On Tue, Dec 11, 2001 at 12:09:51AM -0800, Mike Bresnahan wrote:

> I believe at least one of the copy protection schemes used, Safedisc, writes
> a cryptographic signature on the CD that cannot be read or written using
> "normal" means.  I'm not sure what "normal" means exactly, so don't ask.

SafeDisc is a product of Macrovision, see

http://www.macrovision.com/solutions/software/cdrom/pccdrom/

> When the application starts up, it checks for the presense of this signature
> on the CD.  If its not there, the application aborts.  I don't think there
> is any easy way to copy the signature, therefore pirates hex edit the
> executable to remove the code that checks the CD.  They then copy everything
> except the signature.  I believe you can read more at www.gamecopyworld.com

If you read the FAQ, it says that SafeDisc is compatible with 99% of
the CD-ROM players out there.  What I know of these things suggests
two things:

1:	This is a digital watermarking scheme.  They are actually
playing with the encoded data as it goes on to the CD, so it really
*is* tied to the physical layer.  One of the leading researchers in
this area is actually at our own little U of MN (Dr. Ahmed Tewfik.)

2: 	(mix of fact and speculation) The way that seems most likely to me
for this to work is that they manipulate the recorded data so that
there are correctable errors introduced on playback.  If the machine
is played back, the *hardware* will strip the error generating copy
protection, and so the copy will be "better," in the sense that it has
the data, but not the protection code.
	As I said in the last post, CD-R has a huge amount of overhead
-- a good read can have an error rate of as high as 200 errors per
second and still have 100% data accuracy.  So they use some of these
errors themselves.

What I don't know, is how they get their hands on the error data to
determine the unique signature, but those would probably be better
understood if you knew SCSI/IDE programming.  I'm pretty sure that
*some* sort of relevant error info is presented by the hardware, and
that would be how they say "this is the right disc."  Otherwise, they
assume it's the wrong one.



-- 
"Trying to do something with your life is like
sitting down to eat a moose." --Douglas Wood