[dm-crypt] dm-crypt overhead

Lukáš Pohanka numberfour at seznam.cz
Thu Mar 1 15:59:53 CET 2018


Hi Milan,

thanks for clarifications. Unfortunately, we are currently constrained to
use only dmsetup and cannot provision the target device with cryptsetup.
Thus we cannot use LUKS, only plain dm-crypt.
Hopefully it will change in the future. However, does this mean there is
currently no chance of using any form of authenticated encryption in our
case?

Thanks,
Lukas

2018-03-01 15:42 GMT+01:00 Milan Broz <gmazyland at gmail.com>:

> On 03/01/2018 03:24 PM, Lukáš Pohanka wrote:
> > I have a couple of questions regarding dm-crypt and overhead when using
> different encryption algorithms.
> >
> > Firstly, am I right that the aes-xts-plain64 algorithm has no overhead,
> i.e. the size occupied at the target device is exactly the same as it would
> be without the dm-crypt layer?
> >
> > Secondly, when using aes-gcm, is the authentication tag created
> per-sector? This means in this case there is an overhead per each sector
> (depending on the tag size)?
> >
> > Also I couldn't find how the IV is calculated in case of aes-gcm, can
> also -plain64 be used?
> >
> > Thanks in advance for clarifications.
>
> Hi Lukas,
>
> all default FDE modes are length-preserving, so there is no additional
> per-sector metadata space, so it cannot use AEAD.
>
> With LUKS2 I introduced authenticated encryption (still experimental)
> where you can use some authenticated modes,
> but there are many limitations for now.
>
> For the basic info see my FOSDEM slides
> https://fosdem.org/2018/schedule/event/cryptsetup/
>
> and release notes for cryptsetup2
> https://www.kernel.org/pub/linux/utils/cryptsetup/v2.0/v2.0.0-ReleaseNotes
>
> Most of your questions should be answered there.
>
> (There is some paper submitted regarding this, I hope I can make it public
> soon, or mail me privately.)
>
> Milan
>
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