[dm-crypt] Cascading two plain dm-crypt volumes

anderson jackson thewizard at mighty.co.za
Fri Nov 29 00:32:30 CET 2013


Hello, 

I have a small question regarding luks and plain dm-crypt, and I am unsure
what to use. 

I feel that the advantages provided by Luks obviously offers extra security
compared to plain, however I feel uneasy about the obviousness of the fact
that the drive is encrypted. Mainly because a disk with just random data could
have been wiped instead of encrypted. I would like the extra security provided
by luks without it being obvious that the disk is encrypted. To combat this I
was thinking about doing a cascade of two identical ciphers in plain mode, in
this case AES because the AES-NI acceleration will severely limit the
performance penalty of cascading two ciphers, I had the following setup in
mind:

first step: cryptsetup –-cipher=aes-xts-plain –-offset=0 –-key-size=512
open –-type=plain /dev/sdx cascade1, with the first independend password.
Second step: cryptsetup –-cipher=aes-xts-plain –-offset=0 –-key-size=512
open –-type=plain /dev/mapper/cascade1 cascade2, with the second independed
unrelated password.
Third step: nwipe --rounds=1 --noblank --prng=twister --method=random
/dev/mapper/cascade2, this will fill the last block device with random data to
completely fill up the entire disk. 
Fourth step: format the last block device with ext4.

My theory then is, that even when an attacker finds the first passwords, he
will never know he has because the result will be random just as with a wrong
password. In fact all possible passwords will result in random data. The
attacker has no way of knowing if there are cascades and how many. Am I right
to come to this conclusion or should I stick with luks and deal with it being
an obvious encrypted disk?

Kind regards. 



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