Baby Encryption

Posted on Feb 4, 2024

You are after an organised crime group which is responsible for the illegal weapon market in your country. As a secret agent, you have infiltrated the group enough to be included in meetings with clients. During the last negotiation, you found one of the confidential messages for the customer. It contains crucial information about the delivery. Do you think you can decrypt it?

Steps

Unzip the file, password hackthebox (given)
We have two files, a python script and msg.enc which contains this string:

6e0a9372ec49a3f6930ed8723f9df6f6720ed8d89dc4937222ec7214d89d1e0e352ce0aa6ec82bf622227bb70e7fb7352249b7d893c493d8539dec8fb7935d490e7f9d22ec89b7a322ec8fd80e7f8921

This is the python script, chall.py:

import string
from secret import MSG

def encryption(msg):
    ct = []
    for char in msg:
        ct.append((123 * char + 18) % 256)
    return bytes(ct)

ct = encryption(MSG)
f = open('./msg.enc','w')
f.write(ct.hex())
f.close()

Reading/ running the script, we find that the secret module does not exist, and the string library isn’t even used
Writing a decryption script (literally just reversing everything in the above script) - you can also UTF-8 encode it, but the bytes were readble so I didn’t

def decryption(msg):
  ct = []
  for char in msg:
    char = char - 18
    char = 179 * char % 256
    ct.append(char)
  return bytes(ct)

with open('msg.enc') as f:
  ct = bytes.fromhex(f.read())

ct = decryption(ct)
print(ct)

Success!