[dm-crypt] Brute force aes-plain

Kereoz kereoz at kereoz.org
Fri May 18 13:23:13 CEST 2012

On Thu, May 17, 2012 at 09:27:28AM +0200, Arno Wagner wrote:
> Hi,


> > 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 ?
> You could just intsall that old release to be sure. Or maybe just
> get the binary or source package and check that way. But AFAIK
> Debian never changed anything from the package defaults, so these
> two should be it.

I'll give it a go just in case (probably using the Debian snapshots to make sure
I reproduce the same behavior may it be different).
> > 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). 
> Yes. There is a filesystem recognition linrary somewhere
> (used by mount -t auto), that may also be helpful. 

Good to know, I'll check this out. Reiser is fairly easy to recognize though (as
you can just grep the "reiser" string).
> > I don't know how to do the decryption part quick enough for a
> > brute-force approch. Any suggestion would be appreciated.
> Hmm. Use the password hashing from the c-sources of cryptsetup (it is a
> bit more complicated than just direct hashing) and instead of doing
> a mapping, use an external AES implementation (gcrypt, openssl, etc.)
> to decrypt your test-data. Make sure to get the IV right. It should
> be the sector number for "-plain".

Perfect, this is exactly what I needed to know.
> I would suggest to make this work first with generated test-data. (New
> volume, new FS, key e.g. "abc"), and when that works then try
> it on you actual data. May take a few days of programming though.

I'll have a look when I have some time and will let the list know if I get it to

Thank you for your answer.

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