When students were caught participating in an illicit photo ring that was operating out of a high school in Cañon City, Colo., parents and school officials there were quick to say that kind of thing was surely happening all over the country.

They’re not exactly wrong.

With the growth in mobile devices, sexting has become even more prevalent than when it first received widespread attention. Teenagers are certainly not the only people who share nude pictures, but more than a quarter of them have done so, even though they are likely to report feeling bothered by being asked for one, according to the Journal of the American Medical Association. They are following a path started by adults, who report that sexting makes them feel more sexually fulfilled, according to a recent survey by Drexel University.

Regardless of their age, many people who engage in the behavior would like to keep it a secret. And that is where the so-called vault apps come in. Here’s the rundown:

They’ve been around for years.

So-called vault apps have been around at least since Snapchat’s popularity began to rise in 2012, and they have offered a sophisticated suite of privacy tools for about as long.

A writer for the aptly titled Naked Security blog reviewed several vault apps that same year, noting that several of the free versions, like Secret Pictures and Photo Safe, lulled the user into a false sense of security.

But users have been able to pay for high-quality vault apps for some time. In a 2012 post, Ricardo Bilton, a writer for VentureBeat, wrote that a company called NQ Mobile offered a subscription-based vault app that went so far as to create a decoy vault that hides where the real scandalous material is buried.

“It’s a brilliant feature that adds a whole new level of subterfuge to the app’s feature set,” he wrote. “James Bond would be proud.”

If you open up Secret Calculator Folder Free, you’ll be presented with the face of what looks like a simple calculator. But tap in a passcode that you’ve set for the app, and you’re able to access your private stash of photos.

People shop around.

Nowadays, there are several apps available for keeping sexts secret, many of them are free and users aren’t really shy about searching for them. There are several threads on the question-and-answer site Quora dedicated to Android users learning to hide apps and photos on their phones, and users on a Reddit discussion thread mulled over the best options for Apple users.

Although confusingly named, the apps are more popular than you would think. Secret Calculator Folder Free has more than 800 individual reviews. With more than 1,500 individual reviews, Private Photo Vault is the 28th most downloaded photo and video app on the App Store, according to App Annie, a mobile app measurement service.

“A couple of months ago, a friend recommended this app to me when I had a few ‘pics’ I accidentally wound up revealing of my ex-girlfriend,” one user, Appleman7934 wrote. “The decoy password feature is great. Let’s say someone sees photo vault on your phone, and is like what is in that? You tell them the decoy password and it opens up to entire set of fake secret photos so people lose their curiosity.”

Appleman7934 gave the app five stars.

The law is trying to keep up.

But what about underage users? That’s a whole different set of problems.

The illicit photo-sharing ring that was operating in Cañon City High School might seem notable for its size, but Colorado is not the only place where officials are warning about vault apps.

In September, Leslie Rutledge, the attorney general of Arkansas, warned parents of vault apps — like Calculator%, Keep Safe Private Photo Vault, and Best Secret Folder — that are disguised on phones. Ms. Rutledge pleaded for parents to educate themselves about the tool, THV 11 in Little Rock reported.

“Ask them what technology they’re using,” Ms. Rutledge said. “If it looks like a calculator, such as this app, ask them to go further.”

Several of these apps exist in app stores, and officials in Cañon City say that students were using something similar. Officials are now investigating whether bullying or coercion was involved in that case.

Besides violating their school’s code of conduct, the students in Colorado could face possible felony charges: Sending photos of underage subjects, keeping those photos or sharing them with others is a Class 3 felony in Colorado, George Welsh, the superintendent of Cañon City schools, said Friday.

The current version of the law holds teenagers who send photos of themselves responsible.

“The letter of the law says that even if it’s consent, under the age of 18 you cannot give consent to share your naked image,” Mr. Welsh said.

What parents should look for.

Parents concerned about vault apps can take proactive measures by enabling parental controls. For families with iPhones, parents can screen apps before they are downloaded to their children’s iPhones with a feature called Ask to Buy.

By turning on Ask to Buy, whenever a child wants to download an app (whether free or paid), it sends a request to the parent’s iPhone, and the parent can then approve or deny the download. The steps, which are available on Apple’s website, involve setting up each iPhone with Family Sharing and then enabling Ask to Buy for the child’s iPhone.

For families with Android devices, parents can enable parental controlsinside Google Play’s app store to allow children to download apps only at a certain maturity level. For stricter controls, parents can download an app called AppLock on the child’s device and lock down any app that they suspect to be a vault app with a PIN code.

Websites that focus on digital learning for teenagers provide tips for parents on how to spot a hidden app. Common Sense Media, one such website, detailed some of the ways they use vault apps, and other apps that are used to discreetly take photos.

Tools like Stealth Cam, Private Ninja Cam and Top Secret Camera are designed to disguise photo preview screens and activate the camera through a motion sensor, for example.

A post published on TeenSafe.com said that parents should be proactive about looking for warning signs that teenagers are using vault apps: Hiding phone screens, refusing to give over passwords and a sudden increase in device usage are all red flags, according to the site.