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Comment by gblargg | Hacker News Reader

Doesn't the undo functionality cause it to store copyrighted data from the original ROM in the patch?

(Author here, my wife posted this thread; it has no bearing on the question but I am told it is good to disclaim.)

You could quibble about what counts but the core is "yeah".

Ppf3-with-undo stores literal "hey, we expect byte n to be 'foo'. on apply, replace it with 'bar'. on undo, put 'foo' back. remember: the copyrighted byte is 'foo'". Touch every byte in some way and your undo data is the entire original.

Some patches literally touch one byte. Their undo is, indirectly or indirectly, a recipe for retrieving one byte of copyrighted data. Is that fragmentary enough that it is entirely in the clear? I have no idea.

For some reason an XOR scheme escaped me. It's the perfect way, doesn't take any extra space over the size of the modified regions, and avoids even storing the copyrighted work of the ROM patch author. You could distribute the patch without anyone's permission (maybe)! Just include a strong checksum of the affected regions to be sure you're starting with the correct version, and checksums of the resulting data to give the same guarantee for reversing the process.

I think it's similar to adding a note to a book, and then removing that note, leaving you with a the original book. That scenario doesn't violate copyright law as far as I'm aware.

That's not how patching a rom works. You directly replace data and the formats that let you undo have to put a copy of the original data in the patch file. So yes it is a risk.

You don't need a copy of the original data, you can simply store your change as a XOR so undoing it mean XORing it again.

However, if your change involves zeroing something out, you're back to storing the original data again.

Slightly silly idea to solve this: Hash all non-patched areas of the file to produce a key. Use the key to encrypt your XOR value. Thus the key can only be derived by someone who has either the original file, or the patched version.

So? Since when has making backups of the copyrighted stuff that you have the right to use been made illegal?

Employing anti-protection methods to do to has been illegal since 1996 WIPO treaties, and then much more strongly in the 2002 DMCA.

So around 30 years now.

In fairness, games didn’t start employing encryption on the content till the 7th generation. So unless you’re applying ips patches to a 360 game you’re not breaking any DRM

Playing the title you backed up and patched is a different matter entirely

Courts have ruled many things besides encryption count as anti-copying measures, and that the laws apply retroactively, so it’s not so clear such patches even on 360 are legal. Try selling such patches and test it.

A patch wouldn't need to even care about encryption, since it's just looking at the raw file data.

A patch you can undo isn't a backup mechanism, it's data in the patch file when you download it. That's a big copyright risk.

You could design a system that generates undo info while you're patching. But it's easier to make a full backup copy of the original rom, and have no way to actually reverse the patching process.

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