

  key String representing the encryption key

• Return:

  encrypted_text String representing text that has been encrypted

---

padding(text, key_length)

• Description:

  Adds padding to the data if necessary, if no padding needed returns original text and groups

• Parameters:

  text String representing ciphertext

  groups List representing ciphertext

  key_length Int representing length of encryption key

• Return:

  updated_text String representing padded ciphertext

  groups List representing padded ciphertext

---

shift_rows(text, key_length)

• Description:

  Shift the rows of each 4x4 block 1st row: no shift, 2nd row: left shift 1 position, 3rd row: left shift 2 positions, 4th row: left shift 3 positions

• Parameters:

  text String representing ciphertext

  key_length Int representing length of encryption key

• Return:

  shifted_text String representing text that has been shifted accordingly

  shifted_collections 2D list of 4x4 blocks of text that has been shifted accordingly

---

parity_bit(text, key_length)

• Description:

  Adds parity bit to binary ascii value of each char in text Parity bit is needed if there is an odd number of 1s in binary ascii value

• Parameters:

  text String representing padded ciphertext

  key_length Int representing size of encryption key

• Return:

  hex_string String representing hexadecimal conversions of each char with parity bit added

---

output_doubles(title, data, key_size)

• Description:

  Outputs data in 4x4 block where each element is grouped by two to represent a hexadecimal value

• Parameters:

  title String representing which step is being outputted

  data String representing the current data being encrypted

  key_size Int representing size of encryption key

• Return:

  None

---

mix_columns(text, key_length)

• Description:

  Rearranges 2d list (4x4) and passes one column (size 4) to rgf() to be transformed After transformation reorder transformed values to their original positions

• Parameters:

  text String representing ciphertext

  key_length Int representing length of encryption key

• Return:

  one_str String representing transformed hex values in their original positions

---

rgf(value, constant)

• Description:

  Performs multiplication by using Rijndael Galois field

• Parameters:

  value List of hex values representing a single column (size 4) in 2d list

  constant Int representing what multiplicand

• Return:

  finished List representing transformed hex values

---

save_data(text)

• Description:

  Give users a choice to save data by writing to file specified by user

• Parameters:

  text String representing encrypted text to be saved to file

• Return:

  None

---

output_results(pre, post, key)

• Description:

  Outputs the results of encryption

• Parameters:

  pre List representing text to be encrypted

  post String representing encrypted text

  key String representing encryption key

• Return:

  None

Resources

Further information regarding AES can be found by watching this video: AES