Unable to decrypt a message if the unencrypted text is not known

If you take the encrypted cipher resulting from any CBC operation and
decrypt the message using the same IV, key, key size, etc, the decrypted
string is longer than expected and results in gremlins being appended to
the end of the decrypted text.

SlowAES seems to only work in a round-trip scenario where the unencrypted
text is known at the time.

Using Josh Davis' implementation, you can see the issue first hand:

http://josh-davis.org/ecmaScrypt

Password: Test001!
256 Bit
Hex Key: e8cfe0b4e39dcc886ca83116b21db375ca231eeebf1df98f0b38163221385fcd
CBC
Encrypted String:
88ff1654ef2badfc22bc7dfa6c0d5d145cbfa547bce4d87c663473b09ce8d9c3

The result you will get is 'Hello, World! This is a test' but at the end
there will be extra characters at the end.

I tried using the fix submitted here:
http://code.google.com/p/slowaes/issues/detail?id=6

which works correctly unless I pass in a 16 character hex string as the
plain text.

Original issue reported on code.google.com by cougar...@gmail.com on 12 Feb 2010 at 7:35

fdussert / slowaes

Unable to decrypt a message if the unencrypted text is not known #9