You cannot. You need to use Amazon's system, if for no other reason than to ensure your buyers can actually
read the thing...
Also, DRM of any description can be broken, cf. the analog loophole.
Every commercial DRM ebook encryption scheme to date has been broken, save Apple's --and that because nobody buys their books anyway, so who'd waste their time working on it? See:
http://appprenticealf.wordpress.com for the reader perspective. Or search for DRM here on MobileRead.