Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site hp-pcd.UUCP
Path: utzoo!linus!decvax!harpo!seismo!hao!hplabs!hp-pcd!john
From: john@hp-pcd.UUCP (John Eaton)
Newsgroups: net.crypt
Subject: Re: Software Piracy and Coupons - (nf)
Message-ID: <2305@hp-pcd.UUCP>
Date: Fri, 4-Nov-83 03:42:53 EST
Article-I.D.: hp-pcd.2305
Posted: Fri Nov  4 03:42:53 1983
Date-Received: Mon, 7-Nov-83 00:17:49 EST
Sender: netnews@hp-pcd.UUCP
Organization: Hewlett-Packard, Corvallis OR
Lines: 32

#R:utcsrgv:-255700:hp-pcd:42000001:000:1270
hp-pcd!john    Nov  3 08:49:00 1983

Such a scheme might be implemented by lowering the write current to the disk
head to a low enough level to "guarantee" unreliable reads. The tricky part
may be doing it in a production enviorment. If the current is to low then
some customers marginal drives may not pick up anything. If it is to high then
some customers high performance drives may actually be able to read the data.
It would be interesting to see how big the window is where that scheme would
actually work.

Another method that may work would involve changing the timing of the data 
pulses. A disc controller expects to see bit transitions at certain discrete
times and creates a clock signal that places these times in the center of a
square wave.Instead of writing a bit that would occur in the center of the
clock you write one that occurs exactly on the edge of the clock. Depending
on disc speed and various other factors that bit could be picked up as either
a one or a zero.

While such schemes may be a good way to keep your space invaders game out of
the public domain,I would never buy a critical piece of software that I needed
if it were copy protected. There are times when you just cant wait a couple of
days for someone to replace a worn out disc.


John Eaton

hplabs!hp-pcd!john