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.
replyHowever, if your change involves zeroing something out, you're back to storing the original data again.
replySlightly 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.
replySo? Since when has making backups of the copyrighted stuff that you have the right to use been made illegal?
replyEmploying anti-protection methods to do to has been illegal since 1996 WIPO treaties, and then much more strongly in the 2002 DMCA.
replySo around 30 years now.