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

+3 votes
100 views

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
posted Oct 24, 2013 by Majula Joshi

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2 Answers

+1 vote
 
Best answer

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 Oct 24, 2013 by Anderson
+1 vote

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

answer Oct 24, 2013 by Deepak Dasgupta
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