If this is referring to the old B&N > Fictionwise > eReader.com > Palm Digital Media DRM, you had to enter your name and credit card (as of the time you purchased the book). The ebook encryption saved that info as hashed data (I'm probably not saying that right) that was supposed not able to be used to pull the name/credit card from it. Back in 2002 when I first started buying ebooks, this was my DRM of choice and I read a lot about it before I decided to buy books that used it.
I tried a quick Google search, but couldn't find anything about it any more, other than "B&N is changing its DRM" blog posts.
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