pycrypto: output is different each time for same text input?

Question

I seem to have misunderstood something about the way Crypto.Cipher is supposed to work, because I'm getting unexpected results, here is my code..

import hashlib
from Crypto.Cipher import AES
from Crypto import Random

h = hashlib.new('sha256')
h.update('my key')
key = h.digest()

iv = Random.new().read(AES.block_size)
cipher = AES.new(key, AES.MODE_CFB, iv)
txt = 'hello world'

This is the part where I'm confused, because it seems like encrypt will output a different result every time, so how can I decrypt it?

msg = cipher.encrypt(txt)    
>>> '|sx08xf2x12xdex8cDxe7u*'

msg = cipher.encrypt(txt)
>>> 'xa1xed7xb8h

Answer 1

Code that will work:

#!/usr/bin/python3
import hashlib
from Crypto.Cipher import AES
from Crypto import Random

h = hashlib.new('sha256')
h.update(b'my key')
key = h.digest()

iv = Random.new().read(AES.block_size)
cipher = AES.new(key, AES.MODE_CFB, iv)
txt = 'hello world'

msg = cipher.encrypt(txt)
print(msg)

cipher = AES.new(key, AES.MODE_CFB, iv) # Use *same* IV!
origtxt = cipher.decrypt(msg)
print(origtxt)

Also note that manually deriving a symmetric secret using SHA256 is an INCREDIBLY bad idea. Have a look at PBKDF2.

Answer 2

You're not reinitializing the internal state of the crypto engine. When you recreate "cipher" with the same IV every time, it will work.