This idea of yours is a very complicated feature and I hope you have lots of computer experience.
From you comments I have the impression that your eBook is using a "try before you buy" distribution method. -- so bear with me as I walk you through my idea of what would be involved.
Firstly this system would need two eBooks to work properly... If a customer decides to "opt in" the "Try" mode eBook won't work for this procedure and becomes redundant.
1) on your index.html web page the customer has the option to pay for the eBook or "opt in" for a mailing list. (I'll concentrate on the "opt in" feature)...
2) After clicking the "opt in" link -- the customer must be directed to your autoresponder system. (you'll need to control this process to make sure the customer "opts in".
3) After the customer "opts in" you redirect him to the HYPrLock web site security system... (Using the same method that PayPal uses to return to the HYrLock system).
4) The customer would then need to enter his e-mail address to get a download link..
5) After getting the download and Validating his "free" eBook -- he'd get his "free" password.
If you plan your work properly you should be able to collect his e-mail address during the "opt in" process (you must also get the "item number" of the product). Then you could use HYPrLock's shopping cart code to automatically send the customer a "free" e-mail download and password.
All of this hinges on your ability to meld the "opt in" process with the HYPrLock system.
BTW: This system meets all of your security needs.