Re: Deleting Files

Eric Sosman <esosman@acm-dot-org.invalid>
Thu, 30 Nov 2006 23:20:22 -0500
Mary Walker wrote:

"Tor Iver Wilhelmsen" <> wrote in message

"Mary Walker" <> writes:

What's the significance of the DOD format string?

To save REPEATEDLY in patterns that jumble the bytes thouroughly.
Saving ONCE with the same value does not mask the contents for the
purpose of "forensically" get at the data: residues of the old data
will remain in the magnetic media.

I know it's not good practise to ask for example code - but I'm not sure how
to implement this DOD format string in Java and a google search has not
proved helpful so far. And I'm still a bit hazy as to exactly what a DOD
format string actually is but I shall search some more. Any links/examples
would be appreciated.

     Before you get too excited about "DoD format strings," ponder
this fact: Your program is not writing to a physical disk, but to
a file system. If you write a bunch of 0xFFFF's to some part of a
file, and then write 0x0000's, and then 0x4242's, or whatever, how
many times has the physical disk arm moved to the sector in question
and deposited bits there? YOU DON'T KNOW! The file system may have
buffered up your 0xFFFF's and scheduled them to be written to disk
at a later time, and then "done you a favor" by cancelling that write
operation in favor of the "more up-to-date" 0x0000's, and then done
the same thing again with the 0x4242's. You might end up with just
one physical write instead of your three supposed writes. You might
end up with none at all, if the file deletion occurs before the actual
physical writes are done, and the file system "notices" that there's
no point in writing to a file that's about to vanish.

     You may even be dealing with a file system that doesn't use
fixed allocations. Instead of overwriting a bunch of bytes on the
disk, it may instead allocate a new chunk of space and put the
new bytes there, un-linking the old chunk but otherwise leaving it
untouched. "Generational" or "versioned" file systems make their
livings doing this sort of thing, and it can be difficult or even
impossible to "update in place."

     This kind of low-level bit-whacking requires low-level access
to the physical device, uncontaminated by program buffers, file
systems, RAID caches, NFS servers, integrity logs, and all the other
things that make I/O so much pleasanter nowadays than it used to be.
Java is ill-suited to the task. (So are C++, C, Fortran, and even
assembly language; you need to be inside the kernel, and sometimes
even that's not enough.)

     To the best of my (layman's) knowledge, the only reliable ways
to erase data from a disk involve such things as power tools, acid
baths, furnaces, dynamite, and disposal in the craters of active

Eric Sosman

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