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From: tjr@ihnet.ATT.COM (Tom Roberts)
Newsgroups: sci.crypt
Subject: Re: erasing magnetic media
Message-ID: <583@ihnet.ATT.COM>
Date: Fri, 6-Nov-87 10:04:13 EST
Article-I.D.: ihnet.583
Posted: Fri Nov  6 10:04:13 1987
Date-Received: Tue, 10-Nov-87 06:50:56 EST
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 20

> Why won't writing many 1's/0's to a media declassify it?

Two reasons:
	1) on many media (e.g. floppy disks), mechanical alignment is
	   necessary; writing over and over still doesn't GUARANTEE that
	   the entire width of the track was covered with the re-writes.
	   Someone very clever could come along with a very narrow read
	   head and read "between" the tracks, and find some thin region on
	   the media where the original data was written, but the over-writes
	   weren't.
	2) most magnetic systems (e.g. floppy disks) are designed with write
	   field strengths that put the media into the non-linear reqion of
	   the magnetic susceptibility curve. Over-writing does not GUARANTEE
	   that the original data was not further up the hysteriesis curve than
	   the over-write. Someone very clever could come along with a fancy
	   read amplifier, and read the original data, even in the presence of
	   the over-written data "noise".

Tom Roberts
ihnp4!ihnet!tjr