This is a well known fact of data forensic science and why this science
flourishes at all. Overwriting data hardly obscures the data. Even when
sections of data sectors are overwritten many times, changes can very often
be reverse engineered. There are companies out there that make their entire
revenue on just such reconstructions.

In fact even de-gaussing (magnetic relignment of ions) sometimes fails to
remove all residual data. The degaussing machine can fail to cover the
separating bands between data. Imagine you had an eraser and managed to
completely erase everything except the top parts of characters. You could
probably guess what the data said. Imperfections in the media, the optical
writing mechanism and the de-gaussing tools can all contribute to there
being some residual data left.

Bar Biszick-Lockwood (cisa, cissp, csqa)
SDLC & Security Process Standards Expert
GCC SOX 404 Audit and Remediation

QualityIT
206-388-3333
pager message: 4252415391 (at) tmomail (dot) net [email concealed]
barbis (at) qualityit (dot) net [email concealed]

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-----Original Message-----
From: Jared Johnson [mailto:jaredsjazz (at) Yahoo (dot) com [email concealed]]
Sent: Wednesday, July 20, 2005 4:49 PM
To: focus-ms (at) securityfocus (dot) com [email concealed]
Cc: bugtraq (at) securityfocus (dot) com [email concealed]
Subject: Peter Gutmann data deletion theaory?

All,

Do you all agree with Peter Gutman's conclusion on his theory that data can
never really be erased, as noted in his quote below:

"Data overwritten once or twice may be recovered by subtracting what is
expected to be read from a storage location from what is actually read. Data
which is overwritten an arbitrarily large number of times can still be
recovered provided that the new data isn't written to the same location as
the original data (for magnetic media), or that the recovery attempt is
carried out fairly soon after the new data was written (for RAM). For this
reason it is effectively impossible to sanitise storage locations by simple
overwriting them, no matter how many overwrite passes are made or what data
patterns are written. However by using the relatively simple methods
presented in this paper the task of an attacker can be made significantly
more difficult, if not prohibitively expensive."

It seems that the perhaps the only real way to rid your Hard Drives of data
is to burn them.

I'd love to hear some thoughts on this from security and data experts out
there.

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