[dm-crypt] plain: opening with a wrong password

U.Mutlu for-gmane at mutluit.com
Sun Feb 8 11:09:48 CET 2015


Milan Broz wrote, On 02/08/2015 10:55 AM:
> On 02/08/2015 10:23 AM, Arno Wagner wrote:
>> On Sun, Feb 08, 2015 at 09:19:54 CET, Heinz Diehl wrote:
>
>> Form a purely practical perspective, the difference usually negligible.
>> Wile plain dm-crypt mounting fails at the mount-stage due to wrong
>> filesystem signatures, LUKS mounting fails at the decrypt stage.
>
> Beware, there are some combinations of the encryption mode + IV which decrypts
> the first block correctly in both cases, so fs returns correct signature
> but fs is obviously corrupted... if you are not lucky, fsck will run
> and breaks the fs irrecoverably...
>
> This cannot happen with LUKS.
>
> See here that the ext3 device created with ESSIV still have visible signature
> with plain IV:
>
> # echo "password" | cryptsetup create -c aes-cbc-essiv:sha256 -s 256 x /dev/sdb
> # mkfs -t ext3 -q /dev/mapper/x
> # blkid -p /dev/mapper/x
> /dev/mapper/x: UUID="f46ba5d8-8c26-4589-ac09-cb0829f2804f" SEC_TYPE="ext2" VERSION="1.0" TYPE="ext3" USAGE="filesystem"
>
> ... use fs
> # cryptsetup close x
>
> And now thy mistake with plain IV:
>
> # echo "password" | cryptsetup create -c aes-cbc-plain -s 256 x /dev/sdb
> # blkid -p /dev/mapper/x
> /dev/mapper/x: UUID="f46ba5d8-8c26-4589-ac09-cb0829f2804f" SEC_TYPE="ext2" VERSION="1.0" TYPE="ext3" USAGE="filesystem"
>
> # mount /dev/mapper/x /mnt/tst
> mount: wrong fs type, bad option, bad superblock on /dev/mapper/x,
>         missing codepage or helper program, or other error
> ...
>
> DO NOT use plain mode if you are not sure what you are doing. Really.
>
> There is a detached LUKS header which is better, the issues I mentioned in man
> about detached header page are side problems, nothing serious for most users.
> (But obviously depends on your threat model.)
>
> Milan

But isn't it just saying that the mount cannot be done
because something is wrong, ie. wrong/incomplete cipher param was given?

What happens if you repeat the whole with the correct params?

And, should one not use "/dev/sdb1" etc. instead of "/dev/sdb"?

-- 
cu
Uenal




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