Silly question about secure deletion of files

[David Shaw]

On Fri, Jan 23, 2009 at 07:39:25PM -0300, Faramir wrote:
> Robert J. Hansen escribi??:
> ...
> > With a defrag, if you successfully rearrange 95% of the affected blocks
> > then you've substantially improved your drive performance.  Sure, it'll
> > report that it's done 100%, but who cares, really?
> 
>   Not me ;)
> 
> > With disk overwriting, if you successfully overwrite 95% of your
> > sensitive data, you may still be putting yourself at substantial risk.
> > Especially since it will report that it overwrote all your data.
> 
>   Ok, that answers my question.
> 
>   Usually I don't see thing from the same point of view than you, I
> mean, usually I think about "secure deletion" if the file can't be fully
> recovered by the use of a simple recovery tool (let's say, Norton
> unDelete)... but of course, in other context, fragments of information
> can be very dangerous and the attacker can bring the hdd to an
> specialized lab...
>   In other words, if I intend to destroy the PhD thesis of my brother,
> overwritting 50% of it would be more than enough to make him cry like a
> baby. But if I intend to destroy the list of infiltrated agents of the
> Rebel Alliance, even a 5% of the list would make Darth Vader be really
> happy.
> 
>   To David Shaw: I didn't get your point, since both defragmenting and
> overwriting files involve to be able to control what is written and
> where is written... I think both concepts are related somehow.

This is incorrect.  Defragmentation does not need to land on a exact
place on the disk, and overwriting does.  Defragmentation ignores
remapped blocks, while overwriting cannot, etc.

David