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

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.