Write a Linux Python utility aes.py for encrypting and decrypting a single text file. The text file is encrypted on the command line by using the AES standard encryption method.

$python aes.py e filename.txt filename.txt$.$aes $123($for encrypting filename.txt with output file named filename.txt$.$aes$)$

$python aes.py d filename.txt$.$aes filename.txt$.$back AES$\_$key $($for dencrypting filename.txt$.$aes with output file named filename.txt$.$back$)$

Note: Do not wite the program from scratch, just complete the code for the function decrypt of the program template below:

import sys

from cryptography.fernet import Fernet

def encrypt$($infile$,$ outfile$)$:

key $=$ Fernet.generate$\_$key$()$

print$(\text{'}$AES Key: $\text{'},$ key$)$

fernet $=$ Fernet$($key$)$

for line in infile:

name $=$ line.strip$()$

encName $=$ fernet.encrypt$($name$.$encode$())$

outfile.write$($encName$.$decode$()+"$

$")$

with open$("$AES$\_$Key$\_"+$sys$.$argv$[2],\text{'}$w$\text{'})$ as aes$\_$key: aes$\_$key.write$($key$.$decode$())$

infile.close$()$

outfile.close$()$

def decrypt$($infile$,$ outfile, key$)$:

# TODO for homework $3\mathrm{.}1$

infile.close$()$

outfile.close$()$

# Call the main function.

if $\_\_$name$\_\_==\text{'}\_\_$main$\_\_\text{'}$:

if len$($sys$.$argv$)!=5$:

print$("$Usage: python aes.py mode infile.txt outfile.txt key"$)$

exit$()$

operation $=$ sys$.$argv$[1]$

infile $=$ open$($sys$.$argv$[2],\text{'}$r$\text{'})$

outfile $=$ open$($sys$.$argv$[3],\text{'}$w$\text{'})$

key $=$ sys$.$argv$[4]$

if operation $==\text{'}$e$\text{'}$:

# encrypts

encrypt$($infile$,$outfile$)$

elif operation $==\text{'}$d$\text{'}$:

# decrypt

decrypt$($infile$,$outfile,key$)$

else:

# error

print$("$Invalid operation."$)$