On 12/22, Milan Broz wrote:
> Below is very nice example of another "Evil maid" type attacks,
> here directly applied to LUKS CBC disks.
>
> I think it clearly shows known rule:
> If you let your machine out of your sight, it is no longer your machine.
>
> What is important (and blog mentions it)
>
> "It has already been known for a long time that CBC does not prevent
> a malleability attack (targeted manipulation of encrypted data) given
> that the attacker can modify the ciphertext and knows the corresponding
> plaintext as well."
Even more important, in this particular case, is that this "practical
malleability attack" isn't actually very practical at all:
"In the following I assume that we already have access to the
original plaintext and the ciphertext of one file on the system and
that we want to do our manipulations in this file:"
There are a number of other assumptions and variables that must be "just right"
in order for this attack to have even a remote chance of working, e.g.:
"This code can be executed from a Live CD against the encrypted
partition of an Ubuntu 12.04 installation. The position of the
/bin/dash file needs to be adjusted by doing a reference
installation with the same disk layout on a sufficiently similar
hardware."
> BTW blog doesn't mention that CBC is no longer default mode for cryptsetup
> and was replaced by XTS mode.
The original post to f-d [0] that you forwarded does mention this:
"This code can be executed from a Live CD against the encrypted
partition of an Ubuntu 12.04 installation. The position of the
/bin/dash file needs to be adjusted by doing a reference
installation with the same disk layout on a sufficiently similar
hardware. [...] When choosing to encrypt the system with the Ubuntu
12.10 installer, the encryption is set up with mode aes-xts-plain64,
which is not vulnerable to this attack."
It's certainly interesting from a technical perspective but this is
simply not very feasible.
/p
[0]: http://archives.neohapsis.com/archives/fulldisclosure/2013-12/0187.html
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