Subject: Re: securely erasing a hard disk
To: <>
From: Jan Danielsson <>
List: tech-security
Date: 10/21/2005 03:49:21
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Philip Jensen wrote:
> I am wanting to understand the risk outlined in this paper:-
> Is it widely acknowledged that a disk (or file) needs to be
> overwritten up to 30 times in order to remove any traceable
> information on the magnetic media for the file?

   Hmm.. 30 times isn't enough. You'll never be able to get rid of the
first set of data you wrote to the disk by overwriting it. Not the
second set of data either... Probably not third, and perhaps not even
the fourth.

   If you are going to store sensitive data which you do not want to be
recoverable, you should fill the disk with random data a couple of times
before you start using it seriously.

> If so, then does the -P switch for the NetBSD rm command really
> provide the "security" of data erasure people think they are getting? 
> Or should the man page have an addition of "If you are serious about
> removing all traces of this file then ......."?

   The manual page should probably state that someone _really_
determined will be able to recover certain ("early") data no matter how
many times you overwrite it.

> Lastly, how long would it take to retrieve the contents of a disk (or
> file) which has been overwritten with `dd if=/dev/zero of=/dev/rsd0c`?

With limited or unlimited resources?

Kind Regards,
Jan Danielsson
Te audire non possum. Musa sapientum fixa est in aure.

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