gbde blackening feature - how can on disk keys be "destroyed" thoroughly?

David Kreil kreil at ebi.ac.uk
Fri Sep 3 16:18:42 PDT 2004


Dear Vijay,

> I guess I took this off the list. It's OT, in my oppinion.

Oh. Anywhere more appropriate to send it to that you could suggest at all? Now 
also trying freebsd-geom - would that have been the better place to send this 
to to start with?

> I don't know much of anything about data recovery. But, if you can recover  
> data under 20 layers of random writes or 20 iterations of 0s, then how  
> *can* you wipe a hard drive? Short, preferably, of setting fire to it :D

Sigh, tricky, yes. Apparently wiping with >20 repeats of random noise does the 
trick (say from /dev/random or arc4random generated). The difficulty with 
modern file systems / operating systems / disk drives is actually getting the 
patterns written to the magnetic media.

I'm writing to the list because both assessing whether there really is a risk 
and how to fix it requires quite a bot of knowledge that I lack, like knowing 
where to look in the gbde code (maybe I misunderstood?), or writing code that 
is disk driver/hardware caching aware and can hence force a flush.

I'd be most grateful for any help or suggestions.

With best regards,

David.


> >
> > Hi,
> >
> >> From what I can see so far, they are simply overwritten with zeros - is  
> >> that
> > right? If so, the blackening feature would be much weakend, as once can  
> > read
> > up to 20 layers of data even under random data (and more under zeros). I  
> > would
> > be most grateful for comments, or suggestions of where/how one could  
> > extend
> > the code to do a secure wip of the key areas. Also, I know practically  
> > nothing
> > of how I could to best get FreeBSD to physically write to disk
> > (configurability of hardware cache etc permitting).
> >
> > With best regards,
> >
> > David.
> >
> >>
> >> Hello,
> >>
> >> I was wondering whether someone knowledgable about gbde internals could  
> >> tell
> >> me how the keys are being destroyed on request under the "blackening  
> >> feature".
> >> Ideally, I'd like them to be overwritten with random data at least 20  
> >> times
> >> independently, but I suspect it may well be done in a different way.  
> >> I'd be
> >> grateful for learning how the blackening works (and why!).
> >>
> >> With many thanks for your help in advance,
> >>
> >> David Kreil.
> >>
> >
> > ------------------------------------------------------------------------
> > Dr David Philip Kreil                 ("`-''-/").___..--''"`-._
> > Research Fellow                        `6_ 6  )   `-.  (     ).`-.__.`)
> > University of Cambridge                (_Y_.)'  ._   )  `._ `. ``-..-'
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> >
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------------------------------------------------------------------------
Dr David Philip Kreil                 ("`-''-/").___..--''"`-._
Research Fellow                        `6_ 6  )   `-.  (     ).`-.__.`)
University of Cambridge                (_Y_.)'  ._   )  `._ `. ``-..-'
++44 1223 764107, fax 333992         _..`--'_..-_/  /--'_.' ,'
www.inference.phy.cam.ac.uk/dpk20   (il),-''  (li),'  ((!.-'




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