md5 lock/unlock application php/js

I want to create an application that will produce a locking/unlocking "mechanism" for others to secure their hosted scripts/plugins etc with. I imagine doing this with js, since we could create unique code for each user on the fly, and then give them the code to paste into their scripts. The first script would lock the script, so it would need the right md5 key to unlock it. The second script would be the one generating the md5 keys and the user would paste the script into their thank you page, they page they redirect users once they have purchased, signed up etc. I have a graphical example here: [login to view URL] I want to provide a tool for others to secure their scripts using this concept.