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Re: Gutmann's research paper today
From: gimeshell () web de
Date: Wed, 8 Feb 2006 10:11:45 +0100

On Tue, 07 Feb 2006 10:07:38 -0500
Valdis.Kletnieks () vt edu wrote:

DoD 5220.22M only requires 3 passes and verify of each pass - all zeros, all
ones, and all "the same character" (for instance, 'AAAAAAA..' or similar).
That's good for sanitizing disks up to Secret.  For anything higher, physical
destruction is mandated. A "few passes of random scrubbing" is probably
equivalent to 5220.22M for any realistic usage.

But isn't recovering from lower "layers" much easier, if you can predict overwrite-patterns?

After i read "a few passes" another question arised to me:

In his paper he wrote, that securely deleting data from disk is very difficult, because of the fact that write head 
doesn't set polarity of all "magnetic domains":

"Faced with techniques such as MFM, truly deleting data from magnetic media is very difficult. The problem lies in the 
fact that when data is written to the medium, the write head sets the polarity of most, but not all, of the magnetic 
domains. This is partially due to the inability of the writing device to write in exactly the same location each time, 
and partially due to the variations in media sensitivity and field strength over time and among devices."

Probably this statement is right for modern (E)PRML drives, too.

So doesn't incrementing amount of rounds of random writing increase probabilty, that write head sets polarity of _all_ 
magnetic domains sooner or later and thus making secure deleting closer?

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