SAN DIEGO (CBS 8) - A popular type of smart phone app is helping children break the law and hide it from their parents.

They’re called “vault apps” or “ghost apps” and they're no secret to the teenagers who use them.

Just search in the app store for "vault app" and dozens pop up. Kids download them for free onto their smart phones without telling their parents.

Once they're installed, the apps disguise themselves as calculators with names like Hi-Calculator or Calculator Plus.

“Sometimes they look a little different. They conceal themselves. That's what they're meant to do,” said technology expert Nyika Allen.

The vault apps typically operate like a normal calculator but enter a special code and the app unlocks a hidden folder where photos and videos can be secretly stored.

Videos and photos stored in the hidden folder cannot be seen anywhere else on the phone.

Last year in Colorado, police discovered a group of high school students using ghost apps to hide nude photos of their classmates that the students had been sexting to one another.

The students involved could have been charged with distributing child pornography.

“Parents just need to educate themselves, be one step ahead, know a little about the phone,” said Allen.

Experts say the best way to keep kids from downloading ghost apps is to activate parental restrictions on the child's phone.

If you suspect your child has already installed a ghost app, the only way to make sure it's uninstalled is to reset the phone to the factory default.

Then, you can then set the phone to require parental permission before any app can be reinstalled.