Question on shredding a terebyte drive

Dean S. Messing deanm at sharplabs.com
Thu Sep 3 01:12:22 UTC 2009


Bruno Wolff wrote:
> Given 1, this seems like a foolish policy. Just eat the cost as part of
> securing your data. It might be cheaper than having you keep an eye of
> multiple write passes covering several days.

"I have no comment at this time."  :-)


> There was some old documentation that claimed reading the remmants from
> previous writes were recoverable, though I don't remember seeing costs
> estimates that low. I would have expected a lot of human time needed to
> help deal with the incomplete recovery.

I suppose the cost of such equipment is much cheaper now than in the
olden days of 15 years ago.  Also, given my comment below, a sensitive
read head (rather than an MF or AF Microscope) may be all that's
needed in the case of a "zero write-over".

> If one can recover a significant amount of data after writing zeros, one
> is going to be able to do it after writing a single pass of random data
> as well.

I don't think this is correct, based on my reading.  The situation is
a little bit like trying to decrypt a file after adding an unknown
constant numerical value to the data, vs. adding a "one-time pad" of
random numbers.  That's because writing zeros does not completely zero
out the local magnetic orientations so the variations can still be
detected.  A random pattern makes the problem much harder.  Multilple
passes (which is what /usr/bin/shred does) makes it even harder.

Dean




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