Python: TypeError thrown when performing simple ASCII decryption

Question

I am creating an encoding and decoding algorythm based on the python os "ord" function. I have built the encoder which works by multiplying the output of the ord function by a randomly generated number, but the decoder is having some errors. In theory, it should just be the encoder but in reverse, but it's having an error. The reverse should be the "chr()" function.

#Encryption tool
import random
import os
import time
import sys
import tkinter as tk

root = tk.Tk()
canvas1 = tk.Canvas(root, width=400, height=300)
canvas1.pack()

entry1 = tk.Entry(root) 
canvas1.create_window(200, 140, window=entry1)
button1 = tk.Button(text='Encrypt', command=encrypt)


def a():
    with open("encryptiontool.py") as f:
        exec(f.read())


txtlen = 0
shiftnum = random.randint(1, 11)
os.system("afplay /System/Library/Sounds/Hero.aiff")
print("Welcome to SHIFTNUM Encryption Tool! ")


encrypt_input = input("Enter a word to be encrypted: ")
lengthinput = len(list(encrypt_input))
numberlength = int(lengthinput)

if shiftnum == 11:
    os.system("afplay /System/Library/Sounds/Bottle.aiff")
    print("!@**#(*!%#(&^%!&^@%ERROR!(@&^^#$%*^!%$&#^%$")
    sys.exit()

encryptedlist = []

print("NUMSHIFT = ", shiftnum)

while numberlength > 0:
    numberlength = numberlength - 1
    encryptedlist.append(ord(encrypt_input[txtlen]) * shiftnum)
    txtlen = txtlen + 1
          
print(encryptedlist)
os.system("afplay /System/Library/Sounds/Glass.aiff")

I have tried the following code, but the error is the same.

#Decryption tool

txtlen = 0

import re
numshift = int(input("NUMSHIFT = "))
encryptedtxtlist = input("Paste list here: ")
dechartxt = re.sub(r"[^a-zA-Z0-9 ]", "", encryptedtxtlist)
print(dechartxt.split())

listlength = len(dechartxt.split())
lstlen = int(listlength)
dechar = int(dechartxt)
decodedlist = []

while lstlen > 0:
    lstlen = lstlen - 1
    dechar = dechar / numshift
    decodedlist.append(chr(int(dechar[txtlen])))
    txtlen = txtlen + 1

print(decodedlist)

but the output is the same :

NUMSHIFT = 5
Paste list here: ['460']
['460']
Traceback (most recent call last):
  File "/Users/ruben_sutton29/Desktop/pythonproject/decryptiontool.py", line 19, in <module>
    decodedlist.append(chr(int(dechar[txtlen])))
TypeError: 'float' object is not subscriptable

Answer 1

Let's see if we can help simplify this a bit. It appears that using a list to store the ciphertext is over-complicating things for you.

Start with the requirements:

The encryption task:

• Input a plaintext message.
• Derive the ASCII value for each character and multiply by N.
• Store the encrypted offset ASCII value for each character.
• Return the offsets to the user.

The decryption task:

• Accept a ciphertext message, in the form of ASCII offset integers.
• Divide each offset integer by N and store the decrypted ASCII characters.
• Return the plaintext message to the user.

How this can be implemented:

The two documented functions below, encrypt and decrypt follow these simple requirements.

Yes, the functions can be collapsed into single line statements, but have been intentionally left more verbose to aid in beginner clarity.

The primary thing to note is the simplified logic. Rather than passing in a list to the decryption function, simply return and pass in a comma separated string of offset values. The list was the primary tripping-up point.

def encrypt(text: str, n: int) -> str:
    """Encrypt a message, as an ASCII offset.

    Args:
        txt (str): Text to be encrypted.
        n (int): Value of which each character's ASCII value is 
            multiplied.

    Returns:
        str: A comma separated string containing offset integer values.
        
    """
    offsets = []
    for c in text:
        offsets.append(ord(c)*n)
    return ','.join(map(str, offsets)) 

---

def decrypt(text: str, n: int) -> str:
    """Decrypt a message of ASCII offset integers into plaintext.

    Args:
        txt (str): Comma separated integer string to be decrypted.
        n (int): Value of which each integer is divided.

    Returns:
        str: A decrypted plaintext message.
        
    """
    decrypted = []
    for i in text.split(','):
        decrypted.append(chr(int(i)//n))
    return ''.join(decrypted)

---

Demonstration:

# Prompt the user for input.
plaintext = input('Enter your message: ')
n = int(input('Enter the offset value: '))

# Print the original message, for debugging.    
print(f'{plaintext=}')

# Call the encryption function, and print the returned value.
ciphertext = encrypt(plaintext, n)
print(f'{ciphertext=}')

# Call the decryption function, and print the returned value.
plaintext2 = decrypt(ciphertext, n)
print(f'{plaintext2=}')

Output:

Enter your message:  A secret message.
Enter the offset value:  5

plaintext='A secret message.'
ciphertext='325,160,575,505,495,570,505,580,160,545,505,575,575,485,515,505,230'
plaintext2='A secret message.'