Not secure at all!
This app claims to "prevent screenshots altogether" on their desktop version. True, if screenshot is limited to one made by the software on the computer.
Unfortunately, a photo of the readable message may be taken with any camera--I took one with my iPhone just to check it out--so the recipient of the message may make a CYA copy and save or distribute as they choose.
On the iOS version, the words appear piecemeal, but a video of the reading could be made with the same effect.
Too many secrets?
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Confide - Private messenger