Just tested this. In a group with this enabled, indeed, the iOS app results in either a black, transparent, or otherwise missing screenshot of the area of the screen displaying the media. In the few minutes I played with it, I wasn't able to bypass it.
However, the chat is viewable in Telegram Web (K). Web K even offers to download the picture, which actually downloads the file.
Additionally, Telegram Desktop for Mac similarly allows screenshots as usual and isn't affected.
In the past I have seen some other apps implement screenshot prevention on iOS usinf something that "cloaks" the screen when the button combination is detected, and when the OS is about to background the app. This is the method Fido My Account (mobile carrier in Canada) uses (current version allows screenshots, but blocks the info from the app switcher).
The method Telegram is using appears to be rather seamless, as it does not show the screen going black or anything when the screenshot is taken. Stack Overflow seems to discuss two solutions, one using DRM video, and another using some password field hackery. https://stackoverflow.com/questions/18680028/prevent-screen-...
This is the most likely case, as the behaviour is similar to what happens when you take a screenshot of DRM netflix; it appears it is drawn by the hardware and not in the OS framebuffer, and thus shows up as a black box.
Of course this is not really security, but Telegram, despite being open source on the client, does have in the ToS that apps have to implement the "secure" features of secret chats properly, or risk being blocked. Recently they have been sending info out to bot developers that says apps that don't implement the ads in channels will be blocked as well, but I'm not sure if they will really enforce this.
"We ask that you make sure that these sponsored messages are supported and properly displayed in your app by January 1, 2022. Unfortunately, Telegram cannot financially sustain apps that support Telegram Channels but do not display official sponsored messages – such apps will have to be disconnected."
> Just tested this. In a group with this enabled, indeed, the iOS app results in either a black, transparent, or otherwise missing screenshot of the area of the screen displaying the media. In the few minutes I played with it, I wasn't able to bypass it.
Screen recording is handled in an interesting way. If you start the recording while in the Telegram app, it just freezes after the 3-second countdown and never starts. It times out after about 10 seconds. If you switch to another app before that, it starts the recording, but if you try to switch back to Telegram the recording freezes halfway through the app switcher animation. Protected media is also blanked out in the app switcher.
It's different from apps using DRM like Netflix, where it usually just shows a notification saying "failed to start screen recording".
I just tested it again, and as the user below notes, it has some way of interfering with screen recording. However this is not 100% reliable. as I was able to create a screen recording of the media in the group's media overview page and zoom in as usual.
The trick was to start the screen recording out of Telegram, open it (if you're on the actual page it will freeze the recording interestingly) but once you get to the media overview page, the screen recording works again and you can capture whatever you want. I have been able to record everything without problem once incl the chat, but this is probably only effective in some cases.
As it turns out, regular screenshots also work on that page. My testing was not super thorough I suppose, for something I quickly did while trying to fall asleep...
I'd guess it just uses the same iOS framework parts that are used already for Netflix and other streaming services to blank out the content. In that case screen recording would also be taken care of.
However, the chat is viewable in Telegram Web (K). Web K even offers to download the picture, which actually downloads the file.
Additionally, Telegram Desktop for Mac similarly allows screenshots as usual and isn't affected.
In the past I have seen some other apps implement screenshot prevention on iOS usinf something that "cloaks" the screen when the button combination is detected, and when the OS is about to background the app. This is the method Fido My Account (mobile carrier in Canada) uses (current version allows screenshots, but blocks the info from the app switcher).
The method Telegram is using appears to be rather seamless, as it does not show the screen going black or anything when the screenshot is taken. Stack Overflow seems to discuss two solutions, one using DRM video, and another using some password field hackery. https://stackoverflow.com/questions/18680028/prevent-screen-...
This is the most likely case, as the behaviour is similar to what happens when you take a screenshot of DRM netflix; it appears it is drawn by the hardware and not in the OS framebuffer, and thus shows up as a black box.
Of course this is not really security, but Telegram, despite being open source on the client, does have in the ToS that apps have to implement the "secure" features of secret chats properly, or risk being blocked. Recently they have been sending info out to bot developers that says apps that don't implement the ads in channels will be blocked as well, but I'm not sure if they will really enforce this.
"We ask that you make sure that these sponsored messages are supported and properly displayed in your app by January 1, 2022. Unfortunately, Telegram cannot financially sustain apps that support Telegram Channels but do not display official sponsored messages – such apps will have to be disconnected."