The toughest part of designing secure products
It is already obvious that security is hard to do right. Bruce Schneier has written a good essay called: Why Cryptography Is Harder Than It Looks. This essay refers to cryptography, but touches on the subject as a whole. It is still not always clear, however, where the hard-core of security analysis work is, and where exactly the difference from QA, and from other system engineering domains, lies.
I would like to take a shot at explaining the fundamental difference between assuring functionality and assuring security, and pinpoint the toughest part of security analysis.