Python AES Decryption Routine (Code Help)

Question

I developed a code based on information available online regarding an AES Encryption and Decryption routine.

Language: Python 2.7.x

Complete Code -

#!/usr/bin/python

import sys, os
import hashlib
import base64
from Crypto.Cipher import AES

## Variables in computation.
IV = u'1234567890123456'
BLOCK_SIZE = 32
INTERRUPT = u'\u0001'
PAD = u'\u0000'
SECRET = os.urandom(32)
filename=sys.argv[1]

def AddPadding(data, interrupt, pad, block_size):
   new_data = ''.join([data, interrupt])
   new_data_len = len(new_data)
   remaining_len = block_size - new_data_len
   to_pad_len = remaining_len % block_size
   pad_string = pad * to_pad_len
   return ''.join([new_data, pad_string])

def StripPadding(data, interrupt, pad):
   return data.rstrip(pad).rstrip(interrupt)

def encAES(cipher_code, file_data):
   data_padded = AddPadding(file_data, INTERRUPT, PAD, BLOCK_SIZE)
   encrypted = cipher_code.encrypt(data_padded)
   return encrypted

def decAES(cipher_code, file_data):
   decrypted  = cipher_code.decrypt(file_data)
   return StripPadding(decrypted, INTERRUPT, PAD)

def FileSave(fwname, fwdata):
   f = open(fwname, 'w')
   f.write(fwdata)
   f.close

def FileRead(frname):
   f = open(frname, 'rb')
   frdata = f.read()
   return frdata

cipher = AES.new(SECRET, AES.MODE_CBC, IV)

## Encryption
data2encrypt = base64.b64encode(FileRead(filename))
encrypted_data = encAES(cipher, data2encrypt)
encrypted_content = base64.b64encode(encrypted_data)

encrypted_filename = "enc_"+filename
FileSave(encrypted_filename, encrypted_content)
print "Encryption complete. File saved as: "+ encrypted_filename

## Decryption
data2decrypt = base64.b64decode(FileRead(encrypted_filename))
decrypted_data = decAES(cipher, data2decrypt)
decrypted_content = base64.b64decode(decrypted_data)

decrypted_filename = "dec_"+filename
FileSave(decrypted_filename, decrypted_content)
print "Decryption complete. File saved as: "+ decrypted_filename

Now, the encryption routine is working fine but Decryption Routine is giving an error -

Commandline - python test.py sample.txt

ERROR:

Traceback (most recent call last):
  File "test.py", line 65, in <module>
    decrypted_data = decAES(cipher, data2decrypt)
  File "test.py", line 38, in decAES
    return StripPadding(decrypted, INTERRUPT, PAD)
  File "test.py", line 29, in StripPadding
    return data.rstrip(pad).rstrip(interrupt)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe6 in position 3: ordinal not in range(128)

What can be the possible workaround?

You are probably trying to interpret the byte sequence (the result of encryption) as a text string. Use proper encoding like Base64 or HEX. — Oleg Estekhin, Jul 08 '14 at 18:16
@OlegEstekhin: no, the OP is concatenating Unicode and a byte string, triggering an implicit decode. — Martijn Pieters, Jul 08 '14 at 18:30

Martijn Pieters · Accepted Answer · 2014-07-08T18:36:50.793

2

You are concatenating a byte string with a Unicode value, triggering an automatic decode of the bytestring. This fails, as your decrypted text is not decodable as ASCII.

Don't use Unicode INTERRUPT and PAD values here; you are not reading Unicode data from the file here anyway:

INTERRUPT = '\1'
PAD = '\0'

You'll have to create a new instance of the AES object to decrypt; you cannot reuse the object you used for encrypting as it's IV state has been altered by the encryption:

decrypt_cipher = AES.new(SECRET, AES.MODE_CBC, IV)
decrypted_data = decAES(decrypt_cipher, data2decrypt)

With those changes your code works and can encrypt and again decrypt the data.

edited Jul 08 '14 at 18:36

answered Jul 08 '14 at 18:31

Martijn Pieters

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I want this to work with all kind of files. I have to use the same logic for .DAT files and media files (mp4, wtv, wav etc.). – pwn.star Jul 08 '14 at 18:43
1

@pwn.star: then open your files in binary mode, always. Currently your `FileSave` function saves data in *text mode*; that's fine here, but on reading you always read in binary mode, on Windows this'll get you some extra newline characters. Your data will otherwise **always** be bytes, so don't use Unicode strings here. – Martijn Pieters Jul 08 '14 at 18:44

Python AES Decryption Routine (Code Help)

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