ScreenShield — a Third-Party SDK That Somehow Allows iOS Apps to Prevent Screenshots

Wednesday, 10 January 2018

From the announcement of a new version of Confide, a “confidential messenger” app:

ScreenShield is a patent-pending technology that allows you to view an app’s content on your screen but prevents you from taking a screenshot of it. If you try to take a screenshot on Confide, you will now simply capture a blank screen¹. ScreenShield also protects against other forms of screen capture, including iOS 11 screen recording, AirPlay screen mirroring, QuickTime screen recording as well as taking screenshots from the app switcher or by using Xcode.

We initially developed ScreenShield for Confide, but quickly realized that it could be used in a large number of apps — far more than we could build ourselves. That’s why we created ScreenShieldKit — to offer the ScreenShield technology to 3rd-party developers for use in a variety of different apps and categories.

While there’s a lot of technology under the hood that makes ScreenShield possible, the great news is that there are no strange gimmicks for users (e.g., it doesn’t require them to hold their finger on the screen) — it just works as expected. And ScreenShieldKit is simple for developers to integrate into their iOS apps, providing easy to use replacements for UITextView and UIImageView.

It’s an interesting puzzle trying to figure out how they’re doing this. Detecting that a screenshot has been taken is easy — iOS has an API that apps can use to get notified when the screen is recorded in any way. But ScreenShield is detecting it before the screenshot gets taken, so they can blank out the content in their text and image views.

I wasn’t familiar with Confide, so I downloaded it and kicked the tires, and the screenshot prevention works as advertised. Confide also sends a notification to whomever you’re messaging with to warn them that you tried to take a screenshot, a la Snapchat, and they immediately delete the message you tried to capture (I presume so that you can’t try to capture it another way, like, say, by taking a photo of the screen — see below).

My best guess as to how they’re doing this is that they’re using AVPlayer and somehow using FairPlay Streaming to block screenshots and recording. (Where by “my” best guess I mean the best guess of a smart friend who poked around the Confide app bundle.) Have you ever noticed how you can’t take screenshots of streaming video content in apps like Netflix and HBO Go/Now? That’s a feature in iOS (and MacOS — try taking a screenshot of Netflix video playing in Safari) for skittish video providers who don’t want us to capture even a still frame of their precious content. I think ScreenShieldKit is somehow using this to prevent screenshots or video captures of text or images.

If anyone out there has a better or more informed guess, please let me know.

If I’m reading their application correctly, Confide has also filed for a patent for a way to identify when you’re using another device to take a photo of your screen.