Is this a valid byte string for AES Key? - python-3.x

Question

I am working on a little sideproject in Python 3.
My current problem is around AES-based decryption of a file. The content of the file (text) is symmetrical encrypted with AES.
I have imported PyCrypto: https://www.dlitz.net/software/pycrypto/api/current/
The docs specificy only little regarding the symmetrical key:
key (byte string) - The secret key to use in the symmetric cipher. It must be 16 (AES-128), 24 (AES-192), or 32 (AES-256) bytes long.
I have the key and looks like:
0xB0,0x0D,0xDF,0x9D,... (for security reasons I don't report the complete key here)
Anyway, my first question:
What kind of string is that? It looks like ASCII, but I lack deep knowledge about Encodings. Do I need any kind of transformation / decoding?
I wrote a little program to open a file and decrypt it. But PyCrypto throws an error and I spent now 5 hours with trial and error without any progress:
ValueError: AES key must be either 16, 24, or 32 bytes long
So I tried both:
initializing as string:
key = "0xB0,0x0D,0xDF,0x9D,..."
and 2. as byte-string:
key = b"0xB0,0x0D,0xDF,0x9D,..."
No effect.
Any comments or ideas?
Best Regards,
AFX

Answer 1

What you have is a hex string. For example, if you had this:
0x0F, 0x10, 0x1A
Then this, condensed down to be an actual hex string, is:
0F101A
Which, as raw bytes, is:
15, 16, 26
You just need to convert it to a byte array first. Have a look at binascii.unhexlify.

Answer 2

I wanted to provide the solution that worked out for me:
First, get the basics:
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
key = 'insert-key-here'
Now, I build a method to decrypt:
def decrypt_file(key, in_filename, out_filename=None):
""" Decrypts a file using AES (CBC mode) with the
given key.
"""
backend = default_backend()
with open(in_filename, mode="r+b") as infile:
iv = infile.read(16) #Calling .read() will move the iterator exactly as many positions
cipher = Cipher(algorithms.AES(key), modes.CBC(iv), backend=backend)
decryptor = cipher.decryptor()
with open(out_filename, mode="w+b") as outfile:
cipher_text = infile.read() #Only the first 16byte are the iv, the rest is the cipher-text
outfile.write(decryptor.update(cipher_text) + decryptor.finalize())
Here is the key: I had to transform the key to byte-string first
#Transform key to byte-string
key_int = [int(x,0) for x in key.split(',')]
decrypt_byte_key = b''
for x in key_int:
decrypt_byte_key += x.to_bytes(1, 'little')
Finally, you can run it on your file:
decrypt_file(decrypt_byte_key, "input.enc", "output.txt")
Have fun.