Python PyCrypto encrypt/decrypt text files with AES

Question

I already have a working program, but the only thing that doesn't work is the decrypt_file() function I have. I can still copy the encrypted text from the file and put it in my decrypt() function and have it work, but when I try to use my supposed-to-be handy decrypt_file() function it throws an error. Now I know 99.999% sure that my encrypt() and decrypt() functions are fine, but there is something with the bytes and strings conversion when I read and encode the text file that throws an error; I just can't find the hangup. Please help!

My Program:

from Crypto import Random
from Crypto.Cipher import AES

def encrypt(message, key=None, key_size=256):
    def pad(s):
        x = AES.block_size - len(s) % AES.block_size
        return s + ((bytes([x])) * x)

    padded_message = pad(message)

    if key is None:
        key = Random.new().read(key_size // 8)

    iv = Random.new().read(AES.block_size)
    cipher = AES.new(key, AES.MODE_CBC, iv)

    return iv + cipher.encrypt(padded_message)

def decrypt(ciphertext, key):
    unpad = lambda s: s[:-s[-1]]
    iv = ciphertext[:AES.block_size]
    cipher = AES.new(key, AES.MODE_CBC, iv)
    plaintext = unpad(cipher.decrypt(ciphertext))[AES.block_size:]

    return plaintext

def encrypt_file(file_name, key):
    f = open(file_name, 'r')
    plaintext = f.read()
    plaintext = plaintext.encode('utf-8')
    enc = encrypt(plaintext, key)
    f.close()
    f = open(file_name, 'w')
    f.write(str(enc))
    f.close()

def decrypt_file(file_name, key):
    def pad(s):
        x = AES.block_size - len(s) % AES.block_size
        return s + ((str(bytes([x]))) * x)

    f = open(file_name, 'r')
    plaintext = f.read()
    x = AES.block_size - len(plaintext) % AES.block_size
    plaintext += ((bytes([x]))) * x
    dec = decrypt(plaintext, key)
    f.close()
    f = open(file_name, 'w')
    f.write(str(dec))
    f.close()



key = b'\xbf\xc0\x85)\x10nc\x94\x02)j\xdf\xcb\xc4\x94\x9d(\x9e[EX\xc8\xd5\xbfI{\xa2$\x05(\xd5\x18'

encrypt_file('to_enc.txt', key)

The text file I encrypted:

b';c\xb0\xe6Wv5!\xa3\xdd\xf0\xb1\xfd2\x90B\x10\xdf\x00\x82\x83\x9d\xbc2\x91\xa7i M\x13\xdc\xa7'

My error when attempting decrypt_file:

    Traceback (most recent call last):
  File "C:\Python33	esting	est\crypto.py", line 56, in 
    decrypt_file('to_enc.txt', key)
  File "C:\Python33	esting	est\crypto.py", line 45, in decrypt_file
    plaintext += ((bytes([x]))) * x
TypeError: Can't convert 'bytes' object to str implicitly
[Finished in 1.5s]

When I replace line 45 with: plaintext += ((str(bytes([x])))) * x, this is the error I get:

Traceback (most recent call last):
  File "C:\Python33	esting	est\crypto.py", line 56, in 
    decrypt_file('to_enc.txt', key)
  File "C:\Python33	esting	est\crypto.py", line 46, in decrypt_file
    dec = decrypt(plaintext, key)
  File "C:\Python33	esting	est\crypto.py", line 23, in decrypt
    plaintext = unpad(cipher.decrypt(ciphertext))[AES.block_size:]
  File "C:\Python33\lib\site-packages\Crypto\Cipher\blockalgo.py", line 295, in decrypt
    return self._cipher.decrypt(ciphertext)
ValueError: Input strings must be a multiple of 16 in length
[Finished in 1.4s with exit code 1]

Python PyCrypto encrypt/decrypt text files with AES

2 Answers