[Dshield] Secure Data Removal

Josh Tolley josh at raintreeinc.com
Fri Jun 4 15:34:57 GMT 2004

As with all things digital, measurements are made, and a threshold is 
determined to mark the difference between a 1 and a 0. For instance, in 
digital circuits, a potential of more than some predetermined voltage is 
a 1, and anything else is a 0. Although voltages of the actual signals 
vary, as long as all the "1" signals are greater than the given voltage 
and the "0" signals are less than it, your signal remains wholly intact. 
Which is indeed why digital reproduction of analog data is so much 
closer to the real thing than analog reproduction in most cases -- the 
signal can be distorted quite a lot and still remain on the proper side 
of that threshold value, so the signal remains unchanged.

When you overwrite a hard drive with all 0's, you change the strength of 
the magnetism of each bit on the drive to something less than a 
particular threshold. Picture the magnetic strength of each spot on the 
drive as though it were drawn on a graph, with 1's as peaks and 0's as 
valleys -- you've just gone and pushed the graph down a few notches so 
that the entire drive, peaks and valleys alike, are all below that 
threshold value. But there are still peaks and valleys, they're just not 
as big. If you can read those peaks and valleys, you can still recover 
the data.

Josh Tolley
Raintree Systems, Inc.
760 509 9000

> -----Original Message-----
> From: Christophe Rome <asrgchr at yahoo.com>
> To: list at lists.dshield.org
> Date: Fri, 4 Jun 2004 04:23:01 -0700 (PDT)
> Subject: [Dshield] Secure Data Removal
>>A while ago there was a small topic about secure data
>>removal on this list. I remember that overwriting an
>>entire hard drive with O's would erase it completely
>>in a FAST way but NOT a SECURE way. I was wondering
>>now how one could be able to retrieve data from a hard
>>drive that has been completely overwritten with 0's?
>>Or is my assumption wrong?
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