I am a firm believer that there are many privacy techniques you should focus on before encrypted messaging because they will offer you much more “bang for your buck,” things like good passwords, two-factor authentication, and even encrypted email. That said, I still believe that encrypted messaging is a critical part of a well-rounded privacy and security strategy. While the vast majority of our day-to-day conversations may be benign, it can still offer a lot of insight into who we are as people – our routines, likes, and personal thoughts. This information – mundane or not – is worth protecting.
For disappearing messages to work, your conversation partner has to promise they won’t take photos of their screen, and they have to promise to use an app that actually implements the feature instead of just pretending to, and the app developers have to promise to have implemented the code to delete a message when the service says it should
Is there actually a cryptographically-sound and physically-complete method for ensuring that a message is only legible for a temporary duration once it leaves your own device and is delivered to someone elses?
Nope, it is impossible to have messages deleted after a specific time. There is simply no way to do it.