[dm-crypt] SSD disks and cryptsetup-reencrypt

Yves-Alexis Perez corsac at debian.org
Thu Jun 13 07:51:41 CEST 2013


On jeu., 2013-06-13 at 01:43 +0200, Matthias Schniedermeyer wrote:
> On 13.06.2013 00:30, Arno Wagner wrote:
> > 
> > That said, unless you have high-resource attackers to defend
> > against, with something like, say, 8 complete-disk re-encryptions
> > you should be relatively secure. But don't blame me if it turns
> > out you are not.
> 
> Or use one of the newer SSDs that take "the easy way" for secure 
> erasing.
> 
> At last one or more of the current generation controller chips encrypt 
> the contents by default (even without enabling FDE), as the controller 
> has to do some form of scrambling anyway as high-entrophy is better for 
> the flash cells(*). So at least one does AES256 encryption always. When 
> you secure erase such a SSD they typically just generate a new key and 
> not actually erase the flash-cells.

Actually they still need to erase the cells, at least as part of the
garbage collection, since at one point they will be reused. And when you
do a secure erase, the few seconds needed to erase all the cells doesn't
really matter, I guess.

That said, nobody knows exactly what the firmware do.

>  The unknown is if they "drop" the 
> old key in a secure way, but if they do there is no way to decrypt older 
> content even if you desolder the flash-chips.
> 
> Also you have to have to hope that key generation is really random. That 
> is something that can't really be proven (only disproven), so personally 
> that is not something i could rely on.

And one needs to know how it is linked to the ATA password, too.

>  So i classify it as a "nice to 
> have", if it works it is a line of defense otherwise it is "nothing".

Yeah, right now I don't think I'd trust a self-encrypting SSD and would
put luks on top of it anyway. Note that you lose some performances here
since those SEDs work way better on compressible data.
> 
> Problem is i can't remember which one(s) do(es) that, and it's bed time.
> :-)

SandForce (now LSI) controllers. Which can be found in OCZ and some
Intel drives.

Regards,
-- 
Yves-Alexis
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