PDF DRM Print only once

Question

I am looking to create an online gift certificate and sell these via paypal on a website for a restaurant.

The website is Wordpress / PHP based and will be using Gravity Forms / PayPal addon to capture the users details and take payment for the gift certificate.

I can create a PDF certificate which the customer will be able to redeem by printing and visiting the restaurant.

Now the problem lies with management of the gift certificates. eg what is to stop a customer printing off a gift certificate and redeeming it. Then next time they visit the restaurant, printing off the certificate again.

RoyalMail.com has some kind of DRM solution whereby online postage can be printed off once and only once.

How can I programatically protect my PDF files to only allow them to be downloaded / printed once?

For the sake of clarification, I do not want to know whether this is a good idea or not. I know that it is not an ideal solution. All I want to know is how to do it/if it is possible. No solution is fool proof, and people will always be able to 'forge' or duplicate the certificates. I just want to mitigate the possibility of doing so. Naturally there will be an in-house mechanism for checking redeemed certificates.

With regards to comment about spilling coffee on the certificate... AFAIK, if you put cash into the washing machine with your clothes, you have to ask the bank of england to re-issue you the bank note. You cannot re-print your own bank note!

Answer 1

There's nothing to stop a user printing off as many copies of a certificate as they want. But managing the certificates based on their physical copies is a really really bad idea anyway. If someone prints out a coupon and then spills their coffee all over it then do they have to buy another one? And what's to stop someone printing off their single copy and then running that through a photocopier?

If each voucher has a unique ID encoded in it, then all you have to do is record the fact that the voucher has been redeemed. Then it doesn't matter how many physical copies there are, if a voucher has already been redeemed once then you know not to redeem it again if someone else presents you with the same voucher.