Concealment is used to provide services only to those who know how
to access them. Examples include secret hallways with trick doors, menu items
that don't appear on the menu, and programs that don't appear in listings but
operate when properly called.
Complexity: This type of concealment is often referred to as security through
obscurity because the effectiveness of the protection is based on a lack of
knowledge by attackers. This is generally held to be quite weak except in
special circumstances because it's hard to keep a secret about such things,
because such things are often found by cleaver attackers, and because
insiders who have the special knowledge are often the sources of attacks.