[clug] Secure Erase of disks/drives

Michael Cohen michael.cohen at netspeed.com.au
Tue May 8 07:19:03 GMT 2007

Just run dmcrypt/cryptoloop/(insert favourite product here) to be safe and dont
worry about it...

At worse the most you will have to overwrite is the boot sector, and start of
the partition where the encrypted key is stored - to prevent brute force
attacks against the key.

Besides I believe that I read somewhere (wikipedia?) that while on old mmf
drives bits where stored in direct magnetic fields (e.g. north = 1, south = 0)
in modern drive this is more similar to a Q phase modulation so there is a
maginetic carrier which has its phase modulated by the data. This means that in
reality the magnetic pattern produced by a zero looks almost the same as that
of random data with just different phase jitters in the Q constellation. This
is why on modern drivers it makes no difference if you write /dev/zero and
/dev/urandom on the ease at which data may be reconstructed.


On Tue, May 08, 2007 at 04:45:25PM +1000, Robert wrote:
> From that same paper
> Looking at this from the other point of view, with the ever-increasing 
> data density on disk platters and a corresponding reduction in feature 
> size and use of exotic techniques to record data on the medium, it's 
> unlikely that anything can be recovered from any recent drive except 
> perhaps one or two levels via basic error-cancelling techniques. In 
> particular the the drives in use at the time that this paper was 
> originally written have mostly fallen out of use, so the methods that 
> applied specifically to the older, lower-density technology don't apply 
> any more. Conversely, with modern high-density drives, even if you've 
> got 10KB of sensitive data on a drive and can't erase it with 100% 
> certainty, the chances of an adversary being able to find the erased 
> traces of that 10KB in 80GB of other erased traces are close to zero.
> That was written 10 years ago. And the features sizes used on modern 
> disks have gone down by an order of magnitude or two since then.
> I'm not convinced that there is any credible risk here.
> Alex Satrapa wrote:
> >On 08/05/2007, at 15:57 , Alex Satrapa wrote:
> >
> >>The cost is in the order of $100K for someone in the Western world 
> >>using commercially produced equipment, doing the work on a contract 
> >>basis.
> >
> >Though I'll also refer you to the paper by Peter Gutmann again:
> >
> >>Even for a relatively inexperienced user the time to start getting 
> >>images of the data on a drive platter is about 5 minutes. To start 
> >>getting useful images of a particular track requires more than a 
> >>passing knowledge of disk formats ... [and] would take approximately 
> >>2-10 minutes depending on the skill of the operator and the resolution 
> >>required.
> >
> >and
> >
> >>If commercially-available SPM's are considered too expensive, it is 
> >>possible to build a reasonably capable SPM for about US$1400, using a 
> >>PC as a controller
> >
> >So assuming someone in an undervalued economy has access to the tools 
> >and material required to build a more-than-reasonably capable SPM, and 
> >it ends up costing them $5000, and scanning a 300GB drive takes 
> >somewhere in the order of a few days - do you now have reason to be more 
> >paranoid about your data destruction?
> >
> >Alex
> >
> -- 
> Robert
> If the Hunter S Thomson quote "When the going gets weird, the weird turn 
> pro" is true, why aren't I rich?
> -- 
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> linux at lists.samba.org
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