From: Bill Stewart <stewarts@ix.netcom.com>
Date: Tue, 10 Oct 95 00:21:26 PDT
To: cypherpunks@toad.com
Subject: Re: PC disk wipe software
Message-ID: <199510100721.AAA20944@ix.ix.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


>There has been considerable discussion on the MCIP list about how most
>*Mac* programs that purport to do this, like Burn, for example, leave stuff
>over in disk blocks. My understanding is that this is not a trivial
>problem, and it occurs in MUSH-DOS as well....

Doing it _right_ is not a trivial job.  Consider disk controllers that
map out bad blocks so the OS never needs to see them - if you're trying
to remove highly classified data, the fact that Diskwipe won't ever see them
might be a serious problem, even if it is close enough for government work.

For a more realistic scenario, consider Stacker and other disk-compression
software;
you don't know what physical blocks the file was written on, or whether
overwriting
the logical blocks of the file will actually write to the same physical blocks.
In some cases you can assume that it probably won't.  You also can't expect to
write large blocks of 1s or 0s over the disk, because disk compression will
turn them into small blocks with run-lengths or equivalent.  (Writing PRNG
random noise will do, or you can generate counter-based patterns that may work,
or do something cryptographically strong if you want to take the time.)
#---
#                                       Thanks;  Bill
# Bill Stewart, Freelance Information Architect, stewarts@ix.netcom.com
# Phone +1-510-247-0664 Pager/Voicemail 1-408-787-1281
#---