So unrelated to this, what are the best couple resources out there for understanding the PKI system? I understand it in vague terms and I can use it. I just d

So unrelated to this, what are the best couple resources out there for understanding the PKI system? I understand it in vague terms and I can use it. I just don't know enough of the details of it to be comfortable with it. It may be that I would be even less comfortable with it if I knew the details.