What is copy protection?
Copy protection is the Holy Grail of the music and film industries. There have been attempts to make DAT tapes, and then CDs, and DVDs, and now MP3. These industry attempts have not been notable only in their lack of success.
At the heart of the problem is that everything on a computer is a copy – there are no originals! To play a music file, or watch a video, or read a PDF document, it MUST be copied on the computer.
And it follows that, no matter how hard you might try, something that must be copied in order to use it can also be copied! Not exactly rocket science. After all, you can play a music CD and record the output from the speakers if you want to, and no copy protection system on earth can stop you. It’s just a matter of convenience and quality.
So PDF copy protection cannot rely upon the idea that you can stop people copying computer files. It’s just plain stupid.
So how do you obtain PDF copy protection?
Well the first thing to do is make sure that the file that people have access to cannot be used without some other information. If you don’t do that then you have no copy protection at all. Because you cannot prevent people copying the file you have to prevent them being able to make any use of it.
Normally this is done using PDF encryption. This technique means that whilst people can (as happens with all files on computers) readily copy PDF files, they cannot make any use of the copied file because it is still encrypted.
Thus, PDF copy protection is not obtained just by trying to prevent people from copying physical files, but by making sure that the files they copy are of no use unless they also have some additional information that they cannot pass on to other people. The secret here is to make sure that the cryptographic key(s) used to decrypt the files cannot be passed on to others. This is achieved by making sure the decryption key is not seen by people and by ‘locking’ it to individual computers. This is a critical feature of any PDF copy protection product.
Protecting PDF files with passwords is therefore useless – it does not provide any copy protection because in order to view the PDF a user must enter a password – if they know this then they can give it to others along with the ‘protected’ PDF file.