Unless you write your own compiler, look at every line of the source by hand well enough to ensure there no "underhanded C code contest" backdoors, compile the

Unless you write your own compiler, look at every line of the source by hand well enough to ensure there no "underhanded C code contest" backdoors, compile the application yourself, load it onto your phone, and do the same for your friends and share keys with them in person, you don't have this kind of assurance for any communication app. You have to draw a line somewhere and accept some level of trust proportionate to your threat model. iMessage is good enough for most people who aren't Edward Snowden.