Re: secure erasure of files?

[Rogier Wolff]

Martin J. Bligh wrote:
> > Some success is rumored to be able to be achieved by sampling the
> > normal signal, and then subtracting the "expected signal assuming the
> > current sequence of bits that was read". That way you might be able to
> > recover the information that peeks out from below. 
> 
> It's more than rumour - I've seen this done. Dr Solomon's (whatever
> they called their data recovery branch), early 1990's, England.
> Maybe it was easier on older hardware like the MFM / RLL disks,
> and certainly easier to piece together fragmented data with earlier
> file formats.

Maybe the difference is in "what's the goal". For datarecovery we
don't really care about just a couple of bits here and there: We want
to piece together the whole thing. 

If you don't want a piece of your data getting into wrong hands
however, you'd better be safe than sorry.

So I (and the Ibas guy) are talking about practical recovery of a
useful amount of data, while even a couple of bits is in theory
dangerous if you really want it "gone".

> I believe the point of overwriting 3 times (or whatever) is to reduce
> the "subtracted difference" to noise levels where it's no longer useful.

Right. 

		Roger. 

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