I agree with most of what you say, and the general idea is valid. However, the
specifics of
then a full reformat is quite enough to cause them to move on
to the next
machine - they're not going to have the motivation or
equipment to delve
into a randomly selected disk.
is a dangerously naïve approach. With point-and-click easy to use freeware
tools under windows, I can do almost 100% retrieval of files after a full
reformat, and even after reloading the OS and using it for a while, the simple
point-and-click freeware tools can retieve an awful lot of stuff. And if I
have the skills to use more powerful, complex tools, I can do even better,
without needing a lot of money, time, or even strong motivation.
Even for a home user, I'd recommend using a program that securely deletes stuff
by actively over-writing with multiple passes of random data (sdelete and DBAN
are a couple of my favorites). A format is *not* enough. Your general idea
(that it depends on the motivation and resources available to the attacker) is
good, just that your level of paranoia should maybe be turned up a notch :)
I'm not positive which Gutmann piece the OP was referring to, but if it's the
one I'm thinking of, it's a bit dated -- his methods were briefly really
popular as a shortcut to secure deletion, but if they're the ones I think he's
referring to, then they don't work with more modern file systems, so simple
random passes are better, though more costly to implement.
-----Original Message-----
From: Jeremy Epstein [mailto:jeremy.epstein@webmethods.com]
Sent: Thursday, July 21, 2005 2:01 PM
To: Jared Johnson; focus-ms@securityfocus.com
Cc: bugtraq@securityfocus.com
Subject: RE: Peter Gutmann data deletion theaory?
Like anything in security, "it depends". In particular, it
depends on what
the assumed adversary motivations and capabilities are. If
the adversary is
a nation-state with electron microscopes and other expensive
devices, and
the disk is believed to have held highly classified information, it's
clearly true that the only way to destroy the data is to burn
the disk (and
in the right way). If, on the other hand, the adversary is
someone who's
randomly buying used computers in hopes of finding carelessly
deleted files,
then a full reformat is quite enough to cause them to move on
to the next
machine - they're not going to have the motivation or
equipment to delve
into a randomly selected disk.
Where in between these two extremes it's necessary to burn
the disk is an
exercise left to the reader ;-) You really have to do a risk
analysis... If
it's cheaper / easier / less dangerous for the adversary to
dumpster dive to
get hardcopies or bribe someone or hack into the system, then
destroying the
hardware is putting the effort in the wrong place. For a lot
of classified
systems, the assumption is that obtaining used disks is a low
cost attack,
so it's cost effective to use destruction.
--Jeremy
-----Original Message-----
From: Jared Johnson [mailto:jaredsjazz@Yahoo.com]
Sent: Wednesday, July 20, 2005 7:49 PM
To: focus-ms@securityfocus.com
Cc: bugtraq@securityfocus.com
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.
