@supernova wouldnt take "epheremal messages" as anything serious.
nothing stops the other party from making a screen or taking a picture of the screen, its security snakeoil.
all i could help with is keeping your phone clean, and being sure that the chat is gone on your own device in the case it gets seized at a later point in time. Also you need to make sure it gets propperly scrubbed instead of just being "deleted" in the tradional meaning.