Python AES解密例程（代码帮助）

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?

Answer 1

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.