It doesn't matter if its free or not. And it doesn't matter if its just an add on for an app. For example; on the android market there's an app called hide it pro. When you install it, it serves as an audio manager and helps disguise the fact that your trying to hide stuff. You hold the title on the manager and opens a number pad and you enter your password the man purpose of the app is to hide pictures or videos but also has a function to lock individual apps and also messages ect. They have it on the App Store but you can only pics/videos and other little stuff
См. статью: Is there an iPhone app that locks individual apps without jailbraking it?