Destroying Data (Re: Remailer Policies)

Bill Stewart wcs at
Mon Dec 21 08:11:21 PST 1992

Yanek points out that "rm" doesn't destroy data on disks, just inodes,
and that you have to overwrite the data to destroy it.  However,
he suggests that you do this by
> 'cat /dev/null > /dev/hdxxx' which overwrites with zeroes all
as well as overwriting swap space and powering off RAM.  
Won't work, and it's the hard way to do it: 
cat /dev/null produces zero bytes of output (there's a /dev/zero on
some systems which produces lots of bytes of zeros.)

If you're dealing with amateurs (i.e. local cops or maybe FBI, but not NSA/KGB,
who can take the disk apart and use fancy magnetic techniques),
you can get away with overwriting the data once with zeros or other stuff;
a simple approach is to create a file large enough to fill all the
unallocated space on the disk (either using /dev/zero or writing your own);
be sure to be root so you get the last 10% of a BSD file system.
For professionals, overwrite it once with 0s, once with 1s, and once with
some test pattern like Xs or /etc/termcap.  This still won't help
if there are bad blocks which the disk controller is mapping out
transparently to your system, of course, but amateurs can't read them.
Swap space is tougher - you'll need to look at how your kernel allocates it
to know how to fill it all up, or else zap it all after rebooting your system,
if this is practical.

The government's rules for handling, um, special data say that you either need
an officially approved software program, or an Official Big Magnet,
or else you need to physically destroy the magnetic media from the disk.
I don't like having Big Magnets near my lab, but sandblasting has a
real physicality to it, and floppy disks make this satisfying scrunch in a 
paper-shredder :-)  Needless to say, unless you're Ollie North, 
you shouldn't be shredding your data when the cops are busting you ...

			Bill Stewart, somewhere in New Jersey

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