A quick search of the apple app store reveals hundreds of these private, secret photo apps often disguised as calculators or audio managers.
"They've become extraordinarily advanced," said Metro State University security expert Steve Beaty. "It's an application -- it does work, but there is also the dark side of it."
With one app called "Calculator Lock," users type in a secret four-digit code on the calculator and press the equal sign. The app then transforms into a secret vault where users can store photos, messages or browse the internet.