Signal is armed with measures to instil a sense of privacy in users by preventing undisclosed screenshotting and screencasting message threads, and by enabling the sender to set their text, audio and video-based messages to disappear at a set time.
However, text-based messages appear not only in the message thread, like in Wickr or other messaging protocols, but, to a varying degree, a preview of them in the chats’ list as well. The chat list, with any messages shown in preview, may be screenshot or screencast without a system notification presented to the sender of the breach of security implied in the choice of using disappearing messages.
Depending on different factors (size of screen, orientation, system font size settings etc.), anywhere between ~50 (i.e. Pro size iPhone screens with default font size) up to several hundreds of characters (macOS) are displayed without the security measure to notify the sender of screenshotting or screencasting done by the user in receipt.
In fact, the messages although displayed are not considered displayed for purposes of the self-delete timer hence a message may remain on display beyond the time set by the sender, and may permit a full chain of custody of the message itself until and unless the sender deleted them.
Additionally, message previews outside of the app may also appear on the Lock Screen, the Notification Center or in the form of Banners elsewhere that do not seem to be prevented on the sender side.
Is there any elegant and secure way a sender can prevent all such insecure disclosure on the receiving devices of the addressee account?
Although one may choose to work around the issue by including filler text or random characters in excess of the previewable portion of a message; however, it would be both impractical and a lack of assurances of security given defining the theoretical maximum of characters displayed may prove an elusive objective.
While it appears that Signal no longer has the feature available on iOS, Android still has the option labelled “Screen security”
“to prevent people from taking screenshots of your chat, and it is one of the most robust features you might’ve ever seen[; t]he feature is called “screen security[” w]hen enabled, the signal logo will appear to hide your chats in the recent menu with a blue privacy screen[; i]t will also protect anyone from reading your conversations from the recent app menu[; o]ne syllable [(sic!)] they have will block a screenshot in the AP and also in the recent list[; i]f you want to turn off the feature at some point, you just have to follow the same steps to disable it.”