[ pycrypto-Bugs-877442 ] CBC mode broken?

Bugs item #877442, was opened at 2004-01-15 04:52
Message generated for change (Comment added) made by akuchling
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=120937&aid=877442&group_id=20937

Category: None
Group: None
>Status: Closed
>Resolution: Wont Fix
Priority: 5
Submitted By: Joakim Sernbrant (joakims)
Assigned to: A.M. Kuchling (akuchling)
Summary: CBC mode broken?

Initial Comment:
From the api (PEP 272) it is not apparent if you can use 
a cipher object for both encryption and decryption but I 
would guess that is the intention.

Then you need to maintain two IVs for CBC mode; one 
for encrypt and one for decrypt, PyCrypto only keeps 
one.

Python 2.2.3 (#1, Jun 10 2003, 16:41:13)
[GCC 3.3 20030604 (Red Hat Linux 3.3-5)] on linux2
Type "help", "copyright", "credits" or "license" for more 
information.
>>> from Crypto.Cipher import AES
>>> key   = "1234567890123456"
>>> iv    = "abcdefghijklmnop"
>>> plain = "ABCDEFGHIJKLMNOP"
>>> aes = AES.new(key, AES.MODE_CBC, iv)
>>> cipher = aes.encrypt(plain)
>>> print aes.decrypt(cipher) == plain
0
>>> aes2 = AES.new(key, AES.MODE_CBC, iv)
>>> print aes2.decrypt(cipher) == plain
1
>>>

The same probably applies to CFB, and OFB mode.

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>Comment By: A.M. Kuchling (akuchling)
Date: 2004-03-05 14:14

Message:
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user_id=11375

While it's possible to use an object for both encryption and
decryption, it's not a common or suggested usage as it would
be too easy to get the state of the IV confused.  

----------------------------------------------------------------------

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