Re: Erasing File Data

"Sebastian G." <seppi@xxxxxxxxx> (07-05-13 02:45:59):

With /dev/urandom? That's gonna take ages.

Why not /dev/zero? Anyway, this is exactly the same as 'shred'.

So shred is (or at least appears to be) suitable for my
purposes, though I don't know whether the patterns it uses are
state of the art,

Huh? The pattern doesn't matter at all. Where have you been the
last years?

Secure data erasure has never been a problem for me. But if the
pattern doesn't matter, then shred does a good job. Unlike dd with
/dev/urandom, it's pretty fast.

dd with /dev/urandom is only limited by the PRNG (hm? RC4 outpust
200MB/s on my machine), synchronisation with /dev/random and the
entropy estimation in /dev/random. It's much more likely that you
simply hit the limit due to the data transfer rate of your HDD, which
is exactly the same as 'shred'.

In Linux 2.6, an SHA-based generator is used. That _is_ slow -- slower
than my hard-drive. shred on the other hand uses easy-to-compute
patterns. The first and last patterns it writes are random. Between
those, a not-so-random pattern X is written, followed by ~X (i.e. its
complement).


That's how I did it, before I've got enough RAM to not need swap
anymore at all.

You've got 4 GB of RAM? And even then I'm not fully obligated to
believe you.

I've got 1.5 GB of RAM, and that's enough for my purposes.


BTW, I've read somewhere that even RAM contents are restorable for a
few days, after power is turned off. Does anybody know something
about this?

Why don't you read Mr. Gutmann's paper on that issue? Basically you
might have some seconds at best, and for retaining the data much
longer you'll need to deep-freeze it.

Yes, this is where I've read that. But as far as I remember, he wrote
that the data can be restored even a few hours or days later. Well, I
guess, for most encrypted systems, rubberhose cryptanalysis is still the
most effective techinque, rather than carrying a fridge around with you.


Regards,
Ertugrul Söylemez.


--
Security is the one concept, which makes things in your life stay as
they are. Otto is a man, who is afraid of changes in his life; so
naturally he does not employ security.
.

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