Wiping Out Data

Matthew Flaschen matthew.flaschen at gatech.edu
Tue Mar 27 13:17:22 UTC 2007

Felipe Figueiredo wrote:
> On Tuesday 27 March 2007 07:10:35 Matthew Flaschen wrote:
>> That will rewrite the entire drive with gibberish 7 times (this can be
>> adjusted).  That should mostly obliterate all data, then reformat the
> Is it really different from 
> dd if=/dev/zero of=/dev/sdb
> ? I don't get it, why is it necessary to use random bits, instead of zeroing 
> all bytes (including the FATs)?

Simply overwriting the data once does not mean it can't be recovered.
The Department of Defense recommends overwriting 7 times with random
data.  It is my understanding that shred can do this.  Someone said
before that even:

dd if=/dev/random of=/dev/sdb

7 times would not be acceptable, though I'm not sure why.

> I did plenty of undeleting and unformating back in DOS days, and I know it's 
> not even guaranteed to work fully. 

Needless to say, the techniques have improved.  If you're looking for
the exactly engineering explanations of why random rewrites are needed,
I don't know.  You can start at
http://en.wikipedia.org/wiki/Data_remanence .

Matthew Flaschen

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