BabyEncryption.md

BabyEncryption - Very Easy

• given scenario - a confidential message file is found that needs to be decrypted

• extract the contents of the ZIP file using the password 'hackthebox'

• the ZIP file has a 'chall.py' script and the encrypted message 'msg.enc'

• checking the Python script:

  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()

  • the script imports a variable 'MSG' from the file/module 'secret.py'

  • the encryption function iterates over each character of the 'MSG' variable and does the following maths on it - (123 * char + 18) % 256)

  • the % 256 operation is to keep the 'char' in the byte range of 0-255

  • the modulo operation makes sense only if the 'MSG' variable is in bytes; if it was in string type it would not work

  • the result is returned as a list of bytes, and written to the 'msg.enc' file, and converts the bytes into a hexadecimal string

• we can decrypt this in the same way to recover the original message

• decoding the original encryption function step-by-step (used ChatGPT for this to understand the logic):

  • ct = (123 * char + 18) % 256 - subtract 18 from both sides

  • ct - 18 = (123 * char) % 256 - now, we need to find its multiplicative modular inverse

  • the modulo inverse formula, as per this case, is 123 * x = 1 (% 256), where x has to be in the range of 1 to 255 (256 minus 1)

  • using an online inverse modulo calculator, we find that x = 179, so that can be used as the modulo inverse

  • apply the modulo inverse - and multiply by 179 to keep the sides equated

  • ((ct - 18) % 256) * 179 = char - this gives the decryption rule

• we can apply rest of the logic with the decryption rule in our script:

  # read file contents
  with open('./msg.enc','r') as file:
      f = file.read()
  
  b = bytes.fromhex(f)
  # decode from hex to bytes
  
  pt = []
  for byte in b:
      p = (179 * (byte - 18)) % 256
      pt.append(p)
  
  # convert back to bytes and print
  print(bytes(pt))

• running the script gives us the flag