[dm-crypt] Brute force aes-plain

Kereoz kereoz at kereoz.org
Wed May 16 19:03:40 CEST 2012

Hi all,

Quick story:
- are there any knows issues with plain dm-crypt volumes in Debian ? (Other than the
  default changing from aes-cbc-plain to aes-cbc-essiv ?)
- anyone here tried bruteforcing aes-cbc-plain (I got a rather short key) ? 

(Could you please CC me in the replies to this thread as I am not (yet ?) a
subscriber of this mailing list).

Long story:
I recently came back from a one year trip abroad, and got my hands back on an
encrypted hard drive I left there. I was pretty sure I knew the key for this
drive but after trying everything I could think about it is now sitting on my
desk until I find a solution. 

I don't know for sure whether I forgot the key or I am using the wrong
algorithm, as the version of cryptsetup I was using at the time was different
(different Debian release) and I read the defaults have changed. I am fairly
sure I used the '-c aes-plain' option initially but I had no luck with it. I
also tried aes-cbc-essiv and had no luck either. Is there anything else I could
try ?

Now, in the case I just forgot the key, it wasn't very long anyway (~ 10
characters) and I got some ideas about the characters it might contain.
Considering that most chances are that the algorithm is aes-cbc-plain, it is
probably possible. I tried writing a script for this, but there are several
issues :
- cryptsetup takes a while to create a devmapper mapping
- trying to mount the partition also takes a while
- cryptsetup then takes a while to delete the devmapper mapping

When you put that together, it is definitely too slow to bruteforce anything.

Is there anything faster I could use here ? I assume the best solution would be
to extract a couple of blocks from the hard drive, those containing the
filesystem superblock, decrypt it and then try to match the filesystem magic
number (reiser). I don't know how to do the decryption part quick enough for a
brute-force approch. Any suggestion would be appreciated.


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